Plastic particles and fluorescent brightener co-modify Chlorella pyrenoidosa photosynthesis and a machine learning approach predict algae growth.

Global release of plastics exerts various impacts on the ecological cycle, particularly on primary photosynthesis, while the impacts of plastic additives are unknown. As a carrier of fluorescent brightener, plastic particles co-modify Chlorella pyrenoidosa (C. pyrenoidosa) growth and its photosynthetic parameters. In general, adding to the oxidative damage induced by polystyrene, fluorescent brightener-doped polystyrene produces stronger visible light and the amount of negative charge is more likely to cause photodamage in C. pyrenoidosa leading to higher energy dissipation through conditioning than in the control group with a date of ETR (II) inhibition rate of 33 %, Fv/Fm inhibition rate of 8.3 % and Pm inhibition rate of 48.8 %. To elucidate the ecological effect of fluorescent brightener doping in plastic particles, a machine learning method is performed to establish a Gradient Boosting Machine model for predicting the impact of environmental factors on algal growth. Upon validation, the model achieved an average fitting degree of 88 %. Relative concentration of plastic particles and algae claimed the most significant factor by interpretability analysis of the machine learning. Additionally, both Gradient Boosting Machine prediction and experimental results indicate a matching result that plastic additives have an inhibitive effect on algal growth.

Nitrogen starvation leads to TOR kinase-mediated downregulation of fatty acid synthesis in the algae Chlorella sorokiniana and Chlamydomonas reinhardtii.

BACKGROUND: When subject to stress conditions such as nutrient limitation microalgae accumulate triacylglycerol (TAG). Fatty acid, a substrate for TAG synthesis is derived from de novo synthesis or by membrane remodeling. The model industrial alga Chlorellasorokiniana accumulates TAG and other storage compounds under nitrogen (N)-limited growth. Molecular mechanisms underlying these processes are still to be elucidated. RESULT: Previously we used transcriptomics to explore the regulation of TAG synthesis in C. sorokiniana. Surprisingly, our analysis showed that the expression of several key genes encoding enzymes involved in plastidic fatty acid synthesis are significantly repressed. Metabolic labeling with radiolabeled acetate showed that de novo fatty acid synthesis is indeed downregulated under N-limitation. Likewise, inhibition of the Target of Rapamycin kinase (TOR), a key regulator of metabolism and growth, decreased fatty acid synthesis. We compared the changes in proteins and phosphoprotein abundance using a proteomics and phosphoproteomics approach in C. sorokiniana cells under N-limitation or TOR inhibition and found extensive overlap between the N-limited and TOR-inhibited conditions. We also identified changes in the phosphorylation status of TOR complex proteins, TOR-kinase, and RAPTOR, under N-limitation. This indicates that TOR signaling is altered in a nitrogen-dependent manner. We find that TOR-mediated metabolic remodeling of fatty acid synthesis under N-limitation is conserved in the chlorophyte algae Chlorella sorokiniana and Chlamydomonas reinhardtii. CONCLUSION: Our results indicate that under N-limitation there is significant metabolic remodeling, including fatty acid synthesis, mediated by TOR signaling. This process is conserved across chlorophyte algae. Using proteomic and phosphoproteomic analysis, we show that N-limitation affects TOR signaling and this in-turn affects the metabolic status of the cells. This study presents a link between N-limitation, TOR signaling and fatty acid synthesis in green-lineage.

Low-dose ionizing radiation generates a hormetic response to modify lipid metabolism in Chlorella sorokiniana.

Algal biomass is a viable source of chemicals and metabolites for various energy, nutritional, medicinal and agricultural uses. While stresses have commonly been used to induce metabolite accumulation in microalgae in attempts to enhance high-value product yields, this is often very detrimental to growth. Therefore, understanding how to modify metabolism without deleterious consequences is highly beneficial. We demonstrate that low-doses (1-5 Gy) of ionizing radiation in the X-ray range induces a non-toxic, hormetic response in microalgae to promote metabolic activation. We identify specific radiation exposure parameters that give reproducible metabolic responses in Chlorella sorokiniana caused by transcriptional changes. This includes up-regulation of >30 lipid metabolism genes, such as genes encoding an acetyl-CoA carboxylase subunit, phosphatidic acid phosphatase, lysophosphatidic acid acyltransferase, and diacylglycerol acyltransferase. The outcome is an increased lipid yield in stationary phase cultures by 25% in just 24 hours, without any negative effects on cell viability or biomass.

ROS and Ca2+ signaling involved in important lipid changes of Chlorella pyrenoidosa under nitrogen stress conditions.

MAIN CONCLUSION: Nitrogen stress altered important lipid parameters and related genes in Chlorella pyrenoidosa via ROS and Ca2+ signaling. The mutual interference between ROS and Ca2+ signaling was also uncovered. The changed mechanisms of lipid parameters (especially lipid classes and unsaturation of fatty acids) in microalgae are not completely well known under nitrogen stress. Therefore, Chlorella pyrenoidosa was exposed to 0, 0.5, 1 and 1.5 g L-1 NaNO3 for 4 days. Then, the physiological and biochemical changes were measured. It was shown that the total lipid contents, neutral lipid ratios as well as their related genes (accD and DGAT) increased obviously while the polar lipid ratios, degrees of unsaturation as well as their related genes (PGP and desC) decreased significantly in nitrogen stress groups. The obvious correlations supported that gene expressions should be the necessary pathways to regulate the lipid changes in C. pyrenoidosa under nitrogen stress. The changes in ROS and Ca2+ signaling as well as their significant correlations with corresponding genes and lipid parameters were analyzed. The results suggested that ROS and Ca2+ may regulate these gene expressions and lipid changes in C. pyrenoidosa under nitrogen stress conditions. This was verified by the subordinate tests with an ROS inhibitor and calcium reagents. It also uncovered the clues of mutual interference between ROS and Ca2+ signaling. To summarize, this study revealed the signaling pathways of important lipid changes in microalgae under N stress.

Chlorella pyrenoidosa polysaccharides supplementation increases Drosophila melanogaster longevity at high temperature.

Chlorella pyrenoidos polysaccharides (CPPs) are the main active components of Chlorella pyrenoidos. They possess beneficial health properties, such as antioxidant, anti-inflammatory, and immune-enhancing. This study aims to investigate the protective function and mechanism of CPPs against high-temperature stress injury. Results showed that supplementation with 20 mg/mL CPPs significantly extended the lifespan of Drosophila melanogaster under high-temperature stress, improved its motility, and enhanced its resistance to starvation and oxidative stress. These effects were mainly attributed to the activation of Nrf2 signaling and enhanced antioxidant capacity. Additionally, it has been discovered that CPPs supplementation enhanced Drosophila resilience by preventing the disruption of the intestinal barrier and accumulation of reactive oxygen species caused by heat stress. Overall, these studies suggest that CPPs could be a useful natural therapy for preventing heat stress-induced injury.

Comprehensive Analysis of Biomass from Chlorella sorokiniana Cultivated with Industrial Flue Gas as the Carbon Source.

Chlorella sorokiniana, isolated from a pond adjacent to a cement plant, was cultured using flue gas collected directly from kiln emissions using 20 L and 25000 L photobioreactors. Lipids, proteins, and polysaccharides were analyzed to understand their overall composition for potential applications. The lipid content ranged from 17.97% to 21.54% of the dry biomass, with carotenoid concentrations between 8.4 and 9.2 mg/g. Lutein accounted for 55% of the total carotenoids. LC/MS analysis led to the identification of 71 intact triacylglycerols, 8 lysophosphatidylcholines, 10 phosphatidylcholines, 9 monogalactosyldiacylglycerols, 12 digalactosyldiacylglycerols, and 1 sulfoquinovosyl diacylglycerol. Palmitic acid, oleic acid, linoleic acid, and α-linolenic acid were the main fatty acids. Polyunsaturated fatty acid covers ≥ 56% of total fatty acids. Protein isolates and polysaccharides were also extracted. Protein purity was determined to be ≥75% by amino acid analysis, with all essential amino acids present. Monomer analysis of polysaccharides suggested that they are composed of mainly D-(+)-mannose, D-(+)-galactose, and D-(+)-glucose. The results demonstrate that there is no adverse effect on the metabolite profile of C. sorokiniana biomass cultured using flue gas as the primary carbon source, revealing the possibility of utilizing such algal biomass in industrial applications such as animal feed, sources of cosmeceuticals, and as biofuel.

A new microalgal negative carbon technology for landfill leachate treatment: Simultaneous removal of nitrogen and phosphorus.

Replete with ammonia nitrogen and organic pollutants, landfill leachate typically undergoes treatment employing expensive and carbon-intensive integrated techniques. We propose a novel microalgae technology for efficient, low-carbon simultaneous treatment of carbon, nitrogen, and phosphorus in landfill leachate (LL). The microbial composition comprises a mixed microalgae culture with Chlorella accounting for 82.58%. After seven days, the process with an N/P ratio of approximately 14:1 removed 98.81% of NH4+-N, 88.62 % of TN, and 99.55% of TP. Notably, the concentrations of NH4+-N and TP met the discharge standards, while the removal rate of NH4+-N was nearly three times higher than previously reported in relevant studies. The microalgae achieved a removal efficiency of 64.27% for Total Organic Carbon (TOC) and 99.26% for Inorganic Carbon (IC) under mixotrophic cultivation, yielding a biomass of 1.18 g/L. The treatment process employed in this study results in a carbon emissions equivalent of -8.25 kgCO2/kgNremoved, representing a reduction of 33.56 kgCO2 compared to the 2AO + MBR process. In addition, shake flask experiments were conducted to evaluate the biodegradability of leachate after microalgae treatment. After microalgae treatment, the TOCB (Biodegradable Total Organic Carbon)/TOC ratio decreased from 56.54% to 27.71%, with no significant improvement in biodegradability. It establishes a fundamental foundation for further applied research in microalgae treatment of leachate.

Winery wastewater treatment by microalgae Chlorella sorokiniana and characterization of the produced biomass for value-added products.

The microalgae Chlorella sorokiniana was used for the treatment of winery wastewater (WWW). Batch experiments were initially conducted to investigate how biomass acclimatization in different media, dilution of wastewater, and addition of ammonium nitrogen (NH4-N) affect the growth of microalgae and the removal of major pollutants. Afterwards, two sequencing batch reactor (SBR) systems were tested applying different configurations and hydraulic retention times. The biomass collected at the end of the experiments was characterized for proteins, lipids, carbohydrates, amino acid profile, and the existence of lutein, ß-carotene, chlorophyll a, and tocopherols. Batch experiments showed that Chlorella sorokiniana acclimatization to urban wastewater enhanced the removal of NH4-N and total phosphorus (TP). The operation of a two-stage SBR system achieved COD and NH4-N removal equal to 85 ± 9% and 91 ± 20%, respectively, while the use of a single-stage system feeding with anaerobically pretreated WWW resulted to COD and NH4-N removal of 78 ± 9% and 95 ± 9%, respectively. Analyses of biomass showed higher protein content (up to 58.8%) in batch experiments with NH4-N addition as well as in SBR experiments. The cultivation of microalgae under SBR conditions enhanced the production of pigments and tocopherols. The maximum concentrations of 1075 mg kg-1, 45.5 mg kg-1, and 131.2 mg kg-1 were achieved for lutein, ß-carotene, and tocopherols, respectively, in the one-stage system. Our findings suggested that Chlorella sorokiniana cultivation in WWW not only removed nutrients from WWW but also could potentially serve for the production of value-added ingredients used in food industry, cosmetics, and animal feedstock.

Enhancing aggregation of microalgae on polystyrene microplastics by high light: Processes, drivers, and environmental risk assessment.

Microplastics (MPs) are emerging pollutants, causing potential threats to aquatic ecosystems and serious concern in aggregating with microalgae (critical primary producers). When entering water bodies, MPs are expected to sink below the water surface and disperse into varying water compartments with different light intensities. However, how light influences the aggregation processes of algal cells onto MPs and the associated molecular coupling mechanisms and derivative risks remain poorly understood. Herein, we investigated the aggregation behavior between polystyrene microplastics (mPS, 10 µm) and Chlorella pyrenoidosa under low (LL, 15 µmol·m-2·s-1), normal (NL, 55 µmol·m-2·s-1), and high light (HL, 150 µmol·m-2·s-1) conditions from integrated in vivo and in silico assays. The results indicated that under LL, the mPS particles primarily existed independently, whereas under NL and HL, C. pyrenoidosa tightly bounded to mPS by secreting more protein-rich extracellular polymeric substances. Infrared spectroscopy analysis and density functional theory calculation revealed that the aggregation formation was driven by non-covalent interaction involving van der Waals force and hydrogen bond. These processes subsequently enhanced the deposition and adherence capacity of mPS and relieved its phytotoxicity. Overall, our findings advance the practical and theoretical understanding of the ecological impacts of MPs in complex aquatic environments.

Characterization of Crystals in Ciliate Paramecium bursaria Harboring Endosymbiotic Chlorella variabilis.

Protists, including ciliates retain crystals in their cytoplasm. However, their functions and properties remain unclear. To comparatively analyze the crystals of Paramecium bursaria, a ciliate, associated with and without the endosymbiotic Chlorella variabilis, we investigated the isolated crystals using a light microscope and analyzed their length and solubility. A negligible number of crystals was found in P. bursaria cells harboring symbiotic algae. The average crystal length in alga-free and algae-reduced cells was about 6.8 µm and 14.4 µm, respectively. The crystals of alga-free cells were spherical, whereas those of algae-reduced cells were angular in shape. The crystals of alga-free cells immediately dissolved in acids and bases, but not in water or organic solvents, and were stable at - 20 °C for more than 3 weeks. This study, for the first time, reveals that the characteristics of crystals present in the cytoplasm of P. bursaria vary greatly depending on the amount of symbiotic algae.

From biomass to Energy: Investigating Chlorella pyrenoidosa's potential for fuel and carbon materials.

This study investigated the bioenergy potential of Chlorella pyrenoidosa (CP) for use as fuel and carbon material through chemical and thermal characterization. The thermo-kinetic characteristics of Chlorella pyrenoidosa were assessed using isoconversional, linear regression, and non-linear regression approaches. The physicochemical analysis revealed high carbon (53.1 %), volatile (69.35 %), and low moisture (2.19 %), ash content (3.42 %). The results indicated that the non-linear model fitting method was the most accurate with the approximated activation energy (Eα) and pre-exponential Arrhenius constant (Ln A) were 124.92 ± 2.74 kJ/mol and 23.38 ± 4.63 min-1, respectively. Additionally, the inclusion of sodium bicarbonate resulted in a significant increase in BET surface area. FTIR analysis revealed several functional groups beneficial for carbon material, while XRD analysis showed a broad peak correlated with an amorphous structure. This study highlighted the potential of Chlorella pyrenoidosa biomass for various applications, including carbon material and renewable fuel.

Zero-waste biorefining co-products from ultrasonically assisted deep eutectic solvent-pretreated Chlorella biomass: Sustainable production of biodiesel and bio-fertilizer.

Microalgal biomass is gaining increasing attention to produce high-value co-products. This study proposes integrating Chlorella microalgal biomass into a zero-waste biorefining system, aiming to produce biodiesel and biofertilizer. It investigates optimal conditions for ultrasound-assisted deep eutectic solvent (DES) pretreatment and lipid recovery to enhance the extraction of lipids. Optimal DES pretreatment was identified as a 1.6:1 acetic acid-to-choline chloride molar ratio, 0.36 g biomass loading, and 2.50 min of pretreatment. Lipid recovery succeeded with a 10-minute extraction time and a 1:3 methanol-to-butanol volume ratio. These conditions yielded biodiesel-quality lipids at 139.52 mg/g microalgal biomass with superior fuel characteristics. The de-oiled microalgal biomass residue exhibited promise as a lettuce biofertilizer, enhancing photosynthetic pigments but potentially reducing yields by 40 %. The study also notes changes in rhizosphere microbial communities, indicating both stimulatory and inhibitory effects on beneficial microbes. This study has the potential to enhance sustainability in energy, agriculture, and the environment.

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.

High biomass yields of Chlorella protinosa with efficient nitrogen removal from secondary effluent in a membrane photobioreactor.

Further treatment of secondary effluents before their discharge into the receiving water bodies could alleviate water eutrophication. In this study, the Chlorella proteinosa was cultured in a membrane photobioreactor to further remove nitrogen from the secondary effluents. The effect of hydraulic retention time (HRT) on microalgae biomass yields and nutrient removal was studied. The results showed that soluble algal products concentration reduced in the suspension at low HRT, thereby alleviating microalgal growth inhibition. In addition, the lower HRT reduced the nitrogen limitation for Chlorella proteinosa's growth through the phase-out of nitrogen-related functional bacteria. As a result, the productivity for Chlorella proteinosa increased from 6.12 mg/L/day at an HRT of 24 hr to 20.18 mg/L/day at an HRT of 8 hr. The highest removal rates of 19.7 mg/L/day, 23.8 mg/L/day, and 105.4 mg/L/day were achieved at an HRT of 8 hr for total nitrogen (TN), ammonia, and chemical oxygen demand (COD), respectively. However, in terms of removal rate, TN and COD were the largest when HRT is 24 hr, which were 74.5% and 82.6% respectively. The maximum removal rate of ammonia nitrogen was 99.2% when HRT was 8 hr.

Specificity of Photochemical Energy Conversion in Photosystem I from the Green Microalga Chlorella ohadii.

Primary excitation energy transfer and charge separation in photosystem I (PSI) from the extremophile desert green alga Chlorella ohadii grown in low light were studied using broadband femtosecond pump-probe spectroscopy in the spectral range from 400 to 850 nm and in the time range from 50 fs to 500 ps. Photochemical reactions were induced by the excitation into the blue and red edges of the chlorophyll Qy absorption band and compared with similar processes in PSI from the cyanobacterium Synechocystis sp. PCC 6803. When PSI from C. ohadii was excited at 660 nm, the processes of energy redistribution in the light-harvesting antenna complex were observed within a time interval of up to 25 ps, while formation of the stable radical ion pair P700+A1- was kinetically heterogeneous with characteristic times of 25 and 120 ps. When PSI was excited into the red edge of the Qy band at 715 nm, primary charge separation reactions occurred within the time range of 7 ps in half of the complexes. In the remaining complexes, formation of the radical ion pair P700+A1- was limited by the energy transfer and occurred with a characteristic time of 70 ps. Similar photochemical reactions in PSI from Synechocystis 6803 were significantly faster: upon excitation at 680 nm, formation of the primary radical ion pairs occurred with a time of 3 ps in ~30% complexes. Excitation at 720 nm resulted in kinetically unresolvable ultrafast primary charge separation in 50% complexes, and subsequent formation of P700+A1- was observed within 25 ps. The photodynamics of PSI from C. ohadii was noticeably similar to the excitation energy transfer and charge separation in PSI from the microalga Chlamydomonas reinhardtii; however, the dynamics of energy transfer in C. ohadii PSI also included slower components.

Linking the removal of enrofloxacin to the extracellular polymers of microalgae in water bodies: A case study focusing on the shifts in microbial communities.

Microalgae can promote antibiotic removal, which has attracted growing attention. However, its synergistic removal performance with bacteria in antibiotic pollutants is still poorly understood. In this study, firstly, we selected two green algae (Dictyosphaerium sp. and Chlorella sp.) and exposed them to Enrofloxacin (ENR) to observe their extracellular polysaccharides (EPS) concentration dynamic and the removal of antibiotics. Secondly, EPS was extracted and added to in situ lake water (no algae) to investigate its combined effect with bacteria. The results indicate that both Dictyosphaerium sp. and Chlorella sp. exhibited high tolerance to ENR stress. When the biomass of microalgae was low, ENR could significantly stimulate algae to produce EPS. The removal rates of Dictyosphaerium sp. and Chlorella sp. were 15.8% and 10.5%, respectively. The addition of EPS can both alter the microbial community structure in the lake water and promote the removal of ENR. The LEfSe analysis showed that there were significant differences in the microbial marker taxa, which promoted the increase of special functional bacteria for decomposing ENR, between the EPS-added group and the control group. The EPS of Dictyosphaerium sp. increased the abundance of Moraxellaceae and Spirosomaceae, while the EPS of Chlorella sp. increased the abundance of Sphingomonadaceae and Microbacteriaceae. Under the synergistic effect, Chlorella sp. achieved a maximum removal rate of 24.2%, while Dictyosphaerium sp. achieved a maximum removal rate of 28.9%. Our study provides new insights into the removal performance and mechanism of antibiotics by freshwater microalgae in water bodies and contribute to the development of more effective water treatment strategies.

The Impact of Calcium Depletion on Proliferation of Chlorella sorokiniana Strain DSCG150.

This study analyzed the effects of Ca2+ metal ions among culture medium components on the Chlorella sorokiniana strain DSCG150 strain cell growth. The C. sorokiniana strain DSCG150 grew based on a multiple fission cell cycle and growth became stagnant in the absence of metal ions in the medium, particularly Ca2+. Flow cytometry and confocal microscopic image analysis results showed that in the absence of Ca2+, cell growth became stagnant as the cells accumulated into four autospores and could not transform into daughter cells. Genetic analysis showed that the absence of Ca2+ caused upregulation of calmodulin (calA) and cell division control protein 2 (CDC2_1) genes, and downregulation of origin of replication complex subunit 6 (ORC6) and dual specificity protein phosphatase CDC14A (CDC14A) genes. Analysis of gene expression patterns by qRT-PCR showed that the absence of Ca2+ did not affect cell cycle progression up to 4n autospore, but it inhibited Chlorella cell fission (liberation of autospores). The addition of Ca2+ to cells cultivated in the absence of Ca2+ resulted in an increase in n cell population, leading to the resumption of C. sorokiniana growth. These findings suggest that Ca2+ plays a crucial role in the fission process in Chlorella.

Microalgae cultivation trials in a membrane bioreactor operated in heterotrophic, mixotrophic, and phototrophic modes using ammonium-rich wastewater: The study of fouling.

In this work, microalgae cultivation trials were carried out in a membrane bioreactor to investigate fouling when the cultures of Chlorellavulgaris were grown under mixotrophic, heterotrophic, and phototrophic cultivation regimes. The Chlorella cultures were cultivated in wastewater as a source of nutrients that contained a high concentration of ammonium. In mixotrophic cultivation trials, the results showed that the elevated contents of carbohydrates in the soluble microbial product and proteins in extracellular polymeric substances probably initiated membrane fouling. In this case, the highest protein content was also found in extracellular polymeric substances due to the high nitrogen removal rate. Consequently, transmembrane pressure significantly increased compared to the phototrophic and heterotrophic regimes. The data indicated that cake resistance was the main cause of fouling in all cultivations. Higher protein content in the cake layer made the membrane surface more hydrophobic, while carbohydrates had the opposite effect. Compared to a mixotrophic culture, a phototrophic culture had a larger cell size and higher hydrophobicity, leading to less membrane fouling. Based on our previous data, the highest ammonia removal rate was reached in the mixotrophic cultures; nevertheless, membrane fouling appeared to be the fundamental problem.

Characterization and potentiality of plant-derived silver nanoparticles for enhancement of biomass and hydrogen production in Chlorella sp. under nitrogen deprived condition.

Energy is a crucial entity for the development and it has various alternative forms of energy sources. Recently, the synthesis of nanoparticles using benign biocatalyst has attracted increased attention. In this study, silver nanoparticles were synthesized and characterized using Azadirachta indica plant-derived phytochemical as the reducing agent. Biomass of the microalga Chlorella sp. cultivated in BG11 medium increased after exposure to low concentrations of up to 0.48 mg L-1 AgNPs. In addition, algal cells treated with 0.24 mg L-1 AgNPs and cultivated in BG110 medium which contained no nitrogen source showed the highest hydrogen yield of 10.8 mmol L-1, whereas the untreated cells under the same conditions showed very low hydrogen yield of 0.003 mmol L-1. The enhanced hydrogen production observed in the treated cells was consistent with an increase in hydrogenase activity. Treatment of BG110 grown cells with low concentration of green synthesized AgNPs at 0.24 mg L-1 enhanced hydrogenase activity with a 5-fold increase of enzyme activity compared to untreated BG110 grown cells. In addition, to improve photolytic water splitting efficiency for hydrogen production, cells treated with AgNPs at 0.24 mg L-1 showed highest oxygen evolution signifying improvement in photosynthesis. The silver nanoparticles synthesized using phytochemicals derived from plant enhanced both microalgal biomass and hydrogen production with an added advantage of CO2 reduction which could be achieved due to an increase in biomass. Hence, treating microalgae with nanoparticles provided a promising strategy to reduce the atmospheric carbon dioxide as well as increasing production of hydrogen as clean energy.

Performance evaluation of typical flocculants for efficient harvesting of Chlorella sorokiniana under different carbon application modes.

In this study, the growth characteristics of microalgae cultured with different carbon sources were analyzed, and the flocculation characteristics under the influence of carbon sources were evaluated using three typical flocculants. The results showed that the organic carbon sources could significantly increase the content of extracellular proteins in microalgae. Specifically, the extracellular protein concentrations of microalgae cultured with pure BG-11, ethanol, sodium acetate and glucose were 18.2 29.2, 97.3, and 34.7 mg/g, respectively. During the flocculation process, microalgae cultured with sodium acetate exhibited a weak response to the flocculant because of excessive extracellular proteins inhibited flocculation. In addition, the flocculation efficiency was also less than 50.0% cultured with sodium acetate in all pH test ranges when alum and chitosan were used as flocculants. It could be inferred that the flocculant initially happened to charge neutralization with the negatively charged proteins in the solution and then bridged the charges with the microalgae. These findings provide insights into the effects of different carbon sources on microalgal flocculation, promising organic integration of microalgae wastewater treatment and harvesting.