Microalgal System Research Articles

Unexpected increase in microalgal removal of doxylamine induced by bicarbonate addition: synergistic chem-/bio-degradation mechanisms

Microalgae-based approaches serves as promising methods for remediation of pharmaceutical contaminants (PCs) compare to conventional wastewater treatment processes. However, how to decrease hydraulic retention times of the microalgal system currently has been one of the main bottlenecks. This study constructed an unexpected synergistic extra-chem-/intra-bio-degradation system by addition of 5.95 mM bicarbonate in the microalgal system, which achieved complete removal (100%) of a representative PC, doxylamine (DOX) in 96 h compare to that 192 h in the control. Removal capacities and mass balance analyses demonstrated that biodegradation rate per unit microalgal density was significantly increased by 207%. Further analyses using transcriptomic, enzymatic inhibitory test, and high-resolution mass spectrometry revealed that after addition of bicarbonate for metabolism of DOX, a hydrolase (CYP97C1) and a primary amine oxidase (TynA) can catalyze DOX into doxylamine N-oxide and an intermediate (C15H17NO2) with a m/z of 244.1335. Meanwhile, bicarbonate reacted with microalgae-excreted hydrogen peroxide to form more oxidative radicals including superoxide and hydroxyl radicals extracellularly, which promised the extracellular degradation of DOX according to the oxidative radical inhibiting tests. Further investigation showed addition of bicarbonate in microalgal system improved the removal rate of 17 PCs by up to 500.8%. Therefore, this study not only developed an approach to enhance treatment efficiencies of diverse PCs by microalgae within a shorter time, but also carried unique mechanistic insights into the underlying principles.

Antimicrobial and Ecological Potential of Chlorellaceae and Scenedesmaceae with a Focus on Wastewater Treatment and Industry

A complex evaluation of antimicrobial activities of microalgae, including those relevant to wastewater treatment (WWT), in light of the integrated biorefinery concept, is performed. An example of this concept is linking a commercial microalgal system to plants, factories, or farms that emit polluted wastewater (WW). The microalgae would not only metabolize the pollutants—such as nitrogen (N) and phosphorus (P)—from the WW, thus fueling their biomass, but they would exert an antibacterial effect against the pathogenic bacteria there. The biomass then could be harvested and used for biofertilizers, biofuels, and bioplastics and might possibly be utilized as animal feed, antimicrobial and other pharmaceutical agents. A large amount of the research on the antimicrobial activity and WWT potential focuses on the families Chlorellaceae and Scenedesmaceae, which are also some of the most commercially used strains of microalgae. For that reason, they are the species chosen for the current review. Furthermore, the increasing antimicrobial resistance necessitates the search for antibiotic alternatives, and the antibacterial and antifungal activity of Chlorellaceae and Scenedesmaceae is very promising. Microalgae are rich in antibacterial compounds like polyunsaturated fatty acids (PUFAs), polysaccharides, carotenoids, proteins, etc., and for that reason, their extracts possess antimicrobial effects. The in vitro antimicrobial activity of Chlorellaceae and Scenedesmaceae families has varied in a broad range from low to strong activity or no effect. Several strains have fulfilled the criteria for outstanding and high activity, especially C. vulgaris and other Chlorellaceae spp., with an effect equal to or better than the control antibiotics. There were several strains with minimum inhibitory concentrations (MIC) below 80 µg/mL and even 10 and 1.5 µg/mL; some species also had inhibition zones (IZ) over 30 mm, even as high as 48 mm. In vivo results are also promising but scarce, and all this warrants further in vivo and in situ studies—from animal models to clinical and environmental trials. Altogether, important data in the light of the circle economy, the urgent necessity to decrease CO2 emissions to fight climate change, and to curb the harmful influence of future pandemics are presented. This review paves the way for further utilizing the total potential of a microalgal system.

Bio-refinery of palm kernel expeller waste into attached microalgal biodiesel, the life cycle analysis of waste biomass-to-energy

The exploitation of solid carbon waste in the form of palm kernel expeller served two purposes: (i) providing nutrients for microalgal growth and (ii) serving as a medium for the adhesion of microalgal cells. This study examined the life cycle assessment of biodiesel production from attached microalgae, a less explored domain as compared with conventional microalgal system focusing on energy and environmental feasibility aspect. The computed environmental impact indicated that harvesting and lipid production had contributed significantly to the overall impact, though the global warming potential remained low (5.2 × 10−4 kg CO2 eq.). The establishment of scenario analysis had revealed that solvent recovery options resulted in about 48–55 % damage recovery, giving significant decline in the environmental impacts. In terms of energy sustainability, the study revealed a net energy ratio of 0.52, suggesting that the energy output is less than the energy invested. While falling short of ideal energy sustainability, the ratio surpassed that of conventional petroleum diesel and many previous microalgal technologies. Despite challenges, the findings accentuated the importance of continuous improvement in microalgal biofuel processes for enhancing energy efficiency and reducing environmental impact.

A whole process study of dual microalgae cultivation coupled to domestic wastewater treatment and wheat growth

The multiple microalgal collaborative treatment of domestic wastewater has been extensively investigated, but its whole life cycle tracking and consequent potential have not been fully explored. Herein, a dual microalgal system was employed for domestic wastewater treatment, tracking the variation in microalgal growth and pollutants removal from shake flask scale to 18 L photobioreactors scales. The results showed that Chlorella sp. HL and Scenedesmus sp. LX1 combination had superior growth and water purification performance, and the interspecies soluble algal products promoted their growth. Through microalgae mixing ratio and inoculum size optimized, the highest biomass yield (0.42 ± 0.03 g/L) and over 91 % N, P removal rates were achieved in 18 L photobioreactor. Harvested microalgae treated in different forms all promoted wheat growth and suppressed yellow leaf rate. This study provided data support for the whole process tracking of dual microalgal system in treating domestic wastewater and improving wheat growth.

Cytoprotective alginate microcapsule serves as a shield for microalgal encapsulation defensing sulfamethoxazole threats and safeguarding nutrient recovery

Microalgal encapsulation technology is expected to broaden more possibilities for employing microalgae for upgrading conventional biological wastewater treatment. However, only limited and fragmented information is currently available on microalgal encapsulation and pollutant removal. It is ambiguous whether it hold potential for wastewater treatment. Particularly, it remains to be determined whether this technology can provide more possibilities in harsh sewage environments. Here, potential of encapsulated technology to recover nutrients from wastewater was examined, simultaneously compared with commonly adopted suspended system. Results indicate the encapsulated microalgal system showed outstanding advantages in nutrient recovery and defense against antibiotic threats. Moreover, by examining the cellular oxidative stress response and changes of the photosynthetic system, the encapsulated system exhibited potential cytoprotective advantages to microalgal cells for defensing antibiotic threats. Molecular dynamics simulation revealed that the differences among superficial aggregation between the nutrients’ ions and molecular sulfamethoxazole on the cross-linked alginate microcapsule surface dominated the nutrient recovery and cytoprotective functions. Ultimately, the molecular nature of pollutants was found to be the most critical aspect for predicting application of this microalgal microcapsule. Cytoprotective systems created with alginate microcapsules can potentially handle more diverse threats with a single type of surface charge in their outermost layer.

Pharmaceuticals Removal from Wastewater with Microalgae: A Pilot Study

Urban wastewaters contain pharmaceuticals that are not appropriately removed in conventional wastewater treatments, limiting treated water reuse. Microalgae have been shown to remove pharmaceuticals from urban wastewater in laboratory trials, but few studies have been conducted under natural conditions. In this work, pharmaceutical removal was assessed in a pilot-scale microalgal tertiary wastewater treatment in real conditions. Even after secondary treatment, the water contained measurable amounts of pharmaceuticals (an average of 218.4 ng/L) that significantly decreased to 39.83 ng/L at the exit of the microalgal system. Pharmaceuticals’ average removal rates were slightly higher in the summer (79.1%) than in autumn (71.1%). Antibiotics and antipsychotics were better removed (88.8 and 86.4%, respectively) than antihypertensives (75.3%) and others (Bezafibrate and Diclofenac; 64.0%). Physicochemical characteristics of the wastewater influenced pharmaceutical removal; significant positive correlations were observed between anti-hypertensive drug removal and ammonium concentration (r = 0.53; p < 0.05), total nitrogen and total pharmaceutical removal (r = 0.46; p < 0.05), and total nitrogen and antipsychotic drug removal (r = 0.47; p < 0.05). The results demonstrate the effectiveness of microalgal tertiary treatment in the removal of pharmaceuticals.

Immobilized microalgal system: An achievable idea for upgrading current microalgal wastewater treatment.

Efficient wastewater treatment accompanied by sustainable "nutrients/pollutants waste-wastewater-resources/energy nexus" management is acting as a prominent and urgent global issue since severe pollution has occurred increasingly. Diverting wastes from wastewater into the value-added microalgal-biomass stream is a promising goal using biological wastewater treatment technologies. This review proposed an idea of upgrading the current microalgal wastewater treatment by using immobilized microalgal system. Firstly, a systematic analysis of microalgal immobilization technology is displayed through an in-depth discussion on why using immobilized microalgae for wastewater treatment. Subsequently, the main technical approaches employed for microalgal immobilization and pollutant removal mechanisms by immobilized microalgae are summarized. Furthermore, from high-tech technologies to promote large-scale production and application potentials in diverse wastewater and bioreactors to downstream applications lead upgradation closer, the feasibility of upgrading existing microalgal wastewater treatment into immobilized microalgal systems is thoroughly discussed. Eventually, several research directions are proposed toward the future immobilized microalgal system for microalgal wastewater treatment upgrading. Together, it appears that using immobilization for further upgrading the microalgae-based wastewater treatment can be recognized as an achievable alternative to make microalgal wastewater treatment more realistic. The information and perspectives provided in this review also offer a feasible reference for upgrading conventional microalgae-based wastewater treatment.

Characterization and Biofabrication of Silver Nanoparticles Utilizing Isochrysis Extract along with its in vitro Antibacterial and Antioxidant Applications

Abstract: Background: At present, bio-green methods are more expedient than physical and chemical approaches due to their prompt, simple, lucrative, scalable and conservational synthesis of nanoparticles. Aim: This study investigates the extracellular biogenic synthesis of silver nanoparticles (AgNPs) using the ethanolic-hexane extract of Isochrysis sp. (EHEI). Materials and Methods: Biosynthesized AgNPs were nano-characterized by UV-Vis spectroscopy, Field Emission Scanning Electron Microscope (FESEM), Dynamic light scattering (DLS), X-ray diffraction (XRD), Energy Dispersive X-ray (EDX), Fourier Transform Infrared Spectra (FTIR), and Gas Chromatography Mass Spectrometry (GCMS) and finally examined for antibacterial and antioxidant activity. Results: The biosynthesis of AgNPs was evidenced with UV-Vis spectra showing surface plasmon resonance (SPR) at 421 nm; fabrication of highly stable, well dispersed, spherical AgNPs with an average size of 64.47 nm which were evidenced by FESEM. The crystalline nature of AgNPs was apparent from the diffraction peaks of XRD. The Zeta potential value of-22.3 mV demonstrated the colloidal stability of AgNPs. FTIR spectra showed that functional groups existing in fatty acids, lipids, proteins, xanthophylls, polyphenols and amines are accountable for the formation and stabilization of AgNPs. GCMS confirmed that biomolecules like fatty acid ethyl esters (FAEE) are involved in capping, biochemical reduction of Ag+ ions and stabilization of AgNPs. Also, the synthesized NPs displayed high antimicrobial activity against pathogenic G+ve and G-ve bacteria. In vitro antioxidant properties were ascertained by 1,1-diphenyl-2-picryl-hydrazyl (DPPH), hydrogen peroxide and reducing power assays which established that AgNPs have an incomparable scavenging potential than the extracts. Conclusion: Use of such a microalgal system postulates an alternative template for biofabrication of multifunctional nanomaterials in extensive prototype that might be extensively applicable in biomedical sectors. Keywords: Isochrysis, AgNPs, FESEM, GCMS, Antibacterial, Antioxidant.

Effect of macronutrient levels on Chlorella vulgaris cultivation for long duration spaceflights and space settlements

To expand space explorations into space settlements, there is a need for the establishment of a sustainable, closed-loop life support system. Systems involving microalgae are promising, as they could simultaneously revitalize air (i.e., CO2 to O2 conversion), reclaim water (i.e., nutrient biofixation/recycling), and provide food supplement (i.e., biomass). This would entail microalgal cultivation in human-derived wastes (both gas and liquid), which could provide challenges on system tolerance, particularly on the levels of nutrients in these wastes. In this work, the effect of macronutrient [Carbon (C), Nitrogen (N), and Phosphorus (P)] levels on the phototrophic growth of Chlorella vulgaris UTEX 2714 was investigated using synthetic wastewater. This was done to determine the range of macronutrient levels that are suitable for sustained microalgal growth. The highest biomass concentration was achieved when the microalga was axenically cultivated in 71 mg C/L, 64 mg N/L, and 13 mg P/L. With an equivalent C:N:P mass ratio of 5.5:5:1, this cultivation condition was significantly different from the ideal C:N:P of 41:7:1, suggesting a C-limited growth environment. This was also supported by 99.9% C consumption, while only consuming 56% and 72% of N and P, respectively. These results indicate that during space cultivation, incremental amount of C must be added to the microalgal system to improve the overall C:N:P of the process and to enhance N and P consumption, while ensuring that the C level is below the inhibitory threshold limit. Based on these results, together with human-derived wastes data from the International Space Station, a microalgal photobioreactor configuration was proposed.

Multiomics and optobiotechnological approaches for the development of microalgal strain for production of aviation biofuel and biorefinery

Demand and consumption of fossil fuels is increasing daily, and oil reserves are depleting. Technological developments are required towards developing sustainable renewable energy sources and microalgae are emerging as a potential candidate for various application-driven research. Molecular understanding attained through omics and system biology approach empowering researchers to modify various metabolic pathways of microalgal system for efficient extraction of biofuel and important biomolecules. This review furnish insight into different “advanced approaches” like optogenetics, systems biology and multi-omics for enhanced production of FAS (Fatty Acid Synthesis) and lipids in microalgae and their associated challenges. These new approaches would be helpful in the path of developing microalgae inspired technological platforms for optobiorefinery, which could be explored as source material to produce biofuels and other valuable bio-compounds on a large scale.

CO2 bio-mitigation using genetically modified algae and biofuel production towards a carbon net-zero society

CO2 sequestration carried by biological methodologies shows enhanced potential and has the advantage that biomass produced from the captured CO2 can be used for different applications. Bio-mitigation of carbons through a micro-algal system addresses a promising and feasible option. This review mostly focused on the role of algae, particular mechanisms, bioreactors in algae cultivation, and genetically modified algae in CO2 fixation and energy generation systems. A combination of CO2 bio-mitigation and biofuel production might deliver an extremely promising alternative to current CO2 mitigation systems. Bio mitigation in which the excess carbon is captured and bio fixation which the carbon is captured by algae or autotrophs and used for producing biofuel. This review revealed that steps for biofuel production from algae include factors affecting, harvesting techniques, oil extraction and transesterification. This review helps environmentalists and researchers to assess the effect of algae-based biorefinery on the green environment.

Residual palm kernel expeller as the support material and alimentation provider in enhancing attached microalgal growth for quality biodiesel production

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.

Single-step integrated technology for enhanced CO2 biofixation and efficient lipid extraction in microalgal system including a water-immiscible solvent

The potential of microalgae for both carbon fixation and energy sustainability has been demonstrated. However, the low CO2 biofixation rate and high cost of lipid extraction restrict its large-scale development. This paper proposes a novel integrated technology for enhanced CO2 biofixation and efficient lipid extraction in microalgal system by adding water-immiscible solvent N-heptane over the culture medium. N-heptane can act as a ‘carbon sink’ and ‘extract agent’ without causing significant damage to microalgal cells. Specifically, N-heptane with the volume percentages of 4, 8, 12, 25 and 42 % was added to Chlorella sp. culture systems. Simulated flue gas containing 15 vol% CO2 in nitrogen gas was used as the sole carbon source. The maximum biomass concentration (0.84 g·L-1) and specific growth rate (0.23 d-1) were achieved with 12 % N-heptane. These values were higher than blank culture by 12.3 % (biomass concentration) and 56.9 % (specific growth rate). The CO2 biofixation rate increased with N-heptane to a maximum value of 220.2 mg·L-1·d-1, which was 64.7 % higher than that of the blank culture. Meanwhile, the system has a potential for the generation of biofuels with a maximum lipid amount of 2.82 g·gAlgae-1 in this single step approach.

Sustainability Evaluation of Immobilized Acid-Adapted Microalgal Technology in Acid Mine Drainage Remediation following Emergy and Carbon Footprint Analysis.

Sustainability evaluation of wastewater treatment helps to reduce greenhouse gas emissions, as it emphasizes the development of green technologies and optimum resource use rather than the end-of-pipe treatment. The conventional approaches for treating acid mine drainages (AMDs) are efficient; however, they need enormous amounts of energy, making them less sustainable and causing greater environmental concern. We recently demonstrated the potential of immobilized acid-adapted microalgal technology for AMD remediation. Here, this novel approach has been evaluated following emergy and carbon footprint analysis for its sustainability in AMD treatment. Our results showed that imported energy inputs contributed significantly (>90%) to the overall emergy and were much lower than in passive and active treatment systems. The microalgal treatment required 2–15 times more renewable inputs than the other two treatment systems. Additionally, the emergy indices indicated higher environmental loading ratio and lower per cent renewability, suggesting the need for adequate renewable inputs in the immobilized microalgal system. The emergy yield ratio for biodiesel production from the microalgal biomass after AMD treatment was >1.0, which indicates a better emergy return on total emergy spent. Based on greenhouse gas emissions, carbon footprint analysis (CFA), was performed using default emission factors, in accordance with the IPCC standards and the National Greenhouse Energy Reporting (NGER) program of Australia. Interestingly, CFA of acid-adapted microalgal technology revealed significant greenhouse gas emissions due to usage of various construction materials as per IPCC, while SCOPE 2 emissions from purchased electricity were evident as per NGER. Our findings indicate that the immobilized microalgal technology is highly sustainable in AMD treatment, and its potential could be realized further by including solar energy into the overall treatment system.

Valorization of Wastewater Resources Into Biofuel and Value-Added Products Using Microalgal System

Wastewater is not a liability, instead considered as a resource for microbial fermentation and value-added products. Most of the wastewater contains various nutrients like nitrates and phosphates apart from the organic constituents that favor microbial growth. Microalgae are unicellular aquatic organisms and are widely used for wastewater treatment. Various cultivation methods such as open, closed, and integrated have been reported for microalgal cultivation to treat wastewater and resource recovery simultaneously. Microalgal growth is affected by various factors such as sunlight, temperature, pH, and nutrients that affect the growth rate of microalgae. Microalgae can consume urea, phosphates, and metals such as magnesium, zinc, lead, cadmium, arsenic, etc. for their growth and reduces the biochemical oxygen demand (BOD). The microalgal biomass produced during the wastewater treatment can be further used to produce carbon-neutral products such as biofuel, feed, bio-fertilizer, bioplastic, and exopolysaccharides. Integration of wastewater treatment with microalgal bio-refinery not only solves the wastewater treatment problem but also generates revenue and supports a sustainable and circular bio-economy. The present review will highlight the current and advanced methods used to integrate microalgae for the complete reclamation of nutrients from industrial wastewater sources and their utilization for value-added compound production. Furthermore, pertaining challenges are briefly discussed along with the techno-economic analysis of current pilot-scale projects worldwide.

Cell leakage minimization by immobilization modulation of Chlorella sorokiniana NCIM 5561 and phosphate removal from wastewater

Microalgal cell leakage from immobilized beads due to mass transfer limitation and inert support matrix pose major problem during its application in the wastewater treatment process. To overcome this, Ca2+-alginate immobilization with water (AWIM—Alginate in Water Immobilized Microalgal system) and Blue Green (BG)-11 media (ABIM—Alginate in BG-11 Immobilized Microalgal system) using Chlorella sorokiniana NCIM 5561 was employed to determine their efficacy. Upon comparison of microalgal growth in the beads, ABIM resulted in better kinetic properties such as biomass yield, specific growth rate and productivity as 933.12 mg L−1, 0.42 day−1 and 77.78 mg L−1 day−1, respectively, as compared to AWIM. Flow cytometry and SEM analysis revealed that ABIM could hold up more cell density as compared to AWIM due to the reduced pore size and growth nutrient localization in the bead matrix. A novel strategy of co-cultivation of yeast along with microalgal cells under immobilized condition was attempted for improved phosphate uptake from the simulated wastewater. The results indicated that the co-cultivation resulted in minimum residual phosphate concentration of 0.75 mg L−1 as compared to ABIM and AWIM with corresponding concentration of 1.02 and 1.71 mg L−1, respectively. However, the rate of uptake was found to be superior with ABIM (0.41 h−1) than co-cultivation (0.29 h−1) and AWIM (0.23 h−1) that correlates to overall removal efficiency of 94.54%, 80.64% and 67.07%, respectively.

Treatment of wastewater reverse osmosis concentrate using alginate-immobilised microalgae: Integrated impact of solution conditions on algal bead performance

Alginate can be used for entrapment of microalgal cells in gel beads to achieve high-rate treatment of wastewater and can overcome the difficulties of cell separation that would occur in suspended microalgae treatment systems. The potential for alginate beads to disintegrate in the presence of high ion concentrations could limit the use of alginate entrapment for treating municipal wastewater reverse osmosis concentrate (ROC). The combined effect of the pH, alkalinity, and salinity of the ROC that impact the physical stability, chemical characteristics, biomass production, and nutrient removal performance of alginate-entrapped Chlorella vulgaris for treating the ROC was investigated. Water adsorption resulting from the loss of calcium from the alginate matrix was the initiating cause of reduction of the algal bead stability. The combination of alkalinity >400 mg/L and pH ≥9.5 led to a >65% reduction in compressive strength and thus disintegration of beads during ROC treatment. However, alginate beads of C. vulgaris were sufficiently stable and were capable of nutrient remediation (up to 100% TP and 85% TN per treatment cycle of 48 h over a 10-day period) and biomass production (up to 340 mg/L/d) when salinity, pH, and alkalinity levels were <8 g TDS/L, 7–9.5, and <400 mg/L, respectively. Empirical models that were developed and validated could enable the prediction of the performance of the algal beads for various ROC compositions. This study enhances the insight and decision-making regarding the feasibility of the alginate-immobilised microalgal system for treating municipal wastewater ROC streams.

Microalgal–Bacterial Synergistic Interactions and Their Potential Influence in Wastewater Treatment: a Review

Microalgae, as the most promising raw material for biodiesel, can absorb and transform N, P, and organic matter in wastewater into cell components such as oil, carbohydrate, and protein. The cultivation of microalgae in wastewater provides the possibility to realize harmless treatment and resource utilization of wastewater and reduce the cost of microalgal culture. Whether in a single microalgal culture process or wastewater system, there always be a large number of bacteria interacting with microalgae, forming a complex microecological system. Although microalgal–bacterial coculture system is feasible features for the higher biomass production and better nutrient adaptability from wastewater, the precise understanding of interaction and coexistence in water bodies is still unclear. Some major challenges are to develop sustainability in the microalgal–bacterial coculture system due to the incomplete knowledge about the mechanism of communication, algal growth promotion, and effect of microalgae on the indigenous bacteria in wastewater. In the present review, nutritional interaction and signal transduction, two main interaction models of algae and bacteria, and the effect of synergism of these two models on microalgal system are summarized. Moreover, the impact of algal–bacterial coculture on algal growth and removal of pollutants in wastewater treatment process was expounded systematically to develop highly efficient consortium systems by covering the sustainability limitations. This work would provide guidance for the establishment of efficient algal–bacterial symbiosis system in the wastewater with complex microecology and nutrient environment to improve the accumulation of microalgal biomass and reduce the cost of microalgal culture.

Trade-offs between effluent quality and ammonia volatilisation with CO2 augmented microalgal treatment of anaerobically digested food-waste centrate

Diversion of food waste from landfill disposal to waste-to-energy facilities has become both an environmentally and economically viable option to support the circular bioeconomy. However, the liquid centrate produced during anaerobic digestion is high in total ammonia, with concentrations ~2000 g m−3, and can release gaseous emissions, including ammonia, methane, CO2 and nitrous oxide, to the atmosphere. Further treatment is required before discharge to sewer, or to the environment. Microalgal wastewater treatment systems augmented with CO2 offer a promising and cost-effective treatment solution for reducing both total ammonia concentrations and ammonia volatilisation. In this study, we investigate the effects of augmenting CO2 on nutrient removal and specifically nitrogen losses, as well as biomass productivity under two difference hydraulic retention times (HRT). Both CO2 addition and HRT affect nitrogen losses, with the percentage removal of total ammonia significantly lower (p < 0.01) when CO2 was added to the treatments, while increased HRT significantly increased (p < 0.05) total ammonia percentage removal. Total nitrogen budgets showed significantly lower (p < 0.01) abiotic nitrogen losses from the system when CO2 was added to the culture but at the expense of effluent quality. Both total suspended solids and volatile suspended solids significantly increased (p < 0.01) under longer HRT (8 days), with CO2 addition, while chlorophyll-a biomass significantly increased (p < 0.01) on longer HRT, regardless of CO2 addition. These results demonstrate that, while CO2 augmentation helped to mitigate ammonia losses to atmosphere, the trade-off was poorer effluent quality. Coupling CO2 augmentation with longer HRT increased biomass production and nutrient removal efficiency. This study provides an insight into how simple operational changes can alleviate some of the trade-offs between atmospheric losses and effluent quality. However, in order to manage the trade-off between reduced atmospheric losses and poorer effluent quality, further optimisation of the operation of the microalgal system treating food-waste centrate is required.

Exogenous Abscisic Acid Supplementation at Early Stationary Growth Phase Triggers Changes in the Regulation of Fatty Acid Biosynthesis in Chlorella vulgaris UMT-M1.

Abscisic acid (ABA) has been known to exist in a microalgal system and serves as one of the chemical stimuli in various biological pathways. Nonetheless, the involvement of ABA in fatty acid biosynthesis, particularly at the transcription level in microalgae is poorly understood. The objective of this study was to determine the effects of exogenous ABA on growth, total oil content, fatty acid composition, and the expression level of beta ketoacyl-ACP synthase I (KAS I) and omega-3 fatty acid desaturase (ω-3 FAD) genes in Chlorella vulgaris UMT-M1. ABA was applied to early stationary C. vulgaris cultures at concentrations of 0, 10, 20, and 80μM for 48h. The results showed that ABA significantly increased biomass production and total oil content. The increment of palmitic (C16:0) and stearic (C18:0) acids was coupled by decrement in linoleic (C18:2) and α-linolenic (C18:3n3) acids. Both KAS I and ω-3 FAD gene expression were downregulated, which was negatively correlated to saturated fatty acid (SFAs), but positively correlated to polyunsaturated fatty acid (PUFA) accumulations. Further analysis of both KAS I and ω-3 FAD promoters revealed the presence of multiple ABA-responsive elements (ABREs) in addition to other phytohormone-responsive elements. However, the role of these phytohormone-responsive elements in regulating KAS I and ω-3 FAD gene expression still remains elusive. This revelation might suggest that phytohormone-responsive gene regulation in C. vulgaris and microalgae as a whole might diverge from higher plants which deserve further scientific research to elucidate its functional roles.