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Nitrous oxide (N2O), with a global warming potential 300 times higher than that of CO2, represents 6.2 % of the total greenhouse gas emission inventory worldwide. Furthermore, N2O is considered the most critical O3- depleting substance emitted in this XXI century. In spite of the environmental relevance of this pollutant, very little research on biotechnologies for the treatment of N2O emissions has been conducted. In this study, the potential of a bubble column (BCR) and an internal loop airlift (ALR) bioreactors of 2.3 L was evaluated for the abatement of N2O from industrial emissions from nitric acid plants along 62 days of operation. The systems were inoculated with a methylotrophic Paracoccus denitrificans strain (DSM 413) and continuously supplied with methanol as a carbon and electron donor source for the anoxic reduction of N2O. The simulated waste gas consisted of a N2 gas stream containing 1 ± 0.1 % of O2 and 3377 ± 312 ppmv of N2O at the inlet of theBCR and 1 ± 0.1 % of O2 with N2O concentration of 3617 ± 342 ppmv at the inlet of the ALR. This N2-laden stream was supplied at a constant flow rate of 110 ml min-1 in each reactor. The performance of the BCR was characterized by a steady state N2O removal efficiency (RE) of 87 ± 3 % with CO2 productions of 308 ± 36 gm-3 d-1 and total suspended solid (TSS) concentrations of 867 ± 109 mg L-1. On the other hand, the ALR showed a N2O RE of 88 ± 2 % with productions of CO2 of 346 ± 28 g m-3 d-1 and TSS concentrations of 874 ±88 mg L-1. This work constitutes, to the best of our knowledge, the first systematic study of a biotechnology for the continuous abatement of N2O from nitric acid plants.

Benzene, toluene, ethyl-benzene and o-xylene degradation was evaluated under anoxic denitrifying conditions. These aromatic hydrocarbons were supplemented as single carbon sources or in dual or quaternary mixtures in order to identify key interactions during the degradation process. The results showed that toluene, together with ethyl-benzene, were the most readily biodegradable compounds under anoxic conditions among the aromatic compounds and their combinations tested. However, ethyl-benzene degradation time was significantly increased when combined with toluene as carbon source. Both benzene and o-xylene demonstrated to be highly recalcitrant compounds for anoxic denitrification, their degradation being rapidly inhibited likely as a result of the accumulation of toxic metabolites excreted during the degradation process. In this sense, mineral medium renewal was required to achieve complete degradation of these pollutants. Any combination of the aromatic compounds revealed an inhibitory interaction during the biodegradation process, an increase in the biodegradation time and therefore a decrease in pollutant degradation rates being observed upon addition of more than one compound.

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Photosynthetic microorganisms such as cyanobacteria, microalgae, and anoxygenic phototrophic bacteria (APB) have emerged as sustainable and economic biotechnology platforms due to their ability to transform energy from light into chemicals through photosynthesis. The light is absorbed by photosynthetic pigment-protein antenna complexes which are composed of pigments such as bacteriochlorophylls (BChl) and carotenoids in APB, and chlorophylls (Chl), phycobiliproteins (PBP), and carotenoids in cyanobacteria and microalgae. These photosynthetic pigments are essential in the physiology of photosynthetic microorganisms and offer significant health benefits due to their potent antioxidant activity, with properties that include anticancer, antiaging, antiproliferative, anti-inflammatory, and neuroprotective effects. This review first provides an overview of current advances in photosynthetic pigment synthesis and the latest strategies to increase pigment content in cyanobacteria, microalgae, and APB. It then delves into the pigment production process, covering biosynthetic pathways, critical environmental parameters, and extraction methods. Finally, the potential marketability of photosynthetic pigments together with current limitations are discussed., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2025 The Authors. Published by Elsevier B.V. All rights reserved.)

In this study, the performance of a pilot-scale biotrickling filter (BTF) for anoxic hydrogen sulfide (H 2 S) removal from real biogas was evaluated over 226 days. The BTF, inoculated with activated sludge from a nearby wastewater treatment plant, operated in an industrial environment with raw biogas from an anaerobic digester fed with municipal solid waste. The operating strategy was based on controlling nitrate consumption by sulfur-oxidizing nitrate-reducing (SO-NR) bacteria. H 2 S-inlet loads (H 2 S-IL) ranged from 0.43 to 61.73 g S-H 2 S m³ h⁻¹ , with biogas flow rates between 0.25 and 3.5 m³ h⁻¹ and H 2 S concentration peaks of 11,000 ppm v . The pH was maintained at 7.0 using a 25 %v·v⁻¹ NaOH solution. The H 2 S removal efficiency (H 2 S-RE) exceeded 93.9 % for most of the period, with a maximum elimination capacity of 35.17 g S-H 2 S m⁻³ h⁻¹ at an IL of 37.20 g S-H 2 S m⁻³ h⁻¹ (H 2 S-RE > 95 %). Sulfate selectivity was over 85 % during normal operation. High H 2 S-IL led to excessive elemental sulfur accumulation without nitrate, which was later oxidized to sulfate during low H 2 S-IL periods, preventing maintenance shutdowns. Overall, the results demonstrate the efficacy of anoxic BTFs in removing H 2 S under variable biogas conditions at a pilot scale., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Raquel Lebrero reports financial support was provided by University of Valladolid. Raquel Lebrero reports financial support was provided by European Union. Raquel Lebrero reports financial support was provided by Regional Government of Castilla y Leon. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Syngas from the gasification of organic wastes represents a promising feedstock for fostering a sustainable bioeconomy. However, its potential is currently constrained by the low-value products generated. Osmolytes, such as hydroxyectoine, are high-value compounds, however, their biological production as isolated osmolytes is not yet cost-effective. This study utilized shotgun genomics and laboratory validation to find a carboxydotrophic, halotolerant bacterium, Hydrogenibacillus schlegelii, that could produce hydroxyectoine using H 2 , CO and CO 2 as the sole source of energy and carbon. Subsequently, NaCl concentration, temperature and syngas composition were optimized in semi-continuous bioreactors. Optimal conversion of CO into hydroxyectoine occurred at a gas composition of 70 %:10 % CO:H 2 (v/v) (44.8 ± 10.1 mg hydroxyectoine ·g biomass -1 ). NaCl concentrations of 5 % significantly enhanced hydroxyectoine content (46.7 ± 9.5 mg hydroxyectoine ·g biomass -1 ), but negatively affected gas consumption. This study opens new perspectives for the valorisation of syngas into hydroxyectoine, and for new cell platforms for pharmaceutical production based on syngas., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

This study evaluated the influence of nanoparticles in both suspension and solid format on the performance of a microalgal process devoted to photosynthetic biogas purification. The experimental system consisted of an enclosed tubular photobioreactor coupled to a biogas absorption column through a mixing chamber. The high NH 4 + concentration in the inlet mineral medium (530 mg N-NH 4 + L -1 ) and the punctual addition of 115 mL of nanoparticle suspension to the system caused inhibition of the microalgal-bacterial cultivation. Conversely, cultivation broth aeration (0.5 L min -1 air flowrate) allowed the biomethane production fulfilling the EN 16723 (CH 4  > 90 %, CO 2  < 2 %). The nanoparticle suspension performance was superior to that of their solid counterparts in terms of CO 2 removal efficiency at equivalent nanoparticle dose (77 % vs. 49 %). However, parameters such as the nanoparticle suspension dosage and biomass concentration in the photobioreactor should be optimized to further improve biomethane quality before its industrial application., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Today, the use of biogas to produce more sustainable bioproducts is attracting an increasing attention in the quest for a circular economy. This work aims at optimizing the biosynthesis of high value bioproducts such as ectoine and hydroxyectoine from methane using a high mass transfer Taylor flow reactor and a methanotrophic consortium. The influence of the gas residence time (30-240 min) and concentration of microorganisms (0.1-1.8 g TSS·L -1 ) on methane bioconversion and ectoine production was evaluated. The maximum methane bioconversion efficiency reached was ∼90% at a gas residence time of 240 min. Biological limitation in the reactor was observed at concentrations below 0.5 g total suspended solids (TSS)·L -1 . The intracellular ectoine and hydroxyectoine content did not experience large variations with the gas residence time and biomass concentration. The maximum ectoine content was 49.0 ± 16.1 mg EC ·g TSS -1 at a gas residence time of 240 min and a biomass concentration of 0.7 g TSS·L -1 . The maximum hydroxyectoine content was 13.0 ± 3.6 and 12.7 ± 2.2 mg HE ·g TSS -1 at a gas residence time of 240 min and biomass concentrations of 1.8 and 1.2 g TSS·L -1 , respectively. Methylophaga and Methylomicrobium were the dominant methanotrophs in the bioreactor regardless of the gas residence time and biomass concentration. Microorganisms belonging to the genera Paracoccus, Methylophaga, Methylomicrobium and Nitratireductor have been identified as ectoines producers or have been found to possess genes responsible for ectoines synthesis., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Siloxanes (VMS) represent a class of organosilicon compounds known for their adverse effects on both the environment and human health. Their presence in biogas significantly hinders its economic valorisation, highlighting the need for effective treatment methods. This study investigates the performance of three different packing materials in the anoxic biofiltration of VMS (L2, L3, D4 and D5). The materials evaluated included plastic rings (BTF-1), polyurethane foam (BTF-2) and plastic rings combined with activated carbon (80:20) (BTF-3). Among them, BTF-3 exhibited superior performance, achieving maximum VMS removal efficiencies (REs) of 90%, including the complete elimination of L3 and D4, and ∼80% removal of D5, attributed to the presence of activated carbon. However, the abatement of L2 was inferior to that of other VMS (<80%), which was attributed to the activated carbon's affinity for larger molecular weights and critical diameters. In contrast, BTF-1 and BTF-2 supported maximum VMS removals of 40%. Notably, neither increasing the trickling liquid velocity from 2 to 4.5 m h⁻ 1 nor adding Fe-carbon nanoparticles to the solution had any impact on the BTFs' performance. Following the successful results observed in BTF-3, gas residence time was reduced from 60 to 42 min, consequently leading to an increase in the EC from 366 to 509 mg m -3 h -1 (corresponding to an RE = 87%). Despite the different performance of the BTFs, comparable bacterial communities were identified, dominated by the genera Thermomonas, Corynebacterium, Aquimonas, Thauera and Parvibaculum. The results obtained in this study highlighted the potential of activated carbon as packing material for enhancing abatement performance during biotrickling filtration and identified new bacterial genera with potential for VMS degradation., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Utilizing carbon dioxide (CO 2 ) for valuable chemical production is key to a circular economy. Current processes are costly due to limited microorganism use, low-value products, and the need for affordable energy. This study addresses these challenges by using industrial contaminants like thiosulfate (S 2 O 3 2- ) for CO 2 conversion into ectoines. Ectoines, are important ingredients as pharmaceuticals and cosmetics. Here, six microbial genomes were identified as potential candidates to valorize CO 2 and S 2 O 3 2- into ectoine. After laboratory validation at 3 % NaCl, the fastest-growing strain, Guyparkeria halophila, was optimized at 6 %, 9 %, and 15 % NaCl, showing the highest specific ectoine contents (mg Ect  g biomass -1 ) at 15 %. Batch bioreactors, combining optimal conditions, achieved maximum specific ectoine contents of 47 %. These results not only constitute the highest ectoine content so far reported by autotrophs and most of heterotrophs, but also the first proof of a novel valorization platform for CO 2 and S 2 O 3 2- , focused on pharmaceuticals production., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Volatile organic compounds (VOCs) emitted into the atmosphere negatively affect the environment and human health. Biotrickling filtration, an effective technology for treating VOC-laden waste gases, faces challenges in removing hydrophobic VOCs due to their low water solubility and therefore limited bioavailability to microorganisms. Consequently, the addition of (bio)surfactants has proven to be a promising strategy to enhance the removal of hydrophobic VOCs in biotrickling filters (BTFs). Yet, up to now, no single study has ever performed a mass transfer characterization of a BTF under (bio)surfactants addition. In this study, the effect of (bio)surfactant addition on the gas-liquid mass transfer characteristics of two BTFs was measured by using oxygen (O 2 ) as a model gas. Through an empirical correlation, the mass transfer coefficients (k L a) of two hydrophobic VOCs, toluene and hexane, which are of industrial and environmental significance, were estimated. One BTF was filled with expanded perlite, while the other with a mixture of compost and wood chips (C + WC). Both BTFs were operated under different liquid velocities (U L : 0.95 and 1.53 m h -1 ). Saponin, a biological surfactant, and Tween 80, a synthetic surfactant, were added to the recirculating liquid at different critical micelle concentrations (CMCs: 0-3 CMC). The higher interfacial and surface area of the perlite BTF compared to the C + WC BTF led to higher k L aO 2 values regardless of the operational condition: 308 ± 18-612 ± 19 h -1 versus 42 ± 4-177 ± 24 h -1 , respectively. Saponin addition at 0.5 and 1 CMC had positive effects on the perlite BTF, with k L aO 2 values two times higher compared to those at 0 CMC. Tween 80 exhibited a neutral or slightly positive effect on the mass transfer of both BTFs under all conditions. Overall, the CMC, along with the physical characteristics of the packing materials and the operational conditions evaluated explained the results obtained. This study provides fundamental data essential to improve the performance and design of BTFs for hydrophobic VOCs abatement., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Climate change and plastic pollution are likely the most relevant challenges for the environment in the 21st century. Developing cost-effective technologies for the bioconversion of methane (CH 4 ) into polyhydroxyalkanoates (PHAs) could simultaneously mitigate CH 4 emissions and boost the commercialization of biodegradable polymers. Despite the fact that the role of temperature, nitrogen deprivation, CH 4 :O 2 ratio or micronutrients availability on the PHA accumulation capacity of methanotrophs has been carefully explored, there is still a need for optimization of the CH 4 -to-PHA bioconversion process prior to becoming a feasible platform in future biorefineries. In this study, the influence of different cultivation broth pH values (5.5, 7, 8.5 and 10) on bacterial biomass growth, CH 4 bioconversion rate, PHA accumulation capacity and bacterial community structure was investigated in a stirred tank bioreactor under nitrogen deprivation conditions. Higher CH 4 elimination rates were obtained at increasing pH, with a maximum value of 50.4 ± 2.7 g CH 4 ·m -3 ·h -1 observed at pH 8.5. This was likely mediated by an increased ionic strength in the mineral medium, which enhanced the gas-liquid mass transfer. Interestingly, higher PHB accumulations were observed at decreasing pH, with the highest PHB contents recorded at a pH 5.5 (43.7 ± 3.4 %w·w -1 ). The strong selective pressure of low pH towards the growth of Type II methanotrophic bacteria could explain this finding. The genus Methylocystis increased its abundance from 34 % up to 85 and 90 % at pH 5.5 and 7, respectively. On the contrary, Methylocystis was less abundant in the community enriched at pH 8.5 (14 %). The accumulation of intracellular PHB as energy and carbon storage material allowed the maintenance of high CH 4 biodegradation rates during 48 h after complete nitrogen deprivation. The results here obtained demonstrated for the first time a crucial and multifactorial role of pH on the bioconversion performance of CH 4 into PHA., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The present study evaluated the production potential of CH 4 , carboxylic acids and alcohols from a mixed culture enriched using synthetic syngas. The influence of syngas concentration on the microbial community and products productivity and selectivity was investigated. The results demonstrated the enrichment of a mesophilic mixed culture capable of converting CO and H 2 mainly to CH 4 and acetate, along with butyrate. The selectivity values showed that acetate production was enhanced during the first cycle in all conditions tested (up to 20 %), while CH 4 was the main product generated during following cycles. Concretely, CH 4 selectivity remained unaffected by syngas concentration, reaching a stable value of 41.6 ± 2.0 %. On the other hand, butyrate selectivity was only representative at the highest syngas concentration and lower pH values (26.1 ± 5.8 %), where the H 2 consumption was completely inhibited. Thus, pH was identified as a key parameter for both butyrate synthesis and the development of hydrogenotrophic activity., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier Ltd.)

Bioactive coatings are envisaged as a promising biotechnology to tackle the emerging problem of indoor air pollution. This solution could cope with the low concentrations, the wide range of compounds and the hydrophobicity of some indoor air VOCs, which are the most important bottlenecks regarding the implementation of conventional biotechnologies for indoor air treatment. A bioactive coating-based bioreactor was tested in this study for the abatement of different VOCs (n-hexane, toluene and α-pinene) at different empty bed residence times (EBRT) and inlet VOC concentrations. The performance of this reactor was compared with a conventional biofilm-based bioreactor operated with the same microbial inoculum. After an acclimation period, the bioactive coating-based bioreactor achieved abatements of over 50% for hexane, 80% for toluene and 70% for pinene at EBRTs of 112-56 s and inlet concentrations of 9-15 mg m -3 . These results were about 25, 10 and 20% lower than the highest removals recorded in the biofilm-based bioreactor. Both bioreactors experienced a decrease in VOC abatement by ∼25% for hexane, 45% for toluene and 40% for pinene, after reducing the EBRT to 28 s. When inlet VOC concentrations were progressively reduced, VOC abatement efficiencies did not improve. This fact suggested that low EBRTs and low inlet VOCs concentration hindered indoor air pollutant abatement as a result of a limited mass transfer and bioavailability. Metagenomic analyses showed that process operation with toluene, hexane and pinene as the only carbon and energy sources favored an enriched bacterial community represented by the genera Devosia, Mesorhizobium, Sphingobacterium and Mycobacterium, regardless of the bioreactor configuration. Bioactive coatings were used in this work as packing material of a conventional bioreactor, achieving satisfactory VOC abatement similar to a conventional bioreactor., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Nowadays, people spend 80-90% of their time indoors, while recent policies on energy efficient and safe buildings require reduced building ventilation rates and locked windows. These facts have raised a growing concern on indoor air quality, which is currently receiving even more attention than outdoors pollution. Prevention is the first and most cost-effective strategy to improve indoor air quality, but once pollution is generated, a battery of physicochemical technologies is typically implemented to improve air quality with a questionable efficiency and at high operating costs. Biotechnologies have emerged as promising alternatives to abate indoor air pollutants, but current bioreactor configurations and the low concentrations of indoor air pollutants limit their widespread implementation in homes, offices and public buildings. In this context, recent investigations have shown that potted plants can aid in the removal of a wide range of indoor air pollutants, especially volatile organic compounds (VOCs), and can be engineered in aesthetically attractive configurations. The original investigations conducted by NASA, along with recent advances in technology and design, have resulted in a new generation of botanical biofilters with the potential to effectively mitigate indoor air pollution, with increasing public aesthetics acceptance. This article presents a review of the research on active botanical filters as sustainable alternatives to purify indoor air., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Biogas biorefineries have opened up new horizons beyond heat and electricity production in the anaerobic digestion sector. Added-value products such as polyhydroxyalkanoates (PHAs), which are environmentally benign and potential candidates to replace conventional plastics, can be generated from biogas. This work investigated the potential of an innovative two-stage growth-accumulation system for the continuous production of biogas-based polyhydroxybutyrate (PHB) using Methylocystis hirsuta CSC1 as cell factory. The system comprised two turbulent bioreactors in series to enhance methane and oxygen mass transfer: a continuous stirred tank reactor (CSTR) and a bubble column bioreactor (BCB) with internal gas recirculation. The CSTR was devoted to methanotrophic growth under nitrogen balanced growth conditions and the BCB targeted PHB production under nitrogen limiting conditions. Two different operational approaches under different nitrogen loading rates and dilution rates were investigated. A balanced nitrogen loading rate along with a dilution rate (D) of 0.3 day -1 resulted in the most stable operating conditions and a PHB productivity of ~53 g PHB m -3 day -1 . However, higher PHB productivities (~127 g PHB m -3 day -1 ) were achieved using nitrogen excess at a D = 0.2 day -1 . Overall, the high PHB contents (up to 48% w/w) obtained in the CSTR under theoretically nutrient balanced conditions and the poor process stability challenged the hypothetical advantages conferred by multistage vs single-stage process configurations for long-term PHB production., (© 2023 The Authors. Biotechnology and Bioengineering published by Wiley Periodicals LLC.)

The effect of alkali-based pretreatment on the methanization of bioplastics was investigated. The tested bioplastics included PHB [poly(3-hydroxybutyrate)], PHBH [poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)], PHBV [poly(3-hydroxybutyrate-co-3-hydroxyvalerate], PLA (polylactic acid), and a PLA/PCL [poly(caprolactone)] 80/20 blend. Prior to methanization tests, the powdered polymers (500-1000 μm) at a concentration of 50 g/L were subjected to alkaline pretreatment using NaOH 1 M for PLA and PLA/PCL, and NaOH 2 M for PHB-based materials. Following 7 days of pretreatment, the amount of solubilized carbon for PLA and its blend accounted for 92-98% of the total initial carbon, while lower carbon recoveries were recorded for most PHB-based materials (80-93%), as revealed by dissolved total organic carbon analysis. The pretreated bioplastics were then tested for biogas production by means of mesophilic biochemical methane potential tests. Compared to unpretreated PHBs, methanization rates of pretreated PHBs were accelerated by a factor of 2.7 to 9.1 with comparable (430 NmL CH 4 /g material feed) or slightly lower (15% in the case of PHBH) methane yields, despite featuring a 1.4-2.3 times longer lag phases. Both materials, PLA and the PLA/PCL blend, were only extensively digested when pretreated, yielding about 360-380 NmL CH 4 per gram of material fed. Unpretreated PLA-based materials showed nearly zero methanization under the timeframe and experimental conditions tested. Overall, the results suggested that alkaline pretreatment can help to enhance the methanization kinetics of bioplastics., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: O.G.-D., L.J.M.-M.,S.R.-V.,R.L.,R.A.B.,T.B.,R.M. reports financial support was provided by Société des Produits Nestlé S.A. R.A.B. reports a relationship with Société des Produits Nestlé S.A. that includes: employment. T.B. was employed by Société des Produits Nestlé S.A. during the development of this research., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

In recent years, a number of microbial enzymes capable of degrading plastics have been identified. Biocatalytic depolymerization mediated by enzymes has emerged as a potentially more efficient and environmentally friendly alternative to the currently employed methods for plastic treatment and recycling. However, the functional and systematic study of depolymerase enzymes with respect to the degradation of a series of plastic polymers in a single work has not been widely addressed at present. In this study, the ability of a set of enzymes (esterase, arylesterase and cutinase) to degrade commercial biodegradable polymers (PBS, PBAT, PHB, PHBH, PHBV, PCL, PLA and PLA/PCL) and the effect of pre-treatment methods on their degradation rate was assessed. The degradation products were identified and quantified by HPLC and LC-HRMS analysis. Out of the three enzymes, Fusarium solani cutinase (FsCut) showed the highest activity on grinded PBAT, PBS and PCL after 7 days of incubation. FsCut was engineered and heterologous expressed in Escherichia coli , which conferred the bacterium the capability of degrading solid discs of PBAT and to grow in PBS as the sole carbon source of the medium.

The abatement of indoor volatile organic compounds (VOCs) represents a major challenge due to their environmental risk, wide nature and concentration variability. Biotechnologies represent a cost-effective, robust and sustainable platform for the treatment of hazardous VOCs at low and fluctuating concentrations. However, they have been scarcely implemented for indoor air purification. Thus, little is known about the influence of the reactor configuration or the VOC nature and concentration variability on the removal, resilience and the microbial population of bioreactor configurations susceptible to be implemented, both in indoors and industrial environments. The present study aims at comparing the removal performance of four VOCs with different hydrophobicity and molecular structure -acetone, n-hexane, α-pinene and toluene-at two inlet concentrations (5 and 400 mg m -3 ), which mimics the concentrations of contaminated indoor and industrial air. To this aim a stirred tank, flat biofilm and latex-based biocoated flat bioreactor were comparatively evaluated. The results demonstrated the superior performance of the stirred tank reactor for the removal of hydrophilic VOCs at high inlet concentrations, which achieved removals >99% for acetone and toluene. At low concentrations, the removal efficiencies of acetone, toluene and α-pinene were >97% regardless of the bioreactor configuration tested. The most hydrophobic gas, n-hexane, was more efficiently removed in the flat biofilm reactor without latex. The microbial community analyses showed that the presence of VOCs as the only carbon and energy source didn't promote the growth of dominant bacterial members and the populations independently evolved in each reactor configuration and operation mode. The fungal population was more diverse in the biofilm-based bioreactors, although, it was mainly dominated by uncultured fungi from the phylum Cryptomycota., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

This study aims at investigating the influence of operational parameters on biohydrogen production from fruit-vegetable waste (FVW) via lactate-driven dark fermentation. Mesophilic batch fermentations were conducted at different pH (5.5, 6.0, 6.5, 7.0, and non-controlled), total solids (TS) contents (5, 7, and 9%) and initial cell biomass concentrations (18, 180, and 1800 mg VSS/L). Higher hydrogen yields and rates were attained with more neutral pH values and low TS concentrations, whereas higher biomass densities enabled higher production rates and avoided wide variations in hydrogen production. A marked lactate accumulation (still at neutral pH) in the fermentation broth was closely associated with hydrogen inhibition. In contrast, enhanced hydrogen productions matched with much lower lactate accumulations (even it was negligible in some fermentations) along with the acetate and butyrate co-production but not with carbohydrates removal. At pH 7, 5% TS, and 1800 mg VSS/L, 49.5 NmL-H 2 /g VS fed and 976.4 NmL-H 2 /L-h were attained., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

The hazard and operability analysis (HAZOP) is one of the most popular approaches for risk management, although weaknesses such as the limited number of risk factors considered, the inaccuracy of experts' opinions or the limited process knowledge might compromise the quality of the results. In this context, conventional HAZOP analysis can be improved via a Fuzzy Multi-Attribute HAZOP technique. Under a fuzzy logic, Analytic Hierarchy Process and the Technique for Order of Preference by Similarity to Ideal Solution can be combined with Fuzzy Multi-Attribute HAZOP to determine the weight of risk factors and to rank critical hazards. The inherent risks biogas upgrading, such as explosiveness, overpressure, or premature deterioration of equipment, should be identified for planning of critical control points and for enabling a proper maintenance plan. Previous models were applied to a photosynthetic biogas upgrading and a biogas-to-polyhydroxyalkanoates production pilot plant in order to identify and get more information about associated risks of the operation of these valorization biotechnologies, sometimes not fully provided by HAZOP analysis. Biotrickling filter and the polyhydroxyalkanoates production tank were identified as the most critical subsystems, with contributions of 33.3% and 17.8% to the overall risk, respectively (within quartile 1, Q 1 ). Additionally, biogas and recycling/feeding streams clustered a large number of operational risks (up to 83.4% of total risk within Q 1 ). The sensibility analysis demonstrated the reliability and robustness of the final ranking. The results of this analysis will support preventive maintenance by identifying critical monitored points when scaling-up biological biogas upgrading processes., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The feasibility of producing volatile fatty acids (VFAs) from five commercial bioplastics via acidogenic fermentation by a non-pretreated anaerobic sludge was investigated. Mesophilic, anaerobic, acidogenic batch assays at 1, 10 and 20 g/L feed concentrations revealed the feasibility of producing VFAs from polyhydroxyalkanoates (PHA), i.e., PHB and PHBV, but not from PBS, PCL and PLA under the test conditions and time. However, only high PHA substrate concentrations (10-20 g/L) resulted in organic overloading and decreasing the pH of the culture broth down to 4-5, which in turn induced the accumulation of VFAs via kinetic imbalance between acidogenesis and methanogenesis. Gaseous carbon (C-CO 2 and C-CH 4 ) accounted for 8-35% of the total initial carbon, while C-VFAs represented 10-18%, mainly as acetate and butyrate. This study represents the first systematically assessed proof-of-concept to produce VFAs from PHA, which is key for the design of bioplastic-to-bioplastic recycling (bio)technologies., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

In regions highly dependent on fossil fuels imports, biomethane represents a promising biofuel for the transition to a bio-based circular economy. While biomethane is typically produced via anaerobic digestion and upgrading, biomethanation of the synthesis gas (syngas) derived from the gasification of recalcitrant solid waste has emerged as a promising alternative. This work presents a comprehensive and in-depth analysis of the state-of-the-art and most recent advances in the field, compiling the potential of this technology along with the bottlenecks requiring further research. The key design and operational parameters governing syngas production and biomethanation (e.g. organic feedstock, gasifier design, microbiology, bioreactor configuration, etc.) are critically analysed., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The design of efficient cultivation strategies to produce bioplastics from biogas is crucial for the implementation of this biorefinery process. In this work, biogas-based polyhydroxybutyrate (PHB) production and CH 4 biodegradation performance was investigated for the first time in a stirred tank bioreactor inoculated with Methylocystis parvus str. OBBP. Decreasing nitrogen loading rates in continuous mode and alternating feast:famine regimes of 24 h-cycles, and alternating feast:famine regimes of 24 h:24 h and 24 h:48 h were tested. Continuous N feeding did not support an effective PHB production despite the occurrence of nitrogen limiting conditions. Feast-famine cycles of 24 h:24 h (with 50% stoichiometric nitrogen supply) supported the maximum PHB production (20 g-PHB m -3 d -1 ) without compromising the CH 4 -elimination capacity (25 g m -3 h -1 ) of the system. Feast:famine ratios ≤1:2 entailed the deterioration of process performance at stoichiometric nitrogen inputs ≤60%., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The biodegradation of PHB, PHBV, PBS, PBAT, PCL, PLA, and a PLA-PCL blend was compared under aerobic and anaerobic aqueous conditions assessing biodegradation kinetics, extent, carbon fate and particle size influence (in the range of 100-1000 µm). Under standard test conditions, PHB and PBHV were biodegraded anaerobically (83.9 ± 1.3% and 81.2 ± 1.7%, respectively) in 77 days or aerobically (83.0 ± 1.6% and 87.4 ± 7.5%) in 117 days, while PCL was only biodegraded (77.6 ± 2.4%) aerobically in 177 days. Apparent biomass growth accounted for 10 to 30.5% of the total initial carbon depending on the bioplastic and condition. Maximum aerobic and anaerobic biodegradation rates were improved up to 331 and 405%, respectively, at the lowest particle size tested (100-250 µm). This study highlights the usefulness of analysing biodegradation kinetics and carbon fate to improve both the development and testing of biodegradable materials, and waste treatments in the context of a circular bioeconomy., (Copyright © 2021 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Biotechnologies have emerged as a promising solution for indoor air purification with the potential to overcome the inherent limitations of indoor air treatment. These limitations include the low concentrations and variability of pollutants and mass-transfer problems caused by pollutant hydrophobicity. A new latex-based biocoating was herein optimized for the abatement of the volatile organic compounds (VOCs) toluene, trichloroethylene, n-hexane, and α-pinene using acclimated activated sludge dominated by members of the phylum Patescibacteria. The influence of the water content, the presence of water absorbing compounds, the latex pretreatment, the biomass concentration, and the pollutant load was tested on VOC removal efficiency (RE) by varying the formulation of the mixtures. Overall, hexane and trichloroethylene removal was low (<30%), while high REs (>90%) were consistently recorded for toluene and pinene. The assays demonstrated the benefits of operating at high water content in the biocoating, either by including mineral medium or water absorbing compounds in the latex-biomass mixtures. The performance of the latex-based biocoating was likely limited by VOC mass-transfer rather than by biomass concentration in the biocoating. The latex-based biocoating supported a superior toluene and pinene removal than biomass in suspension when VOC loading rate was increased by a factor of 4., (Copyright © 2021 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The capacity of haloalkaliphilic methanotrophic bacteria to synthesize ectoine from CH 4 -biogas represents an opportunity for waste treatment plants to improve their economic revenues and align their processes to the incoming circular economy directives. A techno-economic and sensitivity analysis for the bioconversion of biogas into 10 t ectoine·y -1 was conducted in two stages: (I) bioconversion of CH 4 into ectoine in a bubble column bioreactor and (II) ectoine purification via ion exchange chromatography. The techno-economic analysis showed high investment (4.2 M€) and operational costs (1.4 M€·y -1 ). However, the high margin between the ectoine market value (600-1000 €·kg -1 ) and the estimated ectoine production costs (214 €·kg -1 ) resulted in a high profitability for the process, with a net present value evaluated at 20 years (NPV 20 ) of 33.6 M€. The cost sensitivity analysis conducted revealed a great influence of equipment and consumable costs on the ectoine production costs. In contrast to alternative biogas valorization into heat and electricity or into low added-value bioproducts, biogas bioconversion into ectoine exhibited high robustness toward changes in energy, water, transportation, and labor costs. The worst- and best-case scenarios evaluated showed ectoine break-even prices ranging from 158 to 275 €·kg -1 , ∼3-6 times lower than the current industrial ectoine market value., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

The global environmental pollution by micro- and macro-plastics reveals the consequences of an extensive use of recalcitrant plastic products together with inappropriate waste management practices that fail to sufficiently recycle the broad types of conventional plastic waste. Biobased and biodegradable plastics are experiencing an uprising as their properties offer alternative waste management solutions for a more circular material economy. However, although the production of such bioplastics has advanced on scale, the end-of-life (EOL) (bio)technologies to promote circularity are lacking behind. While composting and biogas plants are the only managed EOL options today, advanced biotechnological recycling technologies for biodegradable bioplastics are still in an embryonic stage. Thus, developing efficient biotechnologies capable of transforming bioplastic waste into high-value chemical building blocks or into the constituents of the original polymer offers promising routes towards life-cycle-engineered products. This review aims at providing a comprehensive state-of-the-art overview of microbial-based processes involved in the complete lifecycle of bioplastics. The current trends in the bioplastic market, the beginning and EOL scenarios of bioplastics, and a critical discussion on the key factors and mechanisms governing microbial degradation are systematically presented. Also, a critical evaluation of terminology and international standards to quantify polymer biodegradability is provided together with the latest biotechnological recycling strategies, including the use of different pre-treatments for (bio)plastic waste. Finally, the challenges and future perspectives for the development of life-cycle-engineered biobased and biodegradable plastic products are discussed., (Copyright © 2021. Published by Elsevier Inc.)

Eliminating volatile siloxanes from gas emissions is increasingly important due to their persistent detrimental economic, societal, and environmental impacts. Although physicochemical technologies are currently the only commercially available abatement methods, recently developed biobased technologies are emerging as a more cost-effective and sustainable alternative to promote the removal of volatile siloxanes., (Copyright © 2021 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The potential of the filamentous N 2 -fixing cyanobacterium Nostoc muscorum for CO 2 capture from high-loaded streams (i.e. flue gas or biogas) combined with the accumulation of glycogen (GL) and polyhydroxybutyrate (PHB), was evaluated under nutrient-sufficient and nutrient-limited conditions. N. muscorum was able to grow under CO 2 contents from 0.03 up to 30% v/v, thus tolerating CO 2 concentrations similar to those found in raw biogas or flue-gas, with maximum CO 2 -fixation rates of 191.9 ± 46 g m -3 d -1 at a biomass concentration of 733.3 ± 207.4 mg TSS L -1 . Despite N. muscorum was inhibited by the presence of H 2 S, the co-inoculation with activated sludge resulted in both CO 2 and H 2 S depletion. Moreover, N. muscorum accumulated GL up to ∼54% dcw under N and P-deprivation, almost 36 times higher than that recorded under nutrients sufficient condition. The addition of 10% extra carbon in the form of valeric acid not only did not hamper the growth of N. muscorum (336.0 ± 113.1 mg TSS L -1 ) but also increased the GL content to ∼58% dcw. On the contrary, a negligible PHB accumulation was found under the tested conditions, likely due to the high CO 2 concentration of 30% v/v in the headspace and therefore the high availability of inorganic carbon for the cultures. N. muscorum cultures achieved VFAs degradations up to ∼78% under controlled pH. These results supported N. muscorum as a sustainable alternative for CO 2 -capture and greenhouse gas mitigation or for photosynthetic biogas upgrading coupled with value added biomass production., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Three innovative operational strategies were successfully evaluated to improve the quality of biomethane in an outdoors pilot scale photobioreactor interconnected to an external absorption unit: i) the use of a greenhouse during winter conditions, ii) a direct CO 2 stripping in the photobioreactor via air stripping during winter conditions and iii) the use of digestate as make-up water during summer conditions. CO 2 concentrations in the biomethane ranged from 0.4% to 6.1% using the greenhouse, from 0.3% to 2.6% when air was injected in the photobioreactor and from 0.4% to 0.9% using digestate as make up water. H 2 S was completely removed under all strategies tested. On the other hand, CH 4 concentrations in biomethane ranged from 89.5% to 98.2%, from 93.0% to 98.2% and from 96.3% to 97.9%, when implementing strategies i), ii) and iii), respectively. The greenhouse was capable of maintaining microalgae productivities of 7.5 g m -2 d -1 during continental weather conditions, while mechanical CO 2 stripping increased the pH in order to support an effective CO 2 and H 2 S removal. Finally, the high evaporation rates during summer conditions allowed maintaining high inorganic carbon concentrations in the cultivation broth using centrate, which provided a cost-effective biogas upgrading., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Indoor air pollution has traditionally received less attention than outdoors pollution despite indoors pollutant levels are typically twice higher, and people spend 80-90% of their life in increasing air-tight buildings. More than 5 million people die every year prematurely from illnesses attributable to poor indoor air quality, which also causes multi-millionaire losses due to reduced employee's productivity, material damages and increased health system expenses. Indoor air pollutants include particulate matter, biological pollutants and over 400 different chemical organic and inorganic compounds, whose concentrations are governed by several outdoor and indoor factors. Prevention of pollutant is not always technically feasible, so the implementation of cost-effective active abatement units is required. Up to date no single physical-chemical technology is capable of coping with all indoor air pollutants in a cost-effective manner. This problem requires the use of sequential technology configurations at the expenses of superior capital and operating costs. In addition, the performance of conventional physical-chemical technologies is still limited by the low concentrations, the diversity and the variability of pollutants in indoor environments. In this context, biotechnologies have emerged as a cost-effective and sustainable platform capable of coping with these limitations based on the biocatalytic action of plants, bacteria, fungi and microalgae. Indeed, biological-based purification systems can improve the energy efficiency of buildings, while providing additional aesthetic and psychological benefits. This review critically assessed the state-of-the-art of the indoor air pollution problem and prevention strategies, along with the recent advances in physical-chemical and biological technologies for indoor pollutants abatement., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper, (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Using the biogas generated from organic waste anaerobic treatment to produce polyhydroxyalkanoates (PHAs) has emerged as an attractive alternative to heat and power generation (CHP) in waste treatment plants. The sustainability of biogas combustion for CHP, biogas bioconversion into PHA and a combination of both scenarios was compared in terms of environmental impact, process economics and social responsibility according to the IChemE Sustainability Metrics. Although PHA production presented higher investment and operational costs, a comparable economic performance was observed in all biogas valorization scenarios regarding net present value (0.77 M€) and internal rate of return (6.4 ± 0.2%) due to the higher market value of biopolymers. The PHA production entailed a significant reduction of atmospheric acidification and odor emissions compared to CHP despite showing higher land, water, chemicals and energy requirements. Job creation associated to biopolymer industry and the increasing public demand for bioproducts were identified as fundamental aspects for enhancing social and local acceptance of waste processing facilities. This study demonstrated that PHA production from biogas constitutes nowadays a realistic alternative to CHP in waste treatment plants and that PHA can be produced at a competitive market price when biogas is used for internal energy provision (4.2 €·kg -1 PHA)., (Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Creating additional value out of biogas during waste treatment has become a priority in past years. Biogas bioconversion into valuable bioproducts such as biopolymers has emerged as a promising strategy. This work assessed the operational feasibility of a bubble column bioreactor (BCB) implemented with gas recirculation and inoculated with a polyhydroxybutyrate (PHB)-producing strain using biogas as substrate. The BCB was initially operated at empty bed residence times (EBRTs) ranging from 30 to 120 min and gas recirculation ratios (R) from 0 to 30 to assess the gas-to-liquid mass transfer and bioconversion of CH 4 . Subsequently, the BCB was continuously operated at a R of 30 and an EBRT of 60 min under excess of nitrogen and nitrogen feast-famine cycles of 24 h:24 h to trigger PHB synthesis. Gas recirculation played a major role in CH 4 gas-liquid transfer, providing almost fourfold higher CH 4 elimination capacities (~41 g CH 4 m -3 h -1 ) at the highest R and EBRT of 60 min. The long-term operation under N excess conditions entailed nitrite accumulation (induced by O 2 limiting conditions) and concurrent methanotrophic activity inhibition above ~60 mg N-NO 2 - L -1 . Adjusting the N-NO 3 - supply to match microbial N demand successfully prevented nitrite accumulation. Finally, the N feast-famine 24 h:24 h strategy supported a stable CH 4 conversion with a removal efficiency of 70% along with a continuous PHB production, which yielded PHB accumulations of 14.5 ± 2.9% (mg PHB mg -1 total suspended solids × 100). These outcomes represent the first step towards the integration of biogas biorefineries into conventional anaerobic digestion plants., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Biogas produced at wastewater treatment plants and landfills contains trace levels of volatile methyl siloxanes (VMS) that are responsible for abrasion, corrosion and erosion of equipment during biogas storage and combustion. This research comparatively evaluated the removal of the most common VMS (L2, L3, D4, and D5) under aerobic conditions in a conventional biotrickling filter (BTF) and a two-phase partitioning BTF (TP-BTF) with silicone oil (at 30%) as organic phase. The TP-BTF showed a superior performance compared to the conventional BTF, increasing the total VMS removal from <30% in the BTF up to ∼70% in the TP-BTF. The highest REs in the TP-BTF were recorded for D4 and D5, reaching values of 80-90%, corresponding to ECs between 0.12 and 0.17 g m -3 .h -1 . Slightly lower values were obtained for L3 (70-80%), and the lowest performance was recorded for L2 (20-60%) due to the high vapor pressure of this siloxane and therefore its lower affinity by the organic phase. Surprisingly, despite the different inocula used, a similar microbial community was found by the end of operation of both BTFs, with KMBC-112, Reynarella and Chitinophaga as the dominant genera., Competing Interests: Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The validation of a control strategy for biogas upgrading via light-driven CO 2 consumption by microalgae and H 2 S oxidation by oxidizing bacteria using the oxygen photosynthetically generated was performed in a semi-industrial scale (9.6 m 3 ) photobioreactor. The control system was able to support CO 2 concentrations lower than 2% with O 2 contents ≤ 1% regardless of the pH in the cultivation broth (ranging from 9.05 to 9.50). Moreover, the control system was efficient to cope with variations in biogas flowrate from 143 to 420 L h -1 , resulting in a biomethane composition of CO 2  < 2.4%, CH 4  > 95.5%, O 2  < 1% and no H 2 S. Despite the poor robustness of this technology against failures in biogas and liquid supply (CH 4 concentration of 67.5 and 70.9% after 2 h of biogas or liquid stoppage, respectively), the control system was capable of restoring biomethane quality in less than 2 h when biogas or liquid supply was resumed., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Trimethylamine (TMA) is an odorous volatile organic compound emitted by industries. Algal-based biotechnologies have been proven as a feasible alternative for wastewater treatment, although their application to abate polluted air emissions is still scarce. This work comparatively assessed the removal of TMA in a conventional bacterial bubble column bioreactor (BC) and a novel algal-bacterial bubble column photobioreactor (PBC). The PBC exhibited a superior TMA abatement performance compared to the conventional BC. In this sense, the BC reached a removal efficiency (RE) and an elimination capacity (EC) of 78% and 12.1 g TMA m -3  h -1 , respectively, while the PBC achieved a RE of 97% and a EC of 16.0 g TMA m -3 ·h -1 at an empty bed residence time (EBRT) of 2 min and a TMA concentration ~500 mg m -3 . The outstanding performance of the PBC allowed to reduce the operating EBRT to 1.5 and 1 min while maintaining high REs of 98 and 94% and ECs of 21.2 and 28.1 g m -3 ·h -1 , respectively. Moreover, the PBC improved the quality of the gas and liquid effluents discharged, showing a net CO 2 consumption and decreasing by ~ 30% the total nitrogen concentration in the liquid effluent via biomass assimilation. A high specialization of the bacterial community was observed in the PBC, Mumia and Aquamicrobium sp. being the most abundant genus within the main phyla identified. GraphicalAbstract.

The valorization of biogas as a feedstock for the generation of added-value bioproducts will play a key role on the sustainability of anaerobic digestion. The present work assessed the influence of key environmental parameters (O 2 :CH 4 ratio, temperature and nitrogen source) on the growth and polyhydroxybutyrate (PHB) synthesis under nitrogen limiting conditions of the type II methanotroph Methylocystis hirsuta CSC1 using biogas as a feedstock. The O 2 :CH 4 ratios tested (1:1, 1.5:1 and 2:1) did not affect significantly M. hirsuta CSC1 growth yields (~5 g TSS mol -1 CH 4 ), although lower CH 4 removal rates were reached under O 2 -limiting conditions (ratio 1:1). The highest PHB content (45 wt%) was achieved at a ratio 2:1 and was threefold higher than those obtained at lower ratios (~15 wt%). The increase in temperature from 15 to 25 °C resulted in increases in the growth yield (from 5 to 6 g TSS mol -1 CH 4 ) and PHB content (from 32 to 40 wt%). Conversely, the lowest PHB content (30 wt%) was reached at 37 °C, together with a negligible growth under nutrient sufficient conditions. The nitrogen source also played a key role on both M. hirsuta CSC1 growth and PHB synthesis. Thus, ammonium resulted in the highest growth yield (7 g TSS mol -1 CH 4 ), although the maximum PHB content was achieved when biomass was previously grown in nitrate as the nitrogen source (41 wt%). Nitrite exerted an inhibitory effect on M. hirsuta CSC1 growth., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Biogas is the byproduct of anaerobic digestion with the highest valorization potential, however its full exploitation is limited by the lack of tax incentives and the inherent presence of pollutants. The development of technologies for biogas conversion into added-value products is crucial in order to ensure the competitiveness of this bioresource. This study constitutes the first proof of concept of upgraded biogas bioconversion into the high profit margin product ectoine. Ectoine represents the most expensive product synthesized by microorganisms with a retail value of 1000 $ kg -1 and a yearly increasing demand that currently entails a total market opportunity of 15000 M€. First, the production of ectoine from upgraded biogas was assessed in batch bioreactors. The presence of H 2 S did not exert a negative effect on the growth of the haloalkaliphilic ectoine producers, and ectoine yields up to 49 mg g biomass -1 were obtained. A second experiment conducted in continuous bubble column bioreactors confirmed the feasibility of the process under continuous mode (with ectoine yields of 109 mg g biomass -1 ). Finally, this study revealed that the removal of toxic compounds (i.e. medium dilution rate of 0.5 day -1 ) and process operation with a consortium composed of methylotrophic/non-methylotrophic ectoine producers enhanced upgraded biogas bioconversion. This research discloses the basis for the application of this innovative technology and could boost the economic performance of anaerobic digestion., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The negative effects of volatile organic compounds (VOCs) on humans' health and the environment have boosted the enforcement of regulations, resulting in the need of effective and environmentally friendly off-gas treatment technologies. In this work, the synergism between microalgae and bacteria was investigated as a sustainable platform to enhance the biological degradation of toluene, herein selected as a model VOC. An innovative algal-bacterial tubular photobioreactor (TPBR) was systematically compared with a conventional biotrickling filter (BTF). The BTF supported average removal efficiencies close to those obtained in the TPBR (86 ± 9% and 88 ± 4%, respectively) at the highest inlet load (∼23 g m 3  h -1 ) and lowest gas residence time (0.75 min). However, the BTF was more sensitive towards the accumulation of secondary metabolites. In this regard, photosynthetic O 2 supplementation (resulting in dissolved oxygen concentrations of ∼7.3 mg O 2 L -1 ) and CO 2 consumption by microalgae (which reduced the impact of acidification) enhanced toluene abatement performance and process stability., (Copyright © 2019 Elsevier B.V. All rights reserved.)

The isolation of highly efficient methanotrophic communities is crucial for the optimization of methane bioconversion into products with a high market value such as polyhydroxyalkanoates (PHA). The research here presented aimed at enriching a methanotrophic consortium from two different inocula (Sphagnum peat moss (Sp) and Sphagnum and activated sludge (M)) able to accumulate PHA while efficiently oxidizing CH 4 . Moreover, the effect of the temperature and phosphorus limitation on the biodegradation rate of CH 4 and the PHA accumulation potential was investigated. Higher CH 4 degradation rates were obtained under P availability at increasing temperature (25, 30 and 37 °C). The biomass enriched from the mixed inoculum always exhibited a superior biodegradation performance regardless of the temperature (a maximum value of 84.3 ± 8.4 mg CH 4  h -1  g biomass -1 was recorded at 37 °C). The results of the PHB production showed that phosphorus limitation is required to promote PHB accumulation, the highest PHB content being observed with the Sphagnum inoculum at 25 °C (13.6 ± 5.6%). The differential specialization of the microbial communities depending on the enrichment temperature supported the key role of this parameter on the results obtained. In all cases after the completion of the enrichment process and of the P limitation tests, Methylocystis, a type II methanotroph known for its ability to accumulate PHA, was the genus that became dominant (reaching percentages from 16 to 46% depending on the enrichment temperature). Thus, the results here obtained demonstrated for the first time the relevance of the temperature used for the enrichment of the methanotrophic bacteria to boost PHA production yields under P limiting condition, highlighting the importance of optimizing culture conditions to improve the cost-efficiency of bioprocesses based on using methane as the primary feedstock for the PHA industrial market., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Genome Scale Metabolic Models (GSMMs) of the recently sequenced Methylocystis hirsuta and two other methanotrophs from the genus Methylocystis have been reconstructed. These organisms are Type II methanotrophs with the ability of accumulating Polyhydroxyalkanoates under nutrient limiting conditions. For the first time, GSMMs have been reconstructed for Type II methanotrophs. These models, combined with experimental biomass and PHB yields of Methylocystis hirsuta, allowed elucidating the methane oxidation mechanism by the enzyme pMMO (particulate methane monooxygenase) in these organisms. In contrast to Type I methanotrophs, which use the "direct coupling mechanism", Type II methanotrophs appear to use the so called "redox arm mechanism". The utilization of the "redox arm mechanism", which involves the coupling between methane oxidation and complex I of the respiratory chain, was confirmed by inhibition of complex I with catechol. Utilization of the "redox arm" mechanism leads to lower biomass yields on methane compared to Type I methanotrophs. However, the ability of Type II methanotrophs to redirect high metabolic carbon fluxes towards acetoacetyl-CoA under nitrogen limiting conditions makes these organisms promising platforms for metabolic engineering., (Copyright © 2019 International Metabolic Engineering Society. Published by Elsevier Inc. All rights reserved.)

The potential of purple phototrophic bacteria (PPB) for the simultaneous treatment of piggery wastewater (PWW) and biogas upgrading was evaluated batchwise in gas-tight photobioreactors. PWW dilution was identified as a key parameter determining the efficiency of wastewater treatment and biomethane quality in PPB photobioreactors. Four times diluted PWW supported the most efficient total organic carbon (TOC) and total nitrogen removals (78% and 13%, respectively), with CH 4 concentrations of 90.8%. The influence of phosphorous concentration (supplementation of 50 mg L -1 of P-PO 4 3- ) on PPB-based PWW treatment coupled to biogas upgrading was investigated. TOC removals of ≈60% and CH 4 concentrations of ≈90.0% were obtained regardless of phosphorus supplementation. Finally, the use of PPB and algal-bacterial consortia supported CH 4 concentrations in the upgraded biogas of 93.3% and 73.6%, respectively, which confirmed the potential PPB for biogas upgrading coupled to PWW treatment., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The performance of photosynthetic biogas upgrading coupled to wastewater treatment was evaluated in an outdoors high rate algal pond (HRAP) interconnected to an absorption column at semi-industrial scale. The influence of biogas flowrate (274, 370 and 459 L h -1 ), liquid to biogas ratio (L/G = 1.2, 2.1 and 3.5), type of wastewater (domestic versus centrate) and hydraulic retention time in the HRAP (HRT) on the quality of the biomethane produced was assessed. The highest CO 2 and H 2 S removal efficiencies (REs) were recorded at the largest L/G due to the higher biogas-liquid mass transfer at increasing liquid flowrates. No significant influence of the biogas flowrate on process performance was observed, while the type of wastewater was identified as a key operational parameter. CO 2 and H 2 S-REs of 99% and 100% at a L/G max  = 3.5 were recorded using centrate. The maximum CH 4 content in the biomethane (90%) was limited by N 2 and O 2 desorption., (Copyright © 2019 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

The influence of the liquid-to-biogas ratio (L/G) and alkalinity on methane quality was evaluated in a 11.7 m 3 outdoors horizontal semi-closed tubular photobioreactor interconnected to a 45-L absorption column (AC). CO 2 concentrations in the upgraded methane ranged from <0.1 to 9.6% at L/G of 2.0 and 0.5, respectively, with maximum CH 4 concentrations of 89.7% at a L/G of 1.0. Moreover, an enhanced CO 2 removal (mediating a decrease in CO 2 concentration from 9.6 to 1.2%) and therefore higher CH 4 contents (increasing from 88.0 to 93.2%) were observed when increasing the alkalinity of the AC cultivation broth from 42 ± 1 mg L -1 to 996 ± 42 mg L -1 . H 2 S was completely removed regardless of the L/G or the alkalinity in AC. The continuous operation of the photobioreactor with optimized operating parameters resulted in contents of CO 2 (<0.1%-1.4%), H 2 S (<0.7 mg m -3 ) and CH 4 (94.1%-98.8%) complying with international regulations for methane injection into natural gas grids., (Copyright © 2019 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Despite the environmental relevance of CH 4 and forthcoming stricter regulations, the development of cost-efficient and environmentally friendly technologies for CH 4 abatement is still limited. To date, one of the most promising solutions for the mitigation of this important GHG consists of the bioconversion of CH 4 into bioproducts with a high profit margin. In this context, methanotrophs have been already proven as cell-factories of some of the most expensive products synthesized by microorganisms. In the case of ectoine (1000 $ kg -1 ), already described methanotrophic genera such as Methylomicrobium can accumulate up to 20% (ectoine wt -1 ) using methane as the only carbon source. Moreover, α-methanotrophs, such as Methylosynus and Methylocystis, are able to store bioplastic concentrations up to 50-60% of their total cell content. More than that, methanotrophs are one of the greatest potential producers of methanol and exopolysaccharides. Although this methanotrophic factory could be enhanced throughout metabolic engineering, the valorization of CH 4 into valuable metabolites has been already consistently demonstrated under continuous and discontinuous mode, producing more than one compound in the same bioprocess, and using both, single strains and specific consortia. This review states the state-of-the-art of this innovative biotechnological platform by assessing its potential and current limitations.

The ability of methanotrophs to rapidly respond to intentional or accidental stress conditions caused by operational failures or process fluctuations is of utmost importance to guarantee the robustness of CH 4 abatement biotechnologies. In this study, the performance of a continuous and two feast-famine (5:5 days feast-famine cycles) stirred tank reactors treating diluted CH 4 emissions (4-5% v/v) was comparatively assessed for 149 days. The robustness of the three bioreactors towards a 5 days CH 4 deprivation episode was thoroughly evaluated at a molecular level (pmoA gene expression level) and correlated to macroscopic process performance. The bioreactors recovered their steady-state abatement performance (in terms of CH 4 elimination capacity and CO 2 production rate) within 1.5-2 h following CH 4 supply resumption concomitantly with a maximum in pmoA gene expression, regardless of the previous operational mode. However, while methanotrophs from the continuous unit maintained higher basal levels of pmoA expression as a strategy for a rapid CH 4 metabolism initiation, the strategy of the feast-famine adapted-methanotrophs consisted on a more accurate regulation of their pmoA transcripts levels along with a higher and/or more rapid induction of the pmoA gene by CH 4 availability., (Copyright © 2018 Elsevier Ltd. All rights reserved.)