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Coupling the cultivation of microalgae with wastewater treatment is a promising technology to recover bioresources from wastewater. However, toxic pollutants in wastewater seriously inhibit the growth of microalgae and the removal of pollutants. Phenolic acids are similar to phytohormones, could potentially relieve the toxicity to microalgae and simultaneously promote pollutant degradation and lipid accumulation. Chlorella and 4-chlorophenol (4-CP) were utilized to simulate the toxic wastewater treatment, and the roles of two typical phenolic acids, such as p-hydroxybenzoic acid (p-HBA) and caffeic acid (CA), were explored. The 0.2 μM concentration of p-HBA or CA improved the specific growth rate by 7.6% by enhancing photosynthesis and DNA replication. The oxidative damage caused by 4-CP was reduced by 30.3-49.7% via the synthesis of more antioxidant enzymes and the direct scavenging of free radicals by phenolic acids. Furthermore, the 4-CP removal rate increased by 27.0%, and toxic 4-CP was degraded into non-toxic compounds. The phenolic acids did not change the 4-CP degradation pathway but accelerated its removal and detoxification by enhancing the expression of 4-CP degradation enzymes. Simultaneously, lipid production increased by 20.5-23.1% due to the upregulation of enzymes related to fatty acid and triacylglycerol synthesis. Trace phenolic acids stimulated the mitogen-activated protein kinase signaling cascade and the calcium signaling pathway to regulate the physiology of the microalgae and protect cells from toxic stress. This study provides a promising new strategy for toxic wastewater treatment and bioresource recovery., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Boron (B) industry and consuming produce large amounts of B-containing wastewater. Low tolerance of microorganisms and plants resulted in the biological removal of B was limited. Microalgae show high adaptability in adverse environments. Whether microalgae able to be utilized in B removal meanwhile produce bioresources, and the B tolerant mechanisms and regulation pathway of microalgae are unclear. In this study, the cell growth, B removal, and lipid/starch production of Chlorella regularis under different levels of B stress (0.5, 10, 25, and 50 mg/L) were examined. The mechanisms of signal perception and response were explored by transcriptome and network analysis. Microalgae tolerated 25 mg/L high B stress, cell growth showed no decline and biomass reach up to 4.5 g/L. Microalgae took in B with 3.35 mg/g and bonded them to protein and carbon components in cells, the B removal capability was higher than some special adsorbents. Microalgae produced 188.65 mg/(L∙d) lipids and 305.35 mg/(L∙d) starch. The mitogen-activated protein-kinase signaling pathway was involved in the B tolerance of microalgae and regulated B efflux, glycolysis, and lipid/starch accumulation to relieve B stress. This study provides potential biological technique for B removal in wastewater and promotes new insight into signal role in toxic pollutants biological treatment., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Integrating microalgae culture and wastewater purification is a promising technology for sustainable bioresource production. However, the challenge is that toxins in wastewater usually limit risk elimination and cause poor bioresource production. Easy-to-biodegrade substrates could alleviate the resistant stress on a bacterial community but we know little about how they function with microalgae. In this study, we tested if Easy-to-biodegrade substrates could simultaneously promote Chlorella to degrade antibiotic amoxicillin (AMO) and produce bioresources. Sodium acetate (NaAC) was used as the representative co-substrate. The results showed NaAC could enhance AMO removal by 76%. The β-lactam structure was destroyed and detoxified to small molecules, due to the up-regulation of hydrolase, oxidoreductase, reductase, and transferase. Chlorella biomass production increased by 36%. The genes encoding the glutathione metabolism and peroxisome pathways were significantly up-regulated to alleviate the antibiotic stress, and the DNA replication pathway was activated. As a result, the production of lipid, carbohydrate, and protein was enhanced by 61%, 122%, and 34%, respectively. This study provides new insights for using microalgae to recover bioresources from toxic wastewater and reveals the critical underlying mechanisms., (Copyright © 2021. Published by Elsevier B.V.)

The cultivation of microalgae using wastewater could bring some major economic benefits; however, the toxics in wastewater typically lead to a reduction in bioresource production. In this study, carbon dots (CDs) could enhance the photosynthetic activity of Chlorella under antibiotic stress because they might optimize photoluminescence by red-shifting incident light. Adding of 1 mg/L CDs increased the specific growth rate of Chlorella by 36.0% (day 8-13) and 52.7% (day 14-18) and significantly increased photosystems II activity. This treatment also increased amoxicillin removal by 18.6%. Thus, the toxicity of residuals was significantly eliminated (P < 0.05). The removal of nitrogen and phosphorous was increased by 14.6% and 9.9%, respectively. The production of pigments, lipids and proteins was increased by 16.6%, 19.5% and 24.8%, respectively. This work provided a new strategy of using CDs to mediate the coupling of microalgal bioresources production and toxic wastewater purification., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Cell-to-cell communication regulates microalgae production via signaling molecules (SMs), but few microalgal SM species are known. Here, we document two new microalgae SMs, benzoic acid (BA) and salicylic acid (SA). Initially, crude SMs were extracted from a microalgae culture in which microalgae grew on heterotrophic-enriched phosphorus nutrition. The extracted SMs enhanced Chlorella growth by ∼72%, promoted nutrient uptake, and up-regulated the mitogen-activated protein-kinase signaling cascade. Fourier transform infrared and nuclear magnetic resonance analyses identified the putative SMs was aromatic carboxylic acids. BA and SA were identified using high-resolution mass spectrometry. BA and SA addition increased cell growth by ∼75% and ∼25%; and improved ATP production by ∼35% and ∼20%. Transcriptomic analysis showed that BA and SA were biosynthesized via CoA-dependent, non-oxidative pathway. The SMs upregulated TCA-cycle enzymes, which promoted carbon assimilation and activated DNA-replicating enzyme, so that accelerated cell division. This study identified two new SMs for microalgae cell communication and provides means to identify other SMs., 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.)

Intracellular phosphorus (P) accumulation can improve microalgal growth and lipid synthesis. However, large excess of P causes cell poisoning. This study utilized a P-fed-batch strategy to investigate its potential to improve the utilization of the excessive P, while avoiding toxic side effects. This strategy contributed to a more complete utilization of the intracellularly stored P, which enhanced the microalgae biomass by 10-15% by upregulating the brassinosteroid growth hormone gene at a P-fed-batch frequency of 2-8. Furthermore, the lipid content increased by 4-16% via upregulation of lipid synthesis-related genes. As a result, the P-fed-batch strategy significantly increased the lipid production by 13-19%. The content of saturated fatty acid increased by ~ 100%, implying improved combustibility and oxidative stability. This is the first study of this P-fed-batch strategy and provides a new concept for the complete utilization of excessive P.

Green microalgae are promising and sustainable bioenergy and biomass feedstocks, that specifically utilize blue and red light for photosynthesis. Using carbon dots (CDs) to optimize photoluminescence is an attractive strategy for enhancing microalgal bioresource production; however, CD synthesis traditionally requires harsh conditions and laborious procedures. Little research has focused on developing CDs with tunable emission that precisely satisfy photosynthetic requirements. In this work, we directionally prepared non-toxic CDs using a simple method, which could adsorb light at spectra 500-600 nm and emit red light at 580-700 nm. CDs significantly promoted microalgae (Chlorella) growth by ~15%. Meanwhile, potential intracellular bioresources, pigments, carbohydrates, proteins, and lipids were generally enhanced. CDs, combined with an extracellular polymeric substance of microalgal cells, served as numerous micro-bulbs for Chlorella irradiation to sustainably provide optimized light. In this context, photosystems I and II were both stimulated. As such, we prepared CDs with tunable emission, which could significantly enhance microalgae and bioresource 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 © 2020 Elsevier Inc. All rights reserved.)

In this study, microalgal biomass waste (Chlorella regularis) was treated while simultaneously producing bioelectricity in a microbial fuel cell (MFC). Algal biomass was the sole electron donor and was enriched with easily biodegradable proteins (46%) and carbohydrates (22%). The generated power density was 0.86 W/m 2 and the columbic efficiency reached ∼61.5%.The power generation could be further increased to 1.07 W/m 2 by using a biomass waste concentration enhancement strategy with maximum chemical oxygen demand (COD) removal of ∼65.2%. Via direct comparison, the power generation and COD removal capability of the algal-fed MFC was close to that of the commercial acetate-fed MFC. The algae-fed MFC presented superior electrochemical characteristics that were attributed to the complicated composition of the biomass anolyte. It possessed a multiple anode respiring bacterial group and diverse microbial community. Hence, this study provides a new strategy for the utilization of microalgal biomass as a bioresource., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

A high phosphorus concentration is widely accepted as favorable for enhancing both microalgae growth and lipid accumulation; however, excessively high P could be counter-productive. In this study, we investigated the effects of increasing P levels (5.4, 25, 45, 150, and 250 mg-P L -1 ) on the heterotrophic cultivation of Chlorella regularis. Microalgae growth was inhibited and cells were severely damaged in response to highly excessive P levels (≥150 mg-P L -1 ). In particular, 250 mg-P L -1 resulted in a ∼40% decrease in cell density and a ∼70% loss of cell viability. Microalgae damage induced by excessive phosphorus included enlarged cell size, deformation of cell walls, and disorganization of organelles. These negative effects were associated with the over-accumulation of polyphosphates within cells, which may further cause binding of P to intracellular components. Although P is an essential nutrient, excessive P lowers cell growth and viability., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Bacterial contamination and biomass harvesting are still challenges associated with coupling of microalgae and wastewater treatment technology. This study investigated aggregation, bacterial growth, lipid production, and pollutant removal during bacteria contaminated Chlorella regularis cultivation under nutrient starvation stress, by supposing the C/N/P ratios of the medium to 14/1.4/1 (MB₂.₅) and 44/1.4/1 (MB₄.₀), respectively. Granules of 500-650 μm were formed in the bacteria contaminated inoculum; however, purified C. regularis were generally suspended freely in the medium, indicating that bacterial presence was a prerequisite for granulation. Extracellular polymeric substance (EPS) analysis showed that polysaccharides were dominant in granules, while protein mainly distributed in the outer layer. Denaturing gradient gel electrophoresis (DGGE) results revealed Sphingobacteriales bacterium and Sphingobacterium sp. are vital organisms involved in the flocculation of microalgae, and nitrifiers (Stenotrophomonas maltophilia) could co-exist in the granular. Both EPS and DGGE results further supported that bacteria played key roles in granulation. C. regularis was always dominant and determined the total biomass concentration during co-cultivation, but bacterial growth was limited owing to nutrient deficiency. Starvation strategy also contributed to enhancement of lipid accumulation, as lipid content in MB₄.₀ with a greater C/N/P led to the greatest increase in the starvation period, and the maximum lipid productivity reached 0.057 g/(L·day). Chemical oxygen demand and nitrogen removal in MB₄.₀ reached 92 and 96%, respectively, after 3 days of cultivation. Thus, cultivation of microalgae in high C/N/P wastewater enabled simultaneous realization of biomass granulation, bacterial overgrowth limitation, enhanced lipid accumulation, and wastewater purification.

Immobilized microalga Chlorella sp. Wu-G23 (G23) was entrapped inside a polymer matrix or appended on the surface of a strong solid carrier. Alginates then formed a microalgae/polymeric matrix granule within the cross-linking solution. The textile wastewater bioremediation technology using immobilized G23 in textile wastewater effectively removed chemical oxygen demand (COD), ammonium nitrogen (NH4+-N) and color after 7 days of cultivation. Batch model results show that the optimal cultivation parameters for simultaneously removing textile wastewater pollutants and accumulating lipids were initial pH 10 with extra urea 1 g/L and K2HPO4 8 mg/L added without aeration. A bioreactor cultivated the immobilized G23 at hydraulic retention time of 48 h with continuously fed textile wastewater for 440 h. Peak removal efficiencies of NH4+-N 80.2%, COD 70.8% and color 77.9% with fatty acid methyl esters 10% were achieved. The color removal occurred through the biosorption mechanism using nonviable suspended and immobilized microalgae biomass. The NH4+-N and COD could be degraded in the cell-free, nonviable microalgae because the microorganism in the textile wastewater could utilize them as a nutrient source. [ABSTRACT FROM AUTHOR]

• Trace p-HBA enables chlorella to triumph dramatically over bacterial contamination. • p-HBA boosted microalgal growth by 32.2 % while inhibiting bacteria by 80 %. • p-HBA upregulated microalgae photosynthesis, MAPK, and oxidative phosphorylation. • p-HBA decreased bacterial glucose absorption and its associated downstream metabolic. • p-HBA increased intracellular products and promoted biomass harvesting. Microalgae represent promising organisms for applications in wastewater resource recovery. Nonetheless, bacterial contamination disrupts the ecological niche dominated by microalgae cells. By modulating plant-pathogen interactions, phytohormones have a positive regulatory role in enhancing plant immunity. In a bioreactor containing 1.0 M p-hydroxybenzoic acid (p-HBA), we tested the hypothesis that phytohormone-like could inhibit bacteria growth while simultaneously promoting microalgal growth. p-HBA increased Chlorella biomass by 32.2 % and upregulated photosynthesis, mitogen-activated protein kinase, and oxidative phosphorylation genes. Simultaneously, bacterial gene copy number decreased ∼80 % and genes involved in flagellum movement, cell wall formation, and DNA replication were found to be silenced. Meanwhile, the increase in the microalgae biomass and the presence of extracellular polymeric substances (EPS) increased bacteria-microalgae floc sedimentation. Microalgae produced 26–82 % more intracellular products. This study establishes a strategy for using a trace amount of p-HBA to assist Chlorella in establishing survival dominance under bacterial contamination. [Display omitted] [ABSTRACT FROM AUTHOR]

• Enrofloxacin caused Chlorella cell density to decrease by 31%. • p-Hydroxybenzoic acid (pHBA) regulated antibiotics resistance in Chlorella. • Chlorella growth rebounded by 23% with the addition of 1 μM pHBA. • pHBA addition increased lipid production by 30% and biodiesel quality improved. • pHBA accelerated photon-electron transmission and improved antioxidant capacity. Phytohormones are widely known to regulate plant growth under environmental stress, however, their roles of microalgae are not clear during the antibiotic wastewater treatment. A hypothesis was proposed that phytohormones were capable to relieve the inhibition of microalgae growth and improve the bioresources recovery from antibiotic wastewater. Chlorella and enrofloxacin (ENR) were utilized to simulate the antibiotic wastewater treatment by microalgae, and explored the roles of p-hydroxybenzoic acid (pHBA). With 10 mg/L ENR and 1 μM pHBA condition, the Chlorella biomass increased by 23%. Moreover, lipid production increased by 30%. Trace amounts of pHBA improved microalgae growth and lipid accumulation by up-regulating photosynthesis, glycolysis, and TCA cycle. Furthermore, pHBA stimulated the signal transduction pathways of microalgae to improve antioxidant defense capability. Therefore, pHBA enhanced tolerance of ENR and improved microalgae lipid production. This study provided a new clue into using microalgae to recover high-value bioresources from antibiotic wastewater. [Display omitted] [ABSTRACT FROM AUTHOR]

The high energy input required for harvesting microalgae means that commercial production of microalgal biodiesel is economically unfeasible. In this study, we investigated the flocculation efficiency and synergistic mechanisms of novel coupled flocculants, AlCl and compound bioflocculants (CBF), to overcome this difficulty. AlCl flocculation was found to be very sensitive to pH, and flocculation efficiency increased from 55 to 95 % when pH increased from 4 to 10. CBF was environmental friendly, less reliant on pH, but had a relatively low flocculation of 75 % in optimum conditions. The harvesting efficiency of Chlorella regularis can achieve a satisfactory level of 96.77 % even in neutral conditions, with a CBF dosage of 0.26 g/L, AlCl dosage of 0.18 g/L, and coagulant aid (CaCl) dosage of 0.12 g/L. Interestingly, compared with the use of single flocculant, the dosage of CBF, AlCl, and coagulant aid (CaCl) were reduced by about 52, 49, and 66 %, respectively. Besides, the aluminum (Al) ion content of the supernatant decreased significantly to a residue of only 0.03 mg/L, therefore meeting the downstream process needs easily. Patching and bridging played key roles in coupled flocculant flocculation, in which AlCl mainly carried out the electrical neutralization. This work provides new insight into an efficient, economical, and environmentally friendly protocol for microalgae harvesting. [ABSTRACT FROM AUTHOR]

Recent studies claimed nitrogen starvation in addition to sufficient phosphorus content to be a “lipid trigger” for microalgae autotrophic cultivation. However, the role of phosphorus in heterotrophic cultivation still remains controversial. Here, we investigated the utilization of a series of phosphorus concentrations of up to 818% in BG11 medium to cultivate Chlorella regularis under nitrogen starvation stress during glucose heterotrophic cultivation. Phosphorus positively influenced microalgal cell synthesis, and the lipid content increased with increasing levels of supplied phosphorus. The excess phosphorus was stored in the form of Poly-P, thus providing energy and participating in the synthesis of cellular materials, which enhanced cell growth and lipid accumulation. The biomass production reached 4.53 g/L under nitrogen starvation with excess phosphorus supply, which was comparable to the effect of a nitrogen-sufficient condition (control). The excessive phosphorus level upregulated the genes ROT3 , BAS1 , CYP735A , FBP , GOT2 , and AOC3 associated with growth. Microalgal lipid content reached 42.3% and lipid productivity reached 310.0 mg/(L·d), values that were 2 and 1.3 times higher than those in the control, respectively. Furthermore, the additional phosphorus supply increased the amounts of saturated fatty acids, while shifting fatty acid pathways. We suggest that higher amounts of phosphorus should be supplied to achieve high microalgal lipid productivity under nitrogen starvation stress during glucose heterotrophic cultivation. This is an effective strategy to enhance biodiesel production, simultaneously achieving high biomass production and efficient lipid accumulation. [ABSTRACT FROM AUTHOR]