Microalgae as a solution of third world energy crisis for biofuels production from wastewater toward carbon neutrality: An updated review

Synergistic enhancement of pulsed light-induced H2 photoproduction in Chlamydomonas cells by optimal sulfite concentration and light waveform

Pulsed light illumination stands out as a noteworthy technique for photosynthetic H2 production, playing a crucial role in eliminating O2 and activating hydrogenase enzymes. However, further improvements are essential to make H2 photoproduction suitable for future commercial applications. In our study, we observed a distinct enhancement in pulsed light-induced H2 photoproduction in the unicellular green alga Chlamydomonas reinhardtii when treated with the optimal concentration of the mild O2 scavenger Na2SO3. This improvement was a result of reduced O2 content, increased hydrogenase enzyme activity, and suppressed H2-uptake activity. Furthermore, our findings indicate that exposing Na2SO3-treated C. reinhardtii to optimal light waveform continues to significantly boost pulsed light-induced H2 photoproduction, attributed to the alleviation of impaired photosystem II activity. Altogether, the combined application of optimal sulfite concentration and light waveform effectively enhances pulsed light-induced photosynthetic H2 production in the green alga C. reinhardtii.

Enhancing CO2 fixation by microalgae in a Photobioreactor: Molecular mechanisms with exogenous carbonic anhydrase

Microalgae biotechnology holds great potential for mitigating CO2 emissions, yet faces challenges in commercialization due to suboptimal photosynthetic efficiency. This study presents an innovative approach to improve CO2 mass transfer efficiency in microalgae using carbonic anhydrase (CA) in an internal LED flexible air-lift photobioreactor. Optimal conditions initial inoculation with 3.55 × 106 cells/mL and 20 % CO2 concentration, complemented by white LED lighting in Chlorella sp. CA regulated intracellular composition, enhancing chlorophyll, lipid, and protein contents. Metabolomics revealed elevated malic and succinic acids, associated with increased Ribulose 1,5-bisphosphate carboxylase oxygenase (RuBisCO) and Acetoacetyl coenzyme A (Acetyl-CoA) activities, facilitating efficient carbon fixation. CA also mitigated cellular oxidative stress by reducing reactive oxygen species (ROS). Furthermore, CA improved extracellular electron acceptor with currents surpassed CK. This CA-based microalgae biotechnology provides a foundation for future commercial applications, addressing CO2 emissions.

From Nature to Technology: Exploring the Potential of Plant-Based Materials and Modified Plants in Biomimetics, Bionics, and Green Innovations

This review explores the extensive applications of plants in areas of biomimetics and bioinspiration, highlighting their role in developing sustainable solutions across various fields such as medicine, materials science, and environmental technology. Plants not only serve essential ecological functions but also provide a rich source of inspiration for innovations in green nanotechnology, biomedicine, and architecture. In the past decade, the focus has shifted towards utilizing plant-based and vegetal waste materials in creating eco-friendly and cost-effective materials with remarkable properties. These materials are employed in making advancements in drug delivery, environmental remediation, and the production of renewable energy. Specifically, the review discusses the use of (nano)bionic plants capable of detecting explosives and environmental contaminants, underscoring their potential in improving quality of life and even in lifesaving applications. The work also refers to the architectural inspirations drawn from the plant world to develop novel design concepts that are both functional and aesthetic. It elaborates on how engineered plants and vegetal waste have been transformed into value-added materials through innovative applications, especially highlighting their roles in wastewater treatment and as electronic components. Moreover, the integration of plants in the synthesis of biocompatible materials for medical applications such as tissue engineering scaffolds and artificial muscles demonstrates their versatility and capacity to replace more traditional synthetic materials, aligning with global sustainability goals. This paper provides a comprehensive overview of the current and potential uses of living plants in technological advancements, advocating for a deeper exploration of vegetal materials to address pressing environmental and technological challenges.

Ensuring carbon neutrality via algae-based wastewater treatment systems: Progress and future perspectives

The emergence of algal biorefineries has garnered considerable attention to researchers owing to their potential to ensure carbon neutrality via mitigation of atmospheric greenhouse gases. Algae-derived biofuels, characterized by their carbon-neutral nature, stand poised to play a pivotal role in advancing sustainable development initiatives aimed at enhancing environmental and societal well-being. In this context, algae-based wastewater treatment systems are greatly appreciated for their efficacy in nutrient removal and simultaneous bioenergy generation. These systems leverage the growth of algae species on wastewater nutrients-including carbon, nitrogen, and phosphorus-alongside carbon dioxide, thus facilitating a multifaceted approach to pollution remediation. This review seeks to delve into the realization of carbon neutrality through algae-mediated wastewater treatment approaches. Through a comprehensive analysis, this review scrutinizes the trajectory of algae-based wastewater treatment via bibliometric analysis. It subsequently examines the case studies and empirical insights pertaining to algae cultivation, treatment performance analysis, cost and life cycle analyses, and the implementation of optimization methodologies rooted in artificial intelligence and machine learning algorithms for algae-based wastewater treatment systems. By synthesizing these diverse perspectives, this study aims to offer valuable insights for the development of future engineering applications predicated on an in-depth understanding of carbon neutrality within the framework of circular economy paradigms.

Toxicological effects and mechanisms of lithium on growth, photosynthesis and antioxidant system in the freshwater microalga Chromochloris zofingiensis

The growing prevalence of lithium (Li) batteries has drawn public attention to Li as an emerging pollutant. The present study investigates the toxicity of Li+ on Chromochloris zofingiensis, examining physiological, biochemical and omics aspects. Results reveal hormesis effects of Li+ on C. zofingiensis growth. At Li+ concentrations below 5 mg L-1, Li+ can enhance chlorophyll content, mitochondrial activity, and antioxidant capacity, leading to increased dry cell weight and cell number. Conversely, when it exceeded 10 mg L-1, Li+ can reduce chlorophyll content, induce oxidative stress, and disrupt chloroplast and mitochondria structure and function, ultimately impeding cell growth. In addition, under 50 mg L-1 Li+ stress, microalgae optimize absorbed light energy use (increasing Fv/Fm and E TR ) and respond to stress by up-regulating genes in starch and lipid biosynthesis pathways, promoting the accumulation of storage components. Weighted gene co-expression network analysis indicates that peptidylprolyl cis/trans isomerase, GTPase and L-ascorbate oxidase might be the key regulators in response to Li+ stress. This research marks the toxic effects and molecular mechanisms of Li+ on freshwater microalga, which would improve our understanding of Li's toxicology and contributing to the establishment of Li pollution standards.

Dual disintegration of microalgae biomass for cost-effective biomethane production: Energy and cost assessment

The present study aims to enhance the biomethane production potential of microalgae via a dual disintegration process. During this process, the microalgae biomass was firstly subjected to cell wall weakening by thermochemical disintegration (TC) (50 to 80 °C), pH adjustment with alkali, NaOH (6 to 10) and time (0 to 10 min) and, secondly, by bacterial disintegration (BD). TC-BD disintegration was comparatively higher (33 %) than BD (24 %), TC (8.5 %), and control (7 %). A more significant VFA accumulation of 2816 mg/L was recorded for TC-BD. Similarly, a greater substrate anaerobic biodegradability was achieved in TC-BD (0.32 g COD /g COD) than BD (0.21 g COD /g COD), TC alone (0.09 gCOD/g COD) and control (0.08 g COD /g COD), respectively. The TC-BD achieves a positive net profit and an energy ratio of + 0.12 GJ/d and 1.03. The proposed dual disintegration has a promising future for commercialization.

Low-Cost Greywater Treatment Using Polyculture Microalgae-Microalgal Growth, Organics, and Nutrient Removal Subject to pH and Temperature Variations During the Treatment

Organics and nutrient removal studies are rarely done using polyculture microalgae, and that too in outdoor conditions, as they are often not deemed effective for wastewater treatment purposes. This study examined the organics and nutrient removal efficiency of polyculture microalgae cultivated in greywater. The reactor was operated in outdoor conditions. Hence, it was subjected to natural pH and temperature variations. A growth rate of 0.05 g L-1 day-1 was observed for temperatures up to 37 °C, beyond which the growth rate declined by 0.07 g L-1 day-1. During the treatment, the pH of the system was observed to be between 7.4 and 8.4. However, the growth rate would again pick up (0.05 g L-1 day-1) when the pH and temperature moved towards the optimum range, indicating that the polycultures adapt very quickly to their environment. The maximum biomass concentration reached 0.82 gL-1. The highest removal efficiency of organic carbon, ammonia, and phosphate was 80.7, 61.9, and 58.4%, respectively. Nitrate and nitrite concentrations remained ≤ 1.3 mgL-1 and ≤ 2 mgL-1, respectively, indicating the absence of nitrification/denitrification and ammonia volatilization. The mass balance of microalgae indicated that the primary removal mechanism of nitrogen and phosphorus removal was assimilation by the microalgae. The study proved polyculture microalgae to be as effective as some monoculture species in wastewater treatment, which require costlier controlled growth conditions. The high organics and nutrient removal by polycultures in outdoor conditions could pave the way to reducing wastewater treatment costs.

Impact of Zero-Valent Iron Nanoparticles and Ampicillin on Adenosine Triphosphate and Lactate Metabolism in the Cyanobacterium Fremyella diplosiphon

In cyanobacteria, the interplay of ATP and lactate dynamics underpins cellular energetics; their pronounced shifts in response to zero-valent iron (nZVI) nanoparticles and ampicillin highlight the nuanced metabolic adaptations to environmental challenges. In this study, we investigated the impact of nZVIs and ampicillin on Fremyella diplosiphon cellular energetics as determined by adenosine triphosphate (ATP) content, intracellular and extracellular lactate levels, and their impact on cell morphology as visualized by transmission electron microscopy. While a significant increase in ATP concentration was observed in 0.8 mg/L ampicillin-treated cells compared to the untreated control, a significant decline was noted in cells treated with 3.2 mg/L nZVIs. ATP levels in the combination regimen of 0.8 mg/L ampicillin and 3.2 mg/L nZVIs were significantly elevated (p < 0.05) compared to the 3.2 mg/L nZVI treatment. Intracellular and extracellular lactate levels were significantly higher in 0.8 mg/L ampicillin, 3.2 mg/L nZVIs, and the combination regimen compared to the untreated control; however, extracellular lactate levels were the highest in cells treated with 3.2 mg/L nZVIs. Visualization of morphological changes indicated increased thylakoid membrane stacks and inter-thylakoidal distances in 3.2 mg/L nZVI-treated cells. Our findings demonstrate a complex interplay of nanoparticle and antibiotic-induced responses, highlighting the differential impact of these stressors on F. diplosiphon metabolism and cellular integrity.

Economic co-production of cellulosic ethanol and microalgal biomass through efficient fixation of fermentation carbon dioxide

An integrated process for the co-production of cellulosic ethanol and microalgal biomass by fixing CO2 generated from bioethanol fermentation is proposed. Specifically, over one-fifth of the fermentative carbon was converted into high-purity CO2 during ethanol production. The optimal concentration of 4 % CO2 was identified for the growth and metabolism of Chlorella sp. BWY-1. A multiple short-term intermittent CO2 supply system was established to efficiently fix and recycle the waste CO2. Using this system, economical co-production of cellulosic ethanol by Zymomonas mobilis and microalgal biomass in biogas slurry wastewater was achieved, resulting in the production of ethanol at a rate of 0.4 g/L/h and a fixed fermentation CO2 of 3.1 g/L/d. Moreover, the amounts of algal biomass and chlorophyll a increased by over 50 % and two-fold, respectively. Through techno-economic analysis, the integrated process demonstrated its cost-effectiveness for cellulosic ethanol production. This study presents an innovative approach to a low-carbon circular bioeconomy.

Effects of environmental microplastic exposure on Chlorella sp. biofilm characteristics and its interaction with nitric oxide signaling

Microalgal biofilm is promising in simultaneous pollutants removal, CO2 fixation, and biomass resource transformation when wastewater is used as culturing medium. Nitric oxide (NO) often accumulates in microalgal cells under wastewater treatment relevant abiotic stresses such as nitrogen deficiency, heavy metals, and antibiotics. However, the influence of emerging contaminants such as microplastics (MPs) on microalgal intracellular NO is still unknown. Moreover, the investigated MPs concentrations among existing studies were mostly several magnitudes higher than in real wastewaters, which could offer limited guidance for the effects of MPs on microalgae at environment-relevant concentrations. Therefore, this study investigated three commonly observed MPs in wastewater at environment-relevant concentrations (10-10,000 μg/L) and explored their impacts on attached Chlorella sp. growth characteristics, nutrients removal, and anti-oxidative responses (including intracellular NO content). The nitrogen source NO3--N at 49 mg/L being 20 % of the nitrogen strength in classic BG-11 medium was selected for MPs exposure experiments because of least intracellular NO accumulation, so that disturbance of intracellular NO by nitrogen availability could be avoided. Under such condition, 10 μg/L polyethylene (PE) MPs displayed most significant microalgal growth inhibition comparing with polyvinyl chloride (PVC) and polyamide (PA) MPs, showing extraordinarily low chlorophyll a/b ratios, and highest superoxide dismutase (SOD) activity and intracellular NO content after 12 days of MPs exposure. PVC MPs exposed cultures displayed highest malonaldehyde (MDA) content because of the toxic characteristics of organochlorines, and most significant correlations of intracellular NO content with conventional anti-oxidative parameters of SOD, CAT (catalase), and MDA. MPs accelerated phosphorus removal, and the type rather than concentration of MPs displayed higher influences, following the trend of PE > PA > PVC. This study expanded the knowledge of microalgal biofilm under environment-relevant concentrations of MPs, and innovatively discovered the significance of intracellular NO as a more sensitive indicator than conventional anti-oxidative parameters under MPs exposure.

The magic of algae-based biochar: advantages, preparation, and applications

Compared with other biomass sources, the use of algae as a raw material to prepare biochar (BC) has important advantages including safety, high yield and economy. The protein content of algae cells is as high as 3.2 mg DCW/L, and the graphitic-N and N-O functional groups generated by the pyrolysis of proteins could effectively activate free radicals. Combined with the generated pore structure, the electron transfer/exchange capability was enhanced, which is conducive to improving its catalytic performance. Algae as a natural N source, the manuscript analyzed the surface properties and physicochemical properties of algae-based BC, and investigated its degradation effect on organic/inorganic pollutants in wastewater. Subsequently, the effect of nitrogen-doped BC on the adsorption/catalysis capacity was discussed. Finally, the directed preparation of algae-based BC applied in different scenarios was summarized. Algae-based BC has the property of N doping, which broadens its application efficiency in the environmental field. Overall, this manuscript reviews how to achieve efficient utilization of algae-based BC in wastewater.

Solar bioreactors used for the industrial production of microalgae

Microalgae are excellent sources of biomass containing several important compounds for human and animal nutrition-proteins, lipids, polysaccharides, pigments and antioxidants as well as bioactive secondary metabolites. In addition, they have a great biotechnological potential for nutraceuticals, and pharmaceuticals as well as for CO2 sequestration, wastewater treatment, and potentially also biofuel and biopolymer production. In this review, the industrial production of the most frequently used microalgae genera-Arthrospira, Chlorella, Dunaliella, Haematococcus, Nannochloropsis, Phaeodactylum, Porphyridium and several other species is discussed as concerns the applicability of the most widely used large-scale systems, solar bioreactors (SBRs)-open ponds, raceways, cascades, sleeves, columns, flat panels, tubular systems and others. Microalgae culturing is a complex process in which bioreactor operating parameters and physiological variables closely interact. The requirements of the biological system-microalgae culture are crucial to select the suitable type of SBR. When designing a cultivation process, the phototrophic production of microalgae biomass, it is necessary to employ SBRs that are adequately designed, built and operated to satisfy the physiological requirements of the selected microalgae species, considering also local climate. Moreover, scaling up microalgae cultures for commercial production requires qualified staff working out a suitable cultivation regime. KEY POINTS: • Large-scale solar bioreactors designed for microalgae culturing. • Most frequently used microalgae genera for commercial production. • Scale-up requires suitable cultivation conditions and well-elaborated protocols.

Mixotrophy, a more promising culture mode: Multi-faceted elaboration of carbon and energy metabolism mechanisms to optimize microalgae culture

Some mixotrophic microalgae appear to exceed the sum of photoautotrophy and heterotrophy in terms of biomass production. This paper mainly reviews the carbon and energy metabolism of microalgae to reveal the synergistic mechanisms of the mixotrophic mode from multiple aspects. It explains the shortcomings of photoautotrophic and heterotrophic growth, highlighting that the mixotrophic mode is not simply the sum of photoautotrophy and heterotrophy. Specifically, microalgae in mixotrophic mode can be divided into separate parts of photoautotrophic and heterotrophic cultures, and the synergistic parts of photoautotrophic culture enhance aerobic respiration and heterotrophic culture enhance the Calvin cycle. Additionally, this review argues that current deficiencies in mixotrophic culture can be improved by uncovering the synergistic mechanism of the mixotrophic mode, aiming to increase biomass growth and improve quality. This approach will enable the full utilization of advantagesin various fields, and provide research directions for future microalgal culture.

Flocculation of oleaginous green algae with Mortierella alpina fungi

Microalgae are promising sources of valuable bioproducts such as biofuels, food, and nutraceuticals. However, harvesting microalgae is challenging due to their small size and low biomass concentrations. To address this challenge, bio-flocculation of starchless mutants of Chlamydomonas reinhardtii (sta6/sta7) was investigated with Mortierella alpina, an oleaginous fungus with high concentrations of arachidonic acid (ARA). Triacylglycerides (TAG) reached 85 % of total lipids in sta6 and sta7 through a nitrogen regime. Scanning electron microscopy determined cell-wall attachment and extra polymeric substances (EPS) to be responsible for flocculation. An algal-fungal biomass ratio around 1:1 (three membranes) was optimal for bio-flocculation (80-85 % flocculation efficiency in 24 h). Nitrogen-deprived sta6/sta7 were flocculated with strains of M. alpina (NVP17b, NVP47, and NVP153) with aggregates exhibiting fatty acid profiles similar to C. reinhardtii, with ARA (3-10 % of total fatty acids). This study showcases M. alpina as a strong bio-flocculation candidate for microalgae and advances a mechanistic understanding of algal-fungal interaction.

Nutrient removal and biogas upgrade using co-cultivation of Chlorella vulgaris and three different bacteria under various GR24 concentrations by induction with 5-deoxystrigol

Algae symbiosis technology shows great potential in the synchronous treatment of biogas slurry and biogas, which has promising applications. For improving nutrients and CO₂ removal rates, the present work constructed four microalgal systems: Chlorella vulgaris (C. vulgaris) monoculture, C. vulgaris-Bacillus licheniformis (B. licheniformis), C. vulgaris-activated sludge, and C. vulgaris-endophytic bacteria (S395-2) to simultaneously treat biogas as well as biogas slurry under GR24 and 5DS induction. Our results showed that the C. vulgaris-endophytic bacteria (S395-2) showed optimal growth performance along with photosynthetic activity under the introduction of GR24 (10^-9 M). Under optimal conditions, CO₂ removal efficiency form biogas, together with chemical oxygen demand, total phosphorus and total nitrogen removal efficiencies from biogas slurry reached 67.25 ± 6.71%, 81.75 ± 7.93%, 83.19 ± 8.32%, and 85.17 ± 8.26%, respectively. The addition of symbiotic bacteria isolated from microalgae can promote the growth of C. vulgaris, and the exogenous addition of GR24 and 5DS can strengthen the purification performance of the algae symbiosis to achieve the maximum removal of conventional pollutants and CO₂.

Potential application of a newly isolated microalga Desmodesmus sp. GXU-A4 for recycling Molasses vinasse

The development of cost-effective and energy-efficient technologies for the stabilization of organic wastewater by microalgae has been essential and sought after. In the current study, GXU-A4 was isolated from an aerobic tank treating molasses vinasse (MV) and identified as Desmodesmus sp. based on its morphology, rbcL, and ITS sequences. It exhibited good growth with a high lipid content and chemical oxygen demand (COD) when grown using MV and the anaerobic digestate of MV (ADMV) as the growth medium. Three distinct COD concentrations for wastewater were established. Accordingly, GXU-A4 removed more than 90% of the COD from molasses vinasse (MV1, MV2, and MV3) with initial COD concentrations of 1193 mgL^-1, 2100 mgL^-1, and 3180 mgL^-1, respectively. MV1 attained the highest COD and color removal rates of 92.48% and 64.63%, respectively, and accumulated 47.32% DW (dry weight) of lipids and 32.62% DW of carbohydrates, respectively. Moreover, GXU-A4 grew rapidly in anaerobic digestate of MV (ADMV1, ADMV2, and ADMV3) with initial COD concentrations of 1433 mgL^-1, 2567 mgL^-1, and 3293 mgL^-1, respectively. Under ADMV3 conditions, the highest biomass reached 13.81 g L^-1 and accumulated 27.43% DW of lipids and 38.70% DW of carbohydrates, respectively. Meanwhile, the removal rates of NH₄-N and chroma in ADMV3 reached 91.10% and 47.89%, respectively, significantly reducing the concentration of ammonia nitrogen and color in ADMV. Thus, the results demonstrate that GXU-A4 has a high fouling tolerance, a rapid growth rate in MV and ADMV, the ability to achieve biomass accumulation and nutrient removal from wastewater, and a high potential for MV recycling.

Production of sustainable biofuels from microalgae with CO2 bio-sequestration and life cycle assessment

Due to anthropogenic emissions, there is an increase in the concentration of carbon dioxide (CO₂) in the atmosphere. Microalgae are versatile, universal, and photosynthetic microorganisms present in nature. Biological CO₂ sequestration using microalgae is a novel concept in CO₂ mitigation strategies. In the current review, the difference between carbon capture and storage (CCS), carbon capture utilization and storage (CCUS), and carbon capture and utilization (CCU) is clarified. The current status of CO₂ sequestration techniques is discussed, including various methods and a comparative analysis of abiotic and biotic sequestration. Particular focus is given to sequestration methods associated with microalgae, including advantages of CO₂ bio-sequestration using microalgae, a summary of microalgae species that tolerate high CO₂ concentrations, biochemistry of microalgal CO₂ biofixation, and elements influencing the microalgal CO₂ sequestration. In addition, this review highlights and summarizes the research efforts made on the production of various biofuels using microalgae. Notably, Chlorella sp. is found to be the most beneficial microalgae, with a sizeable hydrogen (H₂) generation capability ranging from 6.1 to 31.2 mL H₂/g microalgae, as well as the species of C. salina, C. fusca, Parachlorella kessleri, C. homosphaera, C. vacuolate, C. pyrenoidosa, C. sorokiniana, C. lewinii, and C. protothecoides. Lastly, the technical feasibility and life cycle analysis are analyzed. This comprehensive review will pave the way for promoting more aggressive research on microalgae-based CO₂ sequestration.

Emerging microalgae-based biofuels: Technology, life-cycle and scale-up

Microalgae biomass is a versatile feedstock with a variable composition that can be submitted to several conversion routes. Considering the increasing energy demand and the context of third-generation biofuels, algae can fulfill the increasing global demand for energy with the additional benefit of environmental impact mitigation. While biodiesel and biogas are widely consolidated and reviewed, emerging algal-based biofuels such as biohydrogen, biokerosene, and biomethane are cutting-edge technologies in earlier stages of development. In this context, the present study covers their theoretical and practical conversion technologies, environmental hotspots, and cost-effectiveness. Scaling-up considerations are also addressed, mainly through Life Cycle Assessment results and interpretation. Discussions on the current literature for each biofuel directs researchers towards challenges such as optimized pretreatment methods for biohydrogen and optimized catalyst for biokerosene, besides encouraging pilot and industrial scale studies for all biofuels. While presenting studies for larger scales, biomethane still needs continuous operation results to consolidate the technology further. Additionally, environmental improvements on all three routes are discussed in light of life-cycle models, highlighting the ample research opportunities on wastewater-grown microalgae biomass.

Scenedesmus sp. strain SD07 cultivation in municipal wastewater for pollutant removal and production of lipid and exopolysaccharides

In this study, an efficient microalgal strain SD07 was isolated from pond wastewater and identified as Scenedesmus sp. using the 18S rRNA gene sequence analysis. The strain SD07 was grown in a variety of concentrations (25-100%) of municipal wastewater. Scenedesmus sp. strain SD07 grown in 75% diluted wastewater produced a higher amount of biomass (1.93 ± 0.10 g L^-1), and removal of chemical oxygen demand (COD), ammonium (NH₄⁺), total nitrogen (TN) and total phosphate (TP) by 91.36%, 88.41%, 93.26% and 96.32%, respectively from wastewater. The harvested strain SD07 biomass has protein, carbohydrate and lipid contents of 35%, 20.4% and 33%, respectively. Fatty acid profiles revealed that the strain SD07 lipids mainly consist of palmitic acid (40.5%), palmitoleic acid (19%), linoleic acid (17%) and oleic acid (13.2%). Furthermore, strain SD07 cultured in 75% diluted wastewater produced 378 mg L^-1 of exopolysaccharides (EPS). The EPS was utilized as a biostimulant in the cultivation of Solanum lycopersicum under salinity stress. In summary, these findings suggest that this Scenedesmus sp. strain SD07 can be employed for wastewater treatment as well as the production of valuable biomass, high-quality algal oil and EPS.

The long overlooked microalgal nitrous oxide emission: Characteristics, mechanisms, and influencing factors in microalgae-based wastewater treatment scenarios

Microalgae-based wastewater treatment is particularly advantageous in simultaneous CO₂ sequestration and nutrients recovery, and has received increasing recognition and attention in the global context of synergistic pollutants and carbon reduction. However, the fact that microalgae themselves can generate the potent greenhouse gas nitrous oxide (N₂O) has been long overlooked, most previous research mainly regarded microalgae as labile organic carbon source or oxygenic approach that interfere bacterial nitrification-denitrification and the concomitant N₂O production. This study, therefore, summarized the amount and rate of N₂O emission in microalgae-based systems, interpreted in-depth the multiple pathways that lead to NO formation as the key precursor of N₂O, and the pathways that transform NO into N₂O. Reduction of nitrite could take place in either the cytoplasm or the mitochondria to form NO by a series of enzymes, while the NO could be enzymatically reduced to N₂O at the chloroplasts or the mitochondria respectively under light and dark conditions. The influences of abiotic factors on microalgal N₂O emission were analyzed, including nitrogen types and concentrations that directly affect the nitrogen transformation routes, illumination and oxygen conditions that regulate the enzymatic activities related to N₂O generation, and other factors that indirectly interfere N₂O emission via NO regulation. The uncertainty of microalgae-based N₂O emission in wastewater treatment scenarios were emphasized, which would be particularly impacted by the complex competition between microalgae and ammonia oxidizing bacteria or nitrite oxidizing bacteria over ammonium or inorganic carbon source. Future studies should put more efforts in improving the compatibility of N₂O emission results expressions, and adopting consistent NO detection methods for N₂O emission prediction. This review will provide much valuable information on the characteristics and mechanisms of microalgal N₂O emission, and arouse more attention to the non-negligible N₂O emission that may impair overall greenhouse gas reduction efficiency in microalgae-based wastewater treatment systems.

Techno-ecological synergies of hydropower plants: Insights from GHG mitigation

Hydropower is a source of climate-friendly energy; however, its ecological impacts have been criticized. Few studies have considered the greenhouse gas (GHG) emissions resulting from ecosystem restoration. This study proposes a techno-ecological synergy framework based on life cycle assessment (LCA) to evaluate 34 hydropower plants (HPs) in the upper reaches of the Yangtze River from GHG supply and demand side perspectives. Our results show that the demand unit carbon footprint of the 34 HPs ranged from 5.43 to 49.36 g CO₂-eq/kWh, while the imputed GHG emissions from ecosystem restoration occupied 1.22 % to 30.35 %. The unit carbon footprint of large HPs were larger than those of small HPs, and both were positively correlated with the installed capacity of the HPs. All the HPs were unsustainable at the local scale and relied on regional ecosystem supplies. The Sobol' sensitivity analysis and Monte Carlo simulations demonstrated the reliability of our results. Finally, our results were used to consider the related policy implications.

Data-driven analysis on immobilized microalgae system: New upgrading trends for microalgal wastewater treatment

Microalgal immobilization is receiving increasing attention as one of the most viable alternatives for upgrading conventional wastewater treatment. However, an in-depth discussion of the state-of-the-art and limitations of available technologies is currently lacking. More importantly, the reason for the hesitant development of immobilized microalgae for wastewater treatment remains unclear, which hinders its practical application. Thus, comprehensively understanding and evaluating details on immobilized microalgae is urgently needed, especially for the current advances of immobilization of microalgae in wastewater treatment over the last few decades. In this review, scientometric approach is used to explore research hotspots and visualize emerging trends. Data-driven analysis is used to scientifically and methodically determine hotspots in the current research on immobilized microalgal wastewater treatment, along with that the implicit inner connection underlying the frequent co-occurring terms was explored in depth. Four hotspots focusing on immobilized microalgae for wastewater treatment were identified, mainly demonstrating: (1) main factors including light, temperature and immobilization methods would majorly affect the treatment performance of immobilized microalgae; (2) immobilized microalgae membrane bioreactor, immobilized microalgae-based microbial fuel cell and immobilized microalgae-based bed reactor are three dominant treatment systems; (3) immobilized microalgae have a higher robustness and tolerance for treating various types of wastewater; and (4) a complete sustainable circle from wastewater treatment to resource conversion via the immobilized microalgae can be achieved. Finally, several new directions and new perspectives that expose the necessity for fulfilling further research and fundamental gaps are pointed out. Taken together, this review provides helpful information to facilitate the development of innovative and feasible immobilized microalgal technologies thus increasing their viability and sustainability.

Agro-Industrial Wastewaters for Algal Biomass Production, Bio-Based Products, and Biofuels in a Circular Bioeconomy

Recycling bioresources is the only way to sustainably meet a growing world population’s food and energy needs. One of the ways to do so is by using agro-industry wastewater to cultivate microalgae. While the industrial production of microalgae requires large volumes of water, existing agro-industry processes generate large volumes of wastewater with eutrophicating nutrients and organic carbon that must be removed before recycling the water back into the environment. Coupling these two processes can benefit the flourishing microalgal industry, which requires water, and the agro-industry, which could gain extra revenue by converting a waste stream into a bioproduct. Microalgal biomass can be used to produce energy, nutritional biomass, and specialty products. However, there are challenges to establishing stable and circular processes, from microalgae selection and adaptation to pretreating and reclaiming energy from residues. This review discusses the potential of agro-industry residues for microalgal production, with a particular interest in the composition and the use of important primary (raw) and secondary (digestate) effluents generated in large volumes: sugarcane vinasse, palm oil mill effluent, cassava processing waster, abattoir wastewater, dairy processing wastewater, and aquaculture wastewater. It also overviews recent examples of microalgae production in residues and aspects of process integration and possible products, avoiding xenobiotics and heavy metal recycling. As virtually all agro-industries have boilers emitting CO2 that microalgae can use, and many industries could benefit from anaerobic digestion to reclaim energy from the effluents before microalgal cultivation, the use of gaseous effluents is also discussed in the text.

Current perspectives, future challenges and key technologies of biohydrogen production for building a carbon-neutral future: A review

The ever-increasing quantity of greenhouse gases in the atmosphere can be attributed to the rapid increase in the world population as well as the expansion of globalization. Hence, achieving carbon neutrality by 2050 stands as a challenging task to accomplish. Global industrialization had necessitated the need to enhance the current production systems to reduce greenhouse gases emission, whilst promoting the capture of carbon dioxide from atmosphere. Hydrogen is often touted as the fuel of future via substituting fossil-based fuels. In this regard, renewable hydrogen happens to be a niche sector of novel technologies in achieving carbon neutrality. Microalgae-based biohydrogen technologies could be a sustainable and economical approach to produce hydrogen from a renewable source, while simultaneously promoting the absorption of carbon dioxide. This review highlights the current perspectives of biohydrogen production as an alternate source of energy. In addition, future challenges associated with biohydrogen production at large-scale application, storage and transportation are included. Key technologies in producing biohydrogen are finally described in building a carbon-neutral future.

Growth-promoting effects of phytohormones on capillary-driven attached Chlorella sp. biofilm

Using low strength wastewater for microalgae cultivation is challenged by slow growth and biomass harvesting issue in suspended systems, and growth-promoting effects of phytohormones at currently recommended dosages could neither obtain high enough biomass concentrations nor economic feasibility. This study aims to solve the issues of slow growth, biomass harvest, and phytohormone costs altogether by supplementing low dosage phytohormones in an improved capillary-driven attached cultivation device. The device displayed nutrients-condensing properties, and dosages of indole acetic acid (IAA), 6-benzylaminopurine (6-BA), and salicylic acid (SA) for highest microalgal growth were respectively 10-6 M, 10-6 M, and 10-7 M, being at least one order of magnitude lower than in suspended cultures. SA was most effective in growth-promoting (up to 7.0 g/m2 biomass density) and nutrients uptake (up to 98.6 % from the bulk environment), while IAA was most effective in antioxidative defenses. These results provided new insights in cost-effective and harvesting-convenient microalgae production.

Nitrogen recovery from wastewater by microbial assimilation - A review

The increased nitrogen (N) input with low utilization rate in artificial N management has led to massive reactive N (Nr) flows, putting the Earth in a high-risk state. It is essential to recover and recycle Nr during or after Nr removal from wastewater to reduce N input while simultaneously mitigate Nr pollution in addressing the N stress. However, mechanisms for efficient Nr recovery during or after Nr removal remain unclear. Here, the occurrence of N risk and progress in wastewater treatment in recent years as well as challenges of the current technologies for N recovery from wastewater were reviewed. Through analyzing N conversion fluxes in biogeochemical N-cycling networks, microbial N assimilation through photosynthetic and heterotrophic microorganisms was highlighted as promising alternative for synergistic N removal and recovery in wastewater treatment. In addition, the prospects and gaps of Nr recovery from wastewater through microbial assimilation are discussed.

CO2 fixation and cultivation of Thermosynechococcus sp. CL-1 for the production of phycocyanin

Cultivation of Cyanobacteria is preferable for CO2 fixation process due to its efficiency and production of beneficial byproducts like phycocyanin. In this study, Thermosynechococcus sp. CL-1 (TCL-1) was cultivated in a 30 L flat panel photobioreactor using a 3-fold-modified Fitzgerald medium with 113.2 mM dissolved inorganic carbon. The highest CO2 fixation rate of 21.98 ± 1.52 mg/L/h was followed by higher lipid content (49.91 % dry weight content or %dwc) than the generated carbohydrate (24.22 %dwc). TCL-1 also potentially produced phycocyanin that was dominated by C-phycocyanin (98.10 ± 6.67 mg/g) along with a lower amount of allophycocyanin and phycoerythrin under extraction using various types of solvent. Stability of phycocyanin extract was further examined during storage under various temperatures and light illuminations. Extraction with 36 % glucose solvent presented a protective effect to phycocyanin from heat and photo-damage which was proven by the kinetics study of phycocyanin degradation in this study.

Sustainability and carbon neutralization trends in microalgae bioenergy production from wastewater treatment: A review

Reducing CO2 emissions using biomass is gaining popularity as an environmentally friendly strategy. Due to high growth rates, low production costs, and ability to withstand harsh conditions, microalgae have become quite popular. Microalgae may also undertake photosynthesis, converting CO2 and solar energy into sugar before becoming biomass, making them an excellent source of renewable and promising biofuels. CO2 sequestration and biofixation was utilized to compare the synthesis of biodiesel as a third-generation biofuel from various types of wastewater was also used as a source for the algal cultivation. This review article focuses on recent developments, research discoveries in the field of microalgal CO2 capture modification and the optimization of conversion efficiency. This review is intended to serve as a helpful and reference for the use of wastewater treatment with microalgae to collect CO2. The overarching objective of this study is to assist wastewater treatment systems in achieving carbon neutrality.

Bioproducts from microalgae biomass: Technology, sustainability, challenges and opportunities

Microalgae are a potential feedstock for several bioproducts, mainly from its primary and secondary metabolites. Lipids can be converted in high-value polyunsaturated fatty acids (PUFA) such as omega-3, carbohydrates are potential biohydrogen (bioH₂) sources, proteins can be converted into biopolymers (such as bioplastics) and pigments can achieve high concentrations of valuable carotenoids. This work comprehends the current practices for the production of such products from microalgae biomass, with insights on technical performance, environmental and economical sustainability. For each bioproduct, discussion includes insights on bioprocesses, productivity, commercialization, environmental impacts and major challenges. Opportunities for future research, such as wastewater cultivation, arise as environmentally attractive alternatives for sustainable production with high potential for resource recovery and valorization. Still, microalgae biotechnology stands out as an attractive topic for it research and market potential.

Microalgae cultivation strategies using cost-effective nutrient sources: Recent updates and progress towards biofuel production

Scientists are grabbing huge attention as well as consciousness on non-renewable energy sources for the global energy crises because of gradual increase in oil price, fast depletion or low availability of resources, and the release of more toxic-gases (CO₂, SO_x, N_xO) during exhaustion, etc. Due to such hitches, the key need is to find alternative biofuels or feedstocks to replace fossil fuel energy demands worldwide. Currently, microalgae have become intrigued feedstock candidates (3rd generation source of biofuel) to replace nearly 50-60 % of fossil fuels due to high production of biomass and oil, mitigating CO₂ and wastewater remediation. The present work demonstrated the current developments and future perspectives on large-scale algal cultivation strategies for the biorefinery economy. In addition, various advanced cultivation techniques adopted for enhanced biomass production and cost-effective methods for bioenergy production were detailly discussed.

Towards green biomanufacturing of high-value recombinant proteins using promising cell factory: Chlamydomonas reinhardtii chloroplast

Microalgae are cosmopolitan organisms in nature with short life cycles, playing a tremendous role in reducing the pressure of industrial carbon emissions. Besides, microalgae have the unique advantages of being photoautotrophic and harboring both prokaryotic and eukaryotic expression systems, becoming a popular host for recombinant proteins. Currently, numerous advanced molecular tools related to microalgal transgenesis have been explored and established, especially for the model species Chlamydomonas reinhardtii (C. reinhardtii hereafter). The development of genetic tools and the emergence of new strategies further increase the feasibility of developing C. reinhardtii chloroplasts as green factories, and the strong genetic operability of C. reinhardtii endows it with enormous potential as a synthetic biology platform. At present, C. reinhardtii chloroplasts could successfully produce plenty of recombinant proteins, including antigens, antibodies, antimicrobial peptides, protein hormones and enzymes. However, additional techniques and toolkits for chloroplasts need to be developed to achieve efficient and markerless editing of plastid genomes. Mining novel genetic elements and selectable markers will be more intensively studied in the future, and more factors affecting protein expression are urged to be explored. This review focuses on the latest technological progress of selectable markers for Chlamydomonas chloroplast genetic engineering and the factors that affect the efficiency of chloroplast protein expression. Furthermore, urgent challenges and prospects for future development are pointed out.

Visible light-driven carbamoyloxylation of the α-C(sp3)-H bond of arylacetones via radical-initiated hydrogen atom transfer

Photocatalytic synthesis has emerged as an efficient route to transform CO₂ into functionalized organic carbamates by photocatalysis. Herein, a catalyst-free carbamoyloxylation of arylacetones with CO₂ and amines under visible light was developed for the synthesis of O-β-oxoalkyl carbamates in yields up to 93%. This protocol proceeded smoothly with the assistance of inexpensive carbon tetrabromide at room temperature under atmospheric CO₂ pressure, leading to simultaneous construction of C-O and C-N bonds. Mechanism studies suggested the photoinduced hydrogen atom transfer (HAT) pathway followed by radical addition or single electron transfer (SET).

Role of nitrogen transport for efficient energy conversion potential in low carbon and high nitrogen/phosphorus wastewater by microalgal-bacterial system

Microalgal-bacterial system (MBS) is potential biotechnology in wastewater treatment because it can remedy defects of conventional processes (e.g., insufficient carbon source and imbalanced elements ratio). However, the mechanisms of nitrogen (N) transport and removal in MBS are still unclear. In this study, it was discovered that MBS was conducive to adsorb NH₄⁺-N and NO₃^--N through electrical neutralization, while extracellular polymeric substances (EPS) could provide binding sites (e.g., -OH and -CH₃) for enhancing N transport and removal. The microalgae-bacteria interaction could accelerate N transport and removal from aqueous solution to cell. More importantly, the microalgal starch biosynthetic metabolism exhibited demonstrating the energy production potential could be boosted via MBS. Overall, the NO₃^--N and NH₄⁺-N removal efficiencies, and energy yield were 82.28%, 94.15%, and 86.81 kJ/L, respectively, which are better than other relevant studies. Altogether, it is meaningful for revealing the applicability of MBS for treating wastewater and producing energy.

Emerging biological wastewater treatment using microalgal-bacterial granules: A review

Aiming at deepening the understanding of the formation and evolution of emerging microalgal-bacterial granule (MBG)-based wastewater treatment systems, the recent advances regarding the formation processes, transfer phenomena, innovative bioreactors development and wastewater treatment performance of MBG-based systems are comprehensively reviewed in this work. Particularly, the successful establishments of MBG-based systems with various inocula are summarized. Besides, as the indispensable factors for biochemical reactions in MBGs, the light and substrates (organic matters, inorganic nutrients, etc) need to undergo complicated and multi-scale transfer processes before being assimilated by microorganisms within MBGs. Therefore, the involved transfer phenomena and mechanisms in MBG-based bioreactors are critically discussed. Subsequently, some recent advances of MBG-based bioreactors, the application of MBG-based systems in treating various synthetic and real wastewater, and the future development directions are discussed. In short, this review helps in promoting the development of MBG-based systems by presenting current research status and future perspectives.

Could termites be hiding a goldmine of obscure yet promising yeasts for energy crisis solutions based on aromatic wastes? A critical state-of-the-art review

Biodiesel is a renewable fuel that can be produced from a range of organic and renewable feedstock including fresh or vegetable oils, animal fats, and oilseed plants. In recent years, the lignin-based aromatic wastes, such as various aromatic waste polymers from agriculture, or organic dye wastewater from textile industry, have attracted much attention in academia, which can be uniquely selected as a potential renewable feedstock for biodiesel product converted by yeast cell factory technology. This current investigation indicated that the highest percentage of lipid accumulation can be achieved as high as 47.25% by an oleaginous yeast strain, Meyerozyma caribbica SSA1654, isolated from a wood-feeding termite gut system, where its synthetic oil conversion ability can reach up to 0.08 (g/l/h) and the fatty acid composition in yeast cells represents over 95% of total fatty acids that are similar to that of vegetable oils. Clearly, the use of oleaginous yeasts, isolated from wood-feeding termites, for synthesizing lipids from aromatics is a clean, efficient, and competitive path to achieve "a sustainable development" towards biodiesel production. However, the lacking of potent oleaginous yeasts to transform lipids from various aromatics, and an unknown metabolic regulation mechanism presented in the natural oleaginous yeast cells are the fundamental challenge we have to face for a potential cell factory development. Under this scope, this review has proposed a novel concept and approach strategy in utilization of oleaginous yeasts as the cell factory to convert aromatic wastes to lipids as the substrate for biodiesel transformation. Therefore, screening robust oleaginous yeast strain(s) from wood-feeding termite gut system with a set of the desirable specific tolerance characteristics is essential. In addition, to reconstruct a desirable metabolic pathway/network to maximize the lipid transformation and accumulation rate from the aromatic wastes with the applications of various "omics" technologies or a synthetic biology approach, where the work agenda will also include to analyze the genome characteristics, to develop a new base mutation gene editing technology, as well as to clarify the influence of the insertion position of aromatic compounds and other biosynthetic pathways in the industrial chassis genome on the expressional level and genome stability. With these unique designs running with a set of the advanced biotech approaches, a novel metabolic pathway using robust oleaginous yeast developed as a cell factory concept can be potentially constructed, integrated and optimized, suggesting that the hypothesis we proposed in utilizing aromatic wastes as a feedstock towards biodiesel product is technically promising and potentially applicable in the near future.

Evaluation of Euglena gracilis 815 as a New Candidate for Biodiesel Production

Euglena comprises over 200 species, of which Euglena gracilis is a model organism with a relatively high fatty acid content, making it an excellent potential source of biodiesel. This study isolated and characterized a new strain named E. gracilis 815. E. gracilis 815 cells were cultivated under light and dark conditions, with either ethanol or glucose as an external carbon source and an autotrophic medium as control. To achieve maximum active substances within a short period i.e., 6 days, the effects of the light condition and carbon source on the accumulation of bioactive ingredients of E. gracilis 815 were explored, especially fatty acids. In comparison with the industrially used E. gracilis Z strain, E. gracilis 815 exhibited high adaptability to different carbon sources and light conditions, with a comparable biomass and lipid yield. The content and composition of fatty acids of E. gracilis 815 were further determined to assess its potential for biodiesel use. Results suggested that E. gracilis 815 has biodiesel potential under glucose addition in dark culture conditions and could be a promising source for producing unsaturated fatty acids. Therefore, E. gracilis 815 is a candidate for short-chain jet fuel, with prospects for a wide variety of applications.