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Microalgae demonstrate significant potential as a source of liquid-based biofuels. However, increasing biomass productivity in existing cultivation systems is a critical prerequisite for their successful integration into large-scale operations. Thus, the current work aimed to accelerate the growth of C. vulgaris via exogenous supplementation of biostimulant derived from onion peel waste. Under the optimal growth conditions, which entailed a biostimulant dosage of 37.5% v/v, a pH of 3, an air flow rate of 0.4 L/min, and a 2% v/v inoculum harvested during the mid-log phase, yielded a maximum biomass concentration of 1.865 g/L. Under the arbitrarily optimized parameters, a comparable growth pattern was evident in the upscaled cultivation of C. vulgaris , underscoring the potential commercial viability of the biostimulant. The biostimulant, characterized through gas chromatography-mass spectrometry (GC-MS) analysis, revealed a composition rich in polyphenolic and organo-sulphur compounds, notably including allyl trisulfide (28.13%), methyl allyl trisulfide (23.04%), and allyl disulfide (20.78%), showcasing potent antioxidant properties. Additionally, microalgae treated with the biostimulant consistently retained their lipid content at 18.44% without any significant reduction. Furthermore, a significant rise in saturated fatty acid (SFA) content was observed, with C16:0 and C18:1 dominating both bench-scale (44.08% and 14.01%) and upscaled (51.12% and 13.07%) microalgae cultures, in contrast to the control group where C18:2 was prevalent. Consequently, SFA contents reached 54.35% and 65.43% in bench-scale and upscaled samples respectively, compared to 33.73% in the control culture. These compositional characteristics align well with the requirements for producing high-quality crude biodiesel. [Display omitted] • Biostimulant supplementation triggered high microalgal biomass up to 1.865 g/L. • Possibility for culture scale-up without decline in growth performance. • Characterizations verified presence of polyphenolic and organo-sulphur elements. • Acquired fatty acid profile satisfied higher diesel standards. [ABSTRACT FROM AUTHOR]

Microalgae showcase an extraordinary capacity for synthesizing high-value phytochemicals (HVPCs), offering substantial potential for diverse applications across various industries. Emerging research suggests that subjecting microalgae to abiotic stress during cultivation and the harvesting stages can further enhance the accumulation of valuable metabolites within their cells, including carotenoids, antioxidants, and vitamins. This study delves into the pivotal impacts of manipulating abiotic stress on microalgae yields, with a particular focus on biomass and selected HVPCs that have received limited attention in the existing literature. Moreover, approaches to utilising abiotic stress to increase HVPCs production while minimising adverse effects on biomass productivity were discussed. The present study also encompasses a techno-economic assessment (TEA) aimed at pinpointing significant bottlenecks in the conversion of microalgae biomass into high-value products and evaluating the desirability of various conversion pathways. The TEA methodology serves as a valuable tool for both researchers and practitioners in the quest to identify sustainable strategies for transforming microalgae biomass into high-value products and goods. Overall, this comprehensive review sheds light on the pivotal role of abiotic stress in microalgae cultivation, promising insights that could lead to more efficient and sustainable approaches for HVPCs production. • Abiotic stresses are crucial for phytochemicals secretion in microalgae. • Impacts of various stresses on HVPCs production are examined. • Possibilities for one-pot production of biomass and HVPCs are discussed. • Techno-economic constraints were gauged and addressed. • Future directions to improve microalgae industry are elucidated. [ABSTRACT FROM AUTHOR]

The present study investigated the application of photocatalytic process using TiO 2 nanotubes (NTs) photocatalyst as an effective pre-treatment method for palm kernel expeller (PKE) in increasing its biodegradability to enhance the microalgal hydrogen production via dark fermentation. A series of TiO 2 NTs photocatalysts were synthesized via employing electrochemical anodization of Ti foil at various voltages each in different types of electrolytes (choline chloride-ethylene glycol (ChCl-EG), choline chloride-urea (ChCl-U), choline chloride-glycerol (ChCl-Gly)). Increasing trend of average NTs diameters at 26.14 ± 1.30, 73.01 ± 1.94 and 122.00 ± 13.32 nm was observed for 25, 30 and 35 V in synthesizing TiO 2 NTs using suitable ChCl-EG electrolyte, respectively. The high 35 V had resulted in noticeable pore ruptures within this TiO 2 NTs photocatalyst. The PKE pre-treated by TiO 2 NTs photocatalyst anodized at optimum 30 V in ChCl-EG electrolyte had demonstrated the highest releases of biochemical oxygen demand (BOD 5) and soluble chemical oxygen demand (SCOD) concentrations at 594 and 607 mg/L, respectively. Moreover, the microalgal efficiency of BOD 5 conversion and hydrogen yield were also consistently maintained from third cycle of TiO 2 NTs photocatalyst's reutilizations, signifying the stability of TiO 2 NTs photocatalyst. [Display omitted] • Anatase phase TiO 2 NTs photocatalyst to pre-treat palm kernel expeller (PKE). • Photocatalytic pre-treated PKE exhibited improved biodegradable organic matter. • TiO 2 NTs in ChCl-EG at 30 V showed enhanced microalgal hydrogen production. • TiO 2 NTs was stable and reusable in releasing biodegradable organics from PKE. [ABSTRACT FROM AUTHOR]

[Display omitted] • Extraction of antioxidative biostimulant from onion waste was optimized via RSM. • 1.76 g/L of biomass yield was achieved using very low microalgae inoculum. • Microalgae lipid content was sustained at 19.42 % without major loss. • The biostimulant showed high DPPH (78.65 %) and ABTS (73.68 %) inhibition. In this study, a novel and cost-effective method for using onion peel as a biostimulant is presented to overcome the challenge of low microalgae biomass productivity. The study successfully optimized and modelled the extraction parameters, including solid-to-solvent (SS) ratio, temperature, and time, using the response surface methodology (RSM) tool. The results indicated a strong correlation between the extraction parameters and microalgae biomass concentration (BC) and lipid content (LC), with R2 values of 0.9440 and 0.9638, respectively. The optimum biostimulant extraction conditions were SS ratio of 0.006 g/mL, temperature of 90 °C at 1.00 h, attaining BC and LC of 1.76 g/L and 19.42 %, respectively, without any negative impact on LC. Findings on bioactivities of biostimulant recovered at optimal extraction conditions supported its biostimulation capacity on microalgae, with reasonable total phenolic content (TPC) of 9.1094 ± 0.748 mg GAE/g dry weight, total flavonoid content (TFC) of 3.1307 ± 0.210 mg RE/g dry weight, and antioxidant activity (AA) of 78.65 ± 3.421 % and 73.68 ± 9.608 % for DPPH and ABTS inhibition, respectively. The results of this study suggest that biostimulant derived from onion waste has great potential for promoting rapid proliferation of microalgae even with extremely low inoculum cells. These findings present a promising roadmap for high-density microalgae farming, and the developed RSM model can be utilized for process optimization while improving the efficiency, quality, and cost-effectiveness of microalgae industry. [ABSTRACT FROM AUTHOR]

Derivation of biofuel from microalgae biomass has been widely researched in the past few decades. Microalgae is capable of producing 58,700 litres oil per hectare that can generate 121,104 litres biodiesel per hectare, which seemingly a promising transition over conventional fossil fuels. Nevertheless, economic sustainability of commercial scale production of microalgae biomass is still in shadows of doubt, especially the cultivation and harvesting process. Apparently, the microalgae cultivation system has evolved from traditional open pond to various modern photobioreactor (PBR) designs. However, with regards to tubular and flat panel PBRs as the most ubiquitous systems for biofuel production at commercial level, extensive discussion on reactor configurations and design betterment was presented in this review, along with precise technical comparison on cost and energy requirements for the cultivation systems. This review intended to serve as guideline for long term adoption of these well-established cultivation technologies in biofuel plants given the numerous economic benefits. Besides that, in attempt to lower the harvesting cost, potential use of various waste biomass as bioflocculants to recover microalgae biomass was introduced in this review. This article also deliberates direction on potential policy interventions to produce microalgae biofuel in a more sustainable and cost-effective manners in near future. • Microalgae-derived biofuel is the potential substitute over fossil fuel. • Cultivation and harvesting steps are the bottlenecks to produce microalgae biofuel. • Wise selection of cultivation technology ensures sustainable biofuel production. • Bioflocculants from waste biomass are cheap, non-toxic, easily extracted. • Suitable policy framework ensures successful microalgae biofuel commercialization. [ABSTRACT FROM AUTHOR]

Albeit the biodiesel production from suspended microalgal system has gained immense interests in recent years, the domineering limitation of being economically infeasible has hindered this technology from partaking into a large-scale operation. To curtail this issue, attached growth system had been introduced by various studies; however, those were still unable to alleviate the socio-economic challenges faced in commercializing the microalgal biomass production. Thus, this study had developed a novel approach in cultivating-cum-harvesting attached Chlorella vulgaris sp. microalgae, whilst using solid organic waste of palm kernel expeller (PKE) as the supporting and alimentation material for microalgal biofilm formation. The effects of three variables, namely, PKE dosage, light intensity, and photoperiod, were initially modelled and later optimized using Response Surface Methodology tool. The derived statistical models could predict the growth performances of attached microalgal biomass and lipid productivity. The optimum growing condition was attained at PKE dosage of 5.67 g/L, light intensity of 197 μmol/m2 s and photoperiod of 8 light and 16 dark hours/cycle, achieving the microalgal density and lipid content of 9.87 ± 0.05 g/g and 3.39 ± 0.28 g/g, respectively, with lipid productivity of 29.6 mg/L day. This optimum condition had led to the intensification of biodiesel quality with a high percentage of monounsaturated fatty acid, i.e., oleic acid (C18:1), encompassing 81.86% of total fatty acid methyl ester components. Given that the positive acquisition of PKE as an excellent supporting material in enhancing the microalgal density and lipid productivity that had resulted in the commercially viable biodiesel quality, this study served as a novel revolution in augmenting the microalgae and solid waste utilities for sustainable energy generation. [Display omitted] • Palm kernel expeller as the nutrients and platform for attached microalgal growth. • Optimum cultivation condition to enhance microalgal density and lipid productivity. • FAME profile unveiled biodiesel quality with 81% of monounsaturated fatty acid. [ABSTRACT FROM AUTHOR]

Microalgae have been recognized as a potential feedstock for biodiesel production due to their fast growth rate and ability to accumulate lipid within their cells. However, high lipid content and biomass productivity are usually difficult to achieve simultaneously. Instead of stressing the microalgae during cultivation stage, the stress environment such as nutrient starvation, salinity and light effect were introduced to Chlorella vulgaris after harvesting to study the effect on their lipid content. From the results attained, one day of nutrient starvation with 6.0 g/L of salinity stress under dark room condition had shown the highest lipid content of 38.8% (dry weight basis). The lipid content was recorded at 40.28% (dry weight basis) when the working volume was scaled up. In addition, the fatty acids identified in the extracted microalgae lipid were mainly consisted of linoleic, linolenic and palmitic acid, which were commonly found in corn oil, soybean oil and sunflower oil. • High biomass productivity and lipid are difficult to achieve simultaneously. • The stress towards the lipid content of microalgae after harvesting was studied. • Triple stress condition resulted the highest lipid content of microalgae. • Fatty acids of microalgae has similar profile as corn, soybean and sunflower oil. [ABSTRACT FROM AUTHOR]

Changing the growth environment for microalgae can overall lead to the fundamental alteration in cellular biochemicals whilst attaching onto palm kernel expeller (PKE) waste to form adhesion complex in easing harvesting at stationary growth phase. This study had initially optimized the PKE dosage, light intensity and photoperiod in maximizing the attached microalgal productivity being attained at 0.72 g/g day. Lipid content increased progressively from pH 3 to pH 11, with the highest value observed at pH 11. Meanwhile, in terms of protein and carbohydrate contents, the highest values were obtained by cultivation medium of pH 5 with 9.92 g and 17.72 g, respectively followed by pH 7 with 9.16 g and 16.36 g, respectively. Moreover, the findings also suggested that the low pH mediums utilized polar interactions in the formation of complexes between PKE and microalgae, whereas at higher pH levels, the non-polar interactions became more significant. The work of attachment was thermodynamically favourable towards the attachment formation with values greater than zero which was also aligned with the microscopic surface topography, i.e., revealing a clustering pattern of microalgae colonizing the PKE surface. These findings contribute to comprehensive understanding of optimizing growth condition and harvesting strategy of attached microalgae in attaining the cellular biochemical components, facilitating the development of efficient and sustainable bioresource utilization. [Display omitted] • Attached microalgae onto palm kernel expeller in producing cellular biochemicals. • PH factor positively militated biochemical components of attached microalgae. • Low dispersive and polar energy surface yielded optimal microalgal productivity. • Microalgae biolayer formed cluster upon reaching stationary growth phase. [ABSTRACT FROM AUTHOR]

The capacity to maximize the proliferation of microalgal cells by means of topologically textured organic solid surfaces under various pH gave rise to the fundamental biophysical analysis of cell-surface attachment in this study. The substrate used in analysis was palm kernel expeller (PKE) in which the microalgal cells had adhered onto its surface. The findings elucidated the relevance of surface properties in terms of surface wettability and surface energy in relation to the attached microalgal growth with pH as the limiting factor. The increase in hydrophobicity of PKE-microalgae attachment was able to facilitate the formation of biofilm better. The pH 5 and pH 11 were found to be the conditions with highest and lowest microalgal growths, respectively, which were in tandem with the highest contact angle value at pH 5 and conversely for pH 11. The work of attachment (W cs) had supported the derived model with positive values being attained for all the pH conditions, corroborating the thermodynamic feasibility. Finally, this study had unveiled the mechanism of microalgal attachment onto the surface of PKE using the aid of extracellular polymeric surfaces (EPS) from microalgae. Also, the hydrophobic nature of PKE enabled excellent attachment alongside with nutrients for microalgae to grow and from layer-by-layer (LbL) assembly. This assembly was then isolated using organosolv method by means of biphasic solvents, namely, methanol and chloroform, to induce detachment. [Display omitted] • Surface wettability property has positively corresponded to microalgal attachment. • Low dispersive and polar energy substrate is ideal for attached microalgal growth. • The work of attachment is in tandem with the microalgal growth trend. • All pH mediums have thermodynamically favoured attached microalgal growth. [ABSTRACT FROM AUTHOR]