Salinity induced oxidative stress enhanced biofuel production potential of microalgae Scenedesmus sp. CCNM 1077

The potential of Pavlovophyceae species as a source of valuable carotenoids and polyunsaturated fatty acids for human consumption

Microalgae are a group of microorganisms, mostly photoautotrophs with high CO2 fixation capacity, that have gained increased attention in the last decades due to their ability to produce a wide range of valuable metabolites, such as carotenoids and polyunsaturated fatty acids, for application in food/feed, pharmaceutical, and cosmeceutical industries. Their increasing relevance has highlighted the importance of identifying and culturing new bioactive-rich microalgae species, as well as of a thorough understanding of the growth conditions to optimize the biomass production and master the biochemical composition according to the desired application. Thus, this review intends to describe the main cell processes behind the production of carotenoids and polyunsaturated fatty acids, in order to understand the possible main triggers responsible for the accumulation of those biocompounds. Their economic value and the biological relevance for human consumption are also summarized. In addition, an extensive review of the impact of culture conditions on microalgae growth performance and their biochemical composition is presented, focusing mainly on the studies involving Pavlovophyceae species. A complementary description of the biochemical composition of these microalgae is also presented, highlighting their potential applications as a promising bioresource of compounds for large-scale production and human and animal consumption.

Enhancing bioremediation potential of microalgae Chlorella vulgaris and Scenedesmus acutus by NaCl for pyrene degradation

Microalgae are increasingly recognized as promising organisms for bioremediation of organic pollutants. This study investigates the potential of enhancing the bioremediation efficiency of pyrene (PYR), a polycyclic aromatic hydrocarbon (PAH), through NaCl induced physiological and biochemical alterations in two microalgae species, Chlorella vulgaris and Scenedesmus acutus. Our findings reveal significant improvement in PYR removal when these microalgae were cultivated in the presence of 0.1% NaCl where PYR removal increased from 54 to 74% for C. vulgaris and from 26 to 75% for S. acutus. However, it was observed that NaCl induced stress had varying effects on the two species. While C. vulgaris exhibited increased PYR removal, it experienced reduced growth and biomass production, as well as lower photosynthetic efficiency when exposed to PYR and PYR + NaCl. In contrast, S. acutus displayed better growth and biomass accumulation under PYR + NaCl conditions, making it a more efficient candidate for enhancing PYR bioremediation in the presence of NaCl. In addition to assessing growth and biochemical content, we also investigated stress biomarkers, such as lipid peroxidation, polyphenol and proline contents. These findings suggest that S. acutus holds promise as an alternative microalgae species for PYR removal in the presence of NaCl, offering potential advantages in terms of bioremediation efficiency and ecological sustainability. This study highlights the importance of understanding the physiological and biochemical responses of microalgae to environmental stressors, which can be harnessed to optimize bioremediation strategies for the removal of organic pollutants like PYR.

Axenic green microalgae for the treatment of textile effluent and the production of biofuel: a promising sustainable approach

An integrated approach to nutrient recycling utilizing microalgae could provide feasible solutions for both environmental control and energy production. In this study, an axenic microalgae strain, Chlorella sorokiniana ASK25 was evaluated for its potential as a biofuel feedstock and textile wastewater (TWW) treatment. The microalgae isolate was grown on TWW supplemented with different proportions of standard BG-11 medium varying from 0 to 100% (v/v). The results showed that TWW supplemented with 20% (v/v) BG11 medium demonstrated promising results in terms of Chlorella sorokiniana ASK25 biomass (3.80 g L-1), lipid production (1.24 g L-1), nutrients (N/P, > 99%) and pollutant removal (chemical oxygen demand (COD), 99.05%). The COD level dropped by 90% after 4 days of cultivation, from 2,593.33 mg L-1 to 215 mg L-1; however, after day 6, the nitrogen (-NO3-1) and total phosphorus (TP) levels were reduced by more than 95%. The biomass-, total lipid- and carbohydrate- production, after 6 days of cultivation were 3.80 g L-1, 1.24 g L-1, and 1.09 g L-1, respectively, which were 2.15-, 2.95- and 3.30-fold higher than Chlorella sorokiniana ASK25 grown in standard BG-11 medium (control). In addition, as per the theoretical mass balances, 1 tonne biomass of Chlorella sorokiniana ASK25 might yield 294.5 kg of biodiesel and 135.7 kg of bioethanol. Palmitic acid, stearic acid, and oleic acid were the dominant fatty acids found in the Chlorella sorokiniana ASK25 lipid. This study illustrates the potential use of TWW as a microalgae feedstock with reduced nutrient supplementation (20% of TWW). Thus, it can be considered a promising feedstock for economical biofuel production.

Soil health improvement by inoculation of indigenous microalgae in saline soil

Using biological methods to improve saline soils is recognized as an eco-friendly and sustainable way. In this study, two indigenous algae YJ-1 and YJ-2 screened from salinized farmland were inoculated into saline soils with different salinization levels to investigate their potential in enhancing soil health by laboratory microcosm experiment. The results showed that individual inoculation of the two algae quickly resulted in the formation of algal crusts, and the chlorophyll content in the saline soils gradually increased with the incubation time. The soil pH decreased significantly from the initial 8.15-9.45 to 6.97-7.56 after 60-day incubation. The exopolysaccharides secretion and the activities of catalase, sucrase, and urease in saline soils also increased. Microalgal inoculation increased soil organic matter storage, while decreasing the available nutrient contents possibly due to the depletion of microalgal growth. PCA and PCC results identified that microalgal biomass as the predominant variable affecting soil quality. Overall, these data revealed the great potential of microalgae in the amelioration of saline soils, especially in pH reduction and enzyme activity enhancement. This study will provide the theoretical foundation for improving saline soils via algalization.

Biochemical and proteomic response of the freshwater green alga Pseudochlorella pringsheimii to iron and salinity stressors

BACKGROUND

Pseudochlorella pringsheimii (Ppr) is a green unicellular alga rich with chlorophyll, carotenoids, and antioxidants. As a widespread organism, Ppr must face, and adapt to, many environmental stresses and these are becoming more frequent and more extreme under the conditions of climate change. We therefore focused on salinity induced by NaCl and iron (Fe) variation stresses, which are commonly encountered by algae in their natural environment.

RESULTS

The relatively low stress levels improved the biomass, growth rate, and biochemical components of Ppr. In addition, the radical-scavenging activity, reducing power, and chelating activity were stimulated by lower iron concentrations and all NaCl concentrations. We believe that the alga has adapted to the stressors by increasing certain biomolecules such as carotenoids, phenolics, proteins, and carbohydrates. These act as antioxidants and osmoregulators to protect cell membranes and other cellular components from the harmful effects of ions. We have used SDS-PAGE and 2D-PAGE in combination with tandem mass spectrometry to identify responsive proteins in the proteomes of stressed vs. non-stressed Ppr. The results of 2D-PAGE analysis showed a total of 67 differentially expressed proteins, and SDS-PAGE identified 559 peptides corresponding to 77 proteins. Of these, 15, 8, and 17 peptides were uniquely identified only under the control, iron, and salinity treatments, respectively. The peptides were classified into 12 functional categories: energy metabolism (the most notable proteins), carbohydrate metabolism, regulation, photosynthesis, protein synthesis, stress proteins, oxido-reductase proteins, transfer proteins, ribonucleic-associated proteins, hypothetical proteins, and unknown proteins. The number of identified peptides was higher under salinity stress compared to iron stress.

CONCLUSIONS

A proposed mechanism for the adaptation of Ppr to stress is discussed based on the collected data. This data could serve as reference material for algal proteomics and the mechanisms involved in mediating stress tolerance.

Effect of seawater on the biomass composition of Spirulina produced at a pilot-scale

The microalga Arthrospira platensis BEA 005B was produced in 11.4 m3 raceway photobioreactors and a culture medium based on commercial fertilisers and either freshwater or seawater. The biomass productivity of the reactors operated at a fixed dilution rate of 0.3 day-1 decreased from 22.9 g·m-2·day-1 when operated using freshwater to 16.3 g·m-2·day-1 when the biomass was produced using seawater. The protein content of the biomass produced in seawater was lower; however, the content of essential amino acids including valine, leucine and isoleucine was higher. Seawater also triggered the production of carotenoids and altered the synthesis and accumulation of fatty acids. For example, the biomass produced using seawater showed a 319% and 210% higher content of oleic and eicosenoic acid, respectively. The results demonstrate that it is possible to produce the selected microalga using seawater after an adaptation period and that the composition of the produced biomass is suitable for food applications.

Adaptive laboratory evolution empowers lipids and biomass overproduction in Chlorella vulgaris for environmental applications

Microalgal strain improvement with commercial features is needed to generate green biological feedstock to produce lipids for bioenergy. Hence, improving algal strain with enhanced lipid content without hindering cellular physiological parameters is pivotal for commercial applications of microalgae. In this report, we demonstrated the adaptive laboratory evolution (ALE) by hypersaline conditions to improve the algal strains for increasing the lipid overproduction capacity of Chlorella vulgaris for environmental applications. The evolved strains (namely E2 and E2.5) without notable impairment in general physiological parameters were scrutinized after 35 cycles. Conventional gravimetric lipid analysis showed that total lipid accumulation was hiked by 2.2-fold in the ALE strains compared to the parental strains. Confocal observation of algal cells stained with Nile-red showed that the abundance of lipid droplets was higher in the evolved strains without any apparent morphological aberrations. Furthermore, evolved strains displayed notable antioxidant potential than the control cells. Interestingly, carbohydrates and protein content were significantly decreased in the evolved cells, indicating that carbon flux was redirected into lipogenesis in the evolved cells. Altogether, our findings demonstrated a potential and feasible strategy for microalgal strain improvement for simultaneous lipids and biomass hyperaccumulation.

Bioprospecting Microalgae from Sewage Water: Assessment of Biochemicals for Biomass Utilization

Microalgal species from sewage treatment plant were identified by 18S rRNA sequencing and were explored for total lipids, carbohydrate, and protein contents, to serve as a potential candidate for biorefinery. Seven unicellular microalgae were identified as Chlorella sorokiniana, Dictyosphaerium sp., Graesiella emersonii belonging to Chlorellaceae and Scenedesmus sp., Desmodesmus sp., Tetranephris brasiliensis, and Coelastrella sp. belonging to Scenedesmaceae family. Biochemical assessment of all isolates revealed total lipid content from 17.49 ± 1.41 to 47.35 ± 0.61% w/w, total carbohydrate content from 12.82 ± 0.19 to 64.29 ± 0.63% w/w, and total protein content from 8.55 ± 0.19 to 16.65 ± 0.20% w/w. FAME analysis of extracted lipid was found to be rich in Hexadecane (C16:0), Tetradecane (C17:0), Octadecane (C18:0), Eicosane (C20:0), Tetracosane (C24:0), Pentacosane (C25:0) fatty acids, the presence of which makes excellent candidate for biodiesel. Being rich in lipid, microalgae Chlorella sorokiniana, Coelastrella sp., and Scenedesmus sp. have high potential for biofuels. Due to the presence of high protein content, Scenedesmus sp. and Chlorella sorokiniana can serve as food or feed supplement, whereas the high carbohydrate content of Dictyosphaerium sp., Coelastrella sp., and Scenedesmus sp. makes them an ideal candidate for fermentative production of alcohol and organic acids. Chlorella sp. and Scenedesmus sp., being dominant microalgae across all seasons, demonstrate remarkable resilience for their cultivation in sewage water and utilization of biomass in biorefineries.

Impacts of fulvic acid and Cr(VI) on metabolism and chromium removal pathways of green microalgae

Green microalgae are highly efficient and cost-effective in the removal of heavy metals from water. However, dissolved organic matter (DOM), such as fulvic acid (FA), can impact their growth and heavy metal accumulation. Nonetheless, the specific mechanisms underlying these effects remain poorly understood. This study investigated the effects of different FA concentrations on the development, metabolism, and chromium (Cr) enrichment of Chlorella vulgaris, a standard green microalga. The findings revealed that low FA concentrations alleviated Cr-induced stress, stimulated microalgal growth, and enhanced energy conservation by suppressing chlorophyll synthesis. The highest chromium enrichment and reduction rates of 38.73% and 57.95% were observed when FA concentration reached 20 mg/L of total organic carbon (TOC). Furthermore, FA facilitated chromium removal by C. vulgaris through extracellular adsorption. Examination of microalgal cell surface functional groups and ultrastructure indicated that FA increased adsorption site electrons by promoting extracellular polymeric substance (EPS) secretion and enhancing the oxygen content of acidic functional groups. As a result, FA contributed to elevated enrichment and reduction rates of Cr in microalgal cells. These findings provide a theoretical basis for the prevention and control of heavy metal pollution in water environments.

Evaluation of the toxicity of clozapine on the freshwater diatom Navicula sp. using the FTIR spectroscopy

Clozapine is an often prescribed neuroactive pharmaceutical and frequently detected in the aquatic environments. However, its toxicity on low trophic level species (i.e., diatoms) and associated mechanisms are seldom reported. In this study, the toxicity of clozapine on a widely distributed freshwater diatom Navicula sp. was evaluated using the FTIR spectroscopy along with biochemical analyses. The diatoms were exposed to various concentrations of clozapine (0, 0.01, 0.05, 0.10, 0.50, 1.00, 2.00, 5.00 mg/L) for 96 h. The results revealed that clozapine reached up to 392.8 μg/g in the cell wall and 550.4 μg/g within the cells at 5.00 mg/L, suggesting that clozapine could be adsorbed extracellularly and accumulated intracellularly in diatoms. In addition, hormetic effects were displayed on the growth and photosynthetic pigments (chlorophyll a and carotenoid) of Navicula sp., with a promotive effect at concentrations less than 1.00 mg/L while an inhibited effect at concentrations over 2 mg/L. Clozapine induced oxidative stress in Navicula sp., accompanied by decreased levels of total antioxidant capacity (T-AOC) (>0.05 mg/L), in which, the activity of superoxide dismutase (SOD) (at 5.00 mg/L) was increased whereas the activity of catalase (CAT) (>0.05 mg/L) was decreased. Furthermore, FTIR spectroscopic analysis showed that exposure to clozapine resulted in accumulation of lipid peroxidation products, increased sparse β-sheet structures, and altered DNA structures in Navicula sp. This study can facilitate the ecological risk assessment of clozapine in the aquatic ecosystems.

Proline, Cysteine and Branched-Chain Amino Acids in Abiotic Stress Response of Land Plants and Microalgae

Proteinogenic amino acids are the building blocks of protein, and plants synthesize all of them. In addition to their importance in plant growth and development, growing evidence underlines the central role played by amino acids and their derivatives in regulating several pathways involved in biotic and abiotic stress responses. In the present review, we illustrate (i) the role of amino acids as an energy source capable of replacing sugars as electron donors to the mitochondrial electron transport chain and (ii) the role of amino acids as precursors of osmolytes as well as (iii) precursors of secondary metabolites. Among the amino acids involved in drought stress response, proline and cysteine play a special role. Besides the large proline accumulation occurring in response to drought stress, proline can export reducing equivalents to sink tissues and organs, and the production of H2S deriving from the metabolism of cysteine can mediate post-translational modifications that target protein cysteines themselves. Although our general understanding of microalgae stress physiology is still fragmentary, a general overview of how unicellular photosynthetic organisms deal with salt stress is also provided because of the growing interest in microalgae in applied sciences.

The combined toxicity of silver nanoparticles and typical personal care products in diatom Navicula sp

Toxicity of silver nanoparticles (AgNPs) at environmentally relevant concentrations has been received an increasing attention, and their influence on the bioavailability of personal care products has been seldom studied. Here, the toxicity of AgNPs in typical diatom Navicula sp. was explored, and their influence on the bioavailability of typical personal care products such as triclosan (TCS) and galaxolide (HHCB) was also investigated. The underlying toxicity mechanisms were explored using liquid chromatography-mass spectrometry (LC-MS)-based metabolomics. Low concentrations of AgNPs (10 and 50 μg L-1) induced no observable responses of Navicula sp., in terms of growth rate, chlorophyll contents, and malondialdehyde accumulation. Furthermore, low doses of AgNPs could attenuate TCS or HHCB toxicity to Navicula sp., which was mainly attributed to the reduced oxidative stress. Metabolomics revealed that the disruption of DNA or RNA synthesis and instability of cytokinin-like substances may be also the reasons for the toxicity of AgNPs and TCS to Navicula sp. The damaged algal photosynthesis exposed to HHCB may be recovered by AgNPs, and the presence of signal chemicals (dehydrophytosphingosine and cardamonin) also showed a recovered algal growth. These results emphasize the potential of metabolomics to reveal toxicity mechanism, providing a new perspective on the aquatic risk assessment of nanoparticles and emerging organic pollutants.

Salinity-Induced Physiochemical Alterations to Enhance Lipid Content in Oleaginous Microalgae Scenedesmus sp. BHU1 via Two-Stage Cultivation for Biodiesel Feedstock

In the recent past, various microalgae have been considered a renewable energy source for biofuel production, and their amount and extent can be enhanced by applying certain types of stress including salinity. Although microalgae growing under salinity stress result in a higher lipid content, they simultaneously reduce in growth and biomass output. To resolve this issue, the physiochemical changes in microalgae Scenedesmus sp. BHU1 have been assessed through two-stage cultivation. In stage-I, the maximum carbohydrate and lipid contents (39.55 and 34.10%) were found at a 0.4 M NaCl concentration, while in stage-II, the maximum carbohydrate and lipid contents (42.16 and 38.10%) were obtained in the 8-day-old culture. However, under increased salinity, Scenedesmus sp. BHU1 exhibited a decrease in photosynthetic attributes, including Chl-a, Chl-b, Fv/Fm, Y(II), Y(NPQ), NPQ, qP, qL, qN, and ETRmax but increased Y(NO) and carotenoids content. Apart from physiological attributes, osmoprotectants, stress biomarkers, and nonenzymatic antioxidants were also studied to elucidate the role of reactive oxygen species (ROS) facilitated lipid synthesis. Furthermore, elemental and mineral ion analysis of microalgal biomass was performed to evaluate the biomass quality for biofuel and cell homeostasis. Based on fluorometry analysis, we found the maximum neutral lipids in the 8-day-old grown culture at stage-II in Scenedesmus sp. BHU1. Furthermore, the use of Fourier-transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy analyses confirmed the presence of higher levels of hydrocarbons and triacylglycerides (TAGs) composed of saturated fatty acids (SFAs) and monounsaturated fatty acids (MUFAs) in the 8-day-old culture. Therefore, Scenedesmus sp. BHU1 can be a promising microalga for potential biodiesel feedstock.

The Metabolism of Reactive Oxygen Species and Their Effects on Lipid Biosynthesis of Microalgae

Microalgae have outstanding abilities to transform carbon dioxide (CO₂) into useful lipids, which makes them extremely promising as renewable sources for manufacturing beneficial compounds. However, during this process, reactive oxygen species (ROS) can be inevitably formed via electron transfers in basal metabolisms. While the excessive accumulation of ROS can have negative effects, it has been supported that proper accumulation of ROS is essential to these organisms. Recent studies have shown that ROS increases are closely related to total lipid in microalgae under stress conditions. However, the exact mechanism behind this phenomenon remains largely unknown. Therefore, this paper aims to introduce the production and elimination of ROS in microalgae. The roles of ROS in three different signaling pathways for lipid biosynthesis are then reviewed: receptor proteins and phosphatases, as well as redox-sensitive transcription factors. Moreover, the strategies and applications of ROS-induced lipid biosynthesis in microalgae are summarized. Finally, future perspectives in this emerging field are also mentioned, appealing to more researchers to further explore the relative mechanisms. This may contribute to improving lipid accumulation in microalgae.

Impact of dehydration on the physiochemical properties of Nostoc calcicola BOT1 and its untargeted metabolic profiling through UHPLC-HRMS

The global population growth has led to a higher demand for food production, necessitating improvements in agricultural productivity. However, abiotic and biotic stresses pose significant challenges, reducing crop yields and impacting economic and social welfare. Drought, in particular, severely constrains agriculture, resulting in unproductive soil, reduced farmland, and jeopardized food security. Recently, the role of cyanobacteria from soil biocrusts in rehabilitating degraded land has gained attention due to their ability to enhance soil fertility and prevent erosion. The present study focused on Nostoc calcicola BOT1, an aquatic, diazotrophic cyanobacterial strain collected from an agricultural field at Banaras Hindu University, Varanasi, India. The aim was to investigate the effects of different dehydration treatments, specifically air drying (AD) and desiccator drying (DD) at various time intervals, on the physicochemical properties of N. calcicola BOT1. The impact of dehydration was assessed by analyzing the photosynthetic efficiency, pigments, biomolecules (carbohydrates, lipids, proteins, osmoprotectants), stress biomarkers, and non-enzymatic antioxidants. Furthermore, an analysis of the metabolic profiles of 96-hour DD and control mats was conducted using UHPLC-HRMS. Notably, there was a significant decrease in amino acid levels, while phenolic content, fatty acids, and lipids increased. These changes in metabolic activity during dehydration highlighted the presence of metabolite pools that contribute to the physiological and biochemical adjustments of N. calcicola BOT1, mitigating the impact of dehydration to some extent. Overall, present study demonstrated the accumulation of biochemical and non-enzymatic antioxidants in dehydrated mats, which could be utilized to stabilize unfavorable environmental conditions. Additionally, the strain N. calcicola BOT1 holds promise as a biofertilizer for semi-arid regions.

Assessing the potential of Chlorella sp. phycoremediation liquid digestates from brewery wastes mixture integrated with bioproduct production

Digestates from different anaerobic digesters are promising substrates for microalgal culture, leading to effective wastewater treatment and the production of microalgal biomass. However, further detailed research is needed before they can be used on a large scale. The aims of this study were to investigate the culture of Chlorella sp. in Digestate_M from anaerobic fermentation of brewer's grains and brewery wastewater (BWW) and to explore the potential use of the biomass produced under different experimental conditions, including diverse cultivation modes and dilution ratios. Cultivation in Digestate_M initiated from 10% (v/v) loading, with 20% BWW, obtained maximum biomass production, reaching 1.36 g L^-1 that was 0.27g L^-1 higher than 1.09 g L^-1 of BG11. In terms of Digestate_M remediation, the maximum removal of ammonia nitrogen (NH₄ ⁺-N), chemical oxygen demand, total nitrogen, and total phosphorus reached 98.20%, 89.98%, 86.98%, and 71.86%, respectively. The maximum lipid, carbohydrate, and protein contents were 41.60%, 32.44%, and 27.72%, respectively. The growth of Chlorella sp. may be inhibited when the Y(II)-F_v/F_m ratio is less than 0.4.

Influences of changing inorganic nitrogen concentration on accumulation and degradation of organic components in indigenous microalgae cultivated with secondary effluent

Climatic changes due to emission of greenhouse gases are a global concern. These emissions occur by combustion of fossil fuels whose drought is near in which case renewable energy is the only alternative. Microalgae are promising sources of sustainable bioenergy production, and utilisation of wastewater as cultures is recommended for economical production cost. In this study, indigenous microalgae, which had adaptability for wastewater samples, were cultivated with a municipal secondary effluent, and influences of changes in inorganic nitrogen (IN) concentration, specifically IN increase, on temporal accumulation and degradation of organic components in indigenous microalgae were investigated. Indigenous microalgae accumulated total lipids and carbohydrates against reduced IN, and increase in superoxide dismutase suggested that the accumulation was possibly induced by generating reactive oxygen species. Continued cultivation of indigenous microalgae under the IN exhausted condition should be avoided because of the resulting total carbohydrate degradation. IN replenishment when IN was decreased but still existed in the culture and that when IN was exhausted in the culture triggered sharp degradation of the total carbohydrate, which possibly utilised to accumulate crude protein and/or chlorophyll a for continuous growth or regrowth. The total carbohydrate was accumulated and recovered after the degradation; meanwhile, two or three days were required for the recovery of the total carbohydrate. In addition, the IN replenishment also resulted in total lipid degradation. Therefore, to produce indigenous microalgae with high and stable total carbohydrate and lipid content, it was critical to prevent IN increase in the culture.

Biotechnological product potential of Auxenochlorella protothecoides including biologically active compounds (BACs) under nitrogen stress conditions

Nitrogen stress can influence microalgae's growth characteristics, and microalgae grown in nitrogen-deficient conditions may produce higher or lower levels of biotechnological products as a result of metabolic changes. In photoautotrophic and heterotrophic cultures, nitrogen limitation has been proven effective in promoting lipid accumulation. In spite of this, no study has demonstrated a significant correlation between lipid content and other biotechnological products such as bioactive compounds (BACs). This research examines a strategy for lipid accumulation as well as the potential production of BACs with antibacterial properties in parallel with that strategy. This concept involved the treatment of the microalga Auxenochlorella protothecoides with low and high concentrations of ammonium (NH₄⁺). This particular experiment reached a maximum lipid content of 59.5% using a 0.8 mM NH₄⁺ concentration, resulting in the yellowing of the chlorophyll levels. Agar diffusion assays were conducted to determine the antibacterial activity of different extracts derived from the biomass when stressed with different levels of nitrogen. Algal extracts prepared by a variety of solvents showed different levels of antibacterial activity against representative strains of both gram-negative (Escherichia coli) and gram-positive (Staphylococcus aureus) bacteria. Among the extracts tested, 500 mg/L ethyl acetate extract had the greatest antibacterial activity against Escherichia coli. In order to identify the components responsible for the extract's antibacterial activity, fatty acid methyl ester (FAME) analysis was performed. It has been suggested that the lipid fraction may be a valuable indicator of these activities since some lipid components are known to possess antimicrobial properties. In this regard, it was found that the amount of polyunsaturated fatty acid (PUFA) significantly decreased by 53.4% under the conditions with the highest antibacterial activity observed.

Effects of fluidised carriers on the community composition, settleability and energy production of indigenous microalgal consortia cultivated in treated wastewater

Fluidised-bed systems are a promising approach to microalgal cultivation, but few studies have considered their application to indigenous microalgal consortia (IMCs), which have high adaptability to wastewater. In this study, IMCs were cultivated in treated wastewater with and without fluidised carriers, and the effects of operating parameters were considered. Microalgae in the culture were confirmed to originate from the carriers, and the IMC presence on the carriers was promoted by decreasing the carrier replacement number and increasing the culture replacement volume. The presence of carriers enabled greater nutrient removal from the treated wastewater by the cultivated IMCs. Without carriers, IMCs in the culture were scattered and showed poor settleability. With carriers, IMCs in the culture exhibited good settleability owing to floc formation. The improved settleability with carriers also increased the energy production from sedimented IMCs.

Enhanced production of bioactive compounds from marine microalgae Tetraselmis tetrathele under salinity and light stresses: A two-stage cultivation strategy

This study leveraged the salinity and light intensity stresses during the stationary phase for enhancing the pigment contents and antioxidant capacity of Tetraselmis tetrathele. The highest pigments content was obtained in cultures under salinity stress (40 g L^-1) illuminated using fluorescent light. Furthermore, the best inhibitory concentration (IC₅₀) for scavenging the 2, 2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radicals was found as 79.53 µg mL^-1 in ethanol extract and cultures under red LED light stress (300 µmol m^-2 s^-1). The highest antioxidant capacity in a ferric-reducing antioxidant power (FRAP) assay (1,778.6 µM Fe⁺²) was found in ethanol extract and cultures under salinity stress illuminated using fluorescent light. Maximum scavenging of the 2.2-diphenyl-1-picrylhydrazyl (DPPH) radical was found in ethyl acetate extracts under light and salinity stresses. These results indicated that abiotic stresses could enhance the pigment and antioxidant components of T. tetrathele, which are value-added compounds in the pharmaceutical, cosmetic, and food industries.

An attempt to simultaneously quantify the polysaccharide, total lipid, protein and pigment in single Cyclotella cryptica cell by Raman spectroscopy

BACKGROUND

At present, the conventional methods for determining photosynthetic products of microalgae are usually based on a large number of cell mass to reach the measurement baseline, and the result can only reveal the average state at the population level, which is not feasible for large-scale and rapid screening of specific phenotypes from a large number of potential microalgae mutants. In recent years, single-cell Raman spectra (SCRS) has been proved to be able to rapidly and simultaneously quantify the biochemical components of microalgae. However, this method has not been reported to analyze the biochemical components of Cyclotella cryptica (C. cryptica). Thus, SCRS was first attempt to determine these four biochemical components in this diatom.

RESULTS

The method based on SCRS was established to simultaneously quantify the contents of polysaccharide, total lipids, protein and Chl-a in C. cryptica, with thirteen Raman bands were found to be the main marker bands for the diatom components. Moreover, Partial Least Square Regression (PLSR) models based on full spectrum can reliably predict these four cellular components, with Pearson correlation coefficient for these components reached 0.949, 0.904, 0.801 and 0.917, respectively. Finally, based on SCRS data of one isogenic sample, the pairwise correlation and dynamic transformation process of these components can be analyzed by Intra-ramanome Correlation Analysis (IRCA), and the results showed silicon starvation could promote the carbon in C. cryptica cells to flow from protein and pigment metabolism to polysaccharide and lipid metabolism.

CONCLUSIONS

First, method for the simultaneous quantification of the polysaccharide, total lipid, protein and pigment in single C. cryptica cell are established. Second, the instant interconversion of intracellular components was constructed through IRCA, which is based on data set of one isogenic population and more precision and timeliness. Finally, total results indicated that silicon deficiency could promote the carbon in C. cryptica cells to flow from protein and pigment metabolism to polysaccharide and lipid metabolism.

Efficient coupling of sulfadiazine removal with microalgae lipid production in a membrane photobioreactor

This study explored the feasibility of a coupled system for antibiotic removal and biofuel production through microalgae cultivation. Initial, batch culture experiments demonstrated that sulfadiazine (SDZ) had an inhibitory effect on Chlorella sp. G-9, and 100.0 mg L-1 SDZ completely inhibited its growth. In order to improve SDZ removal efficiency by microalgae, three membrane photobioreactors (MPBRs) with different hydraulic retention times (HRTs) were established for continuous microalgae cultivation. The efficient coupling of SDZ removal and microalgal lipid production was achieved through the gradual increment of influent SDZ concentration from 0 to 100.0 mg L-1. The reduction in SDZ ranged between 57.8 and 89.7%, 54.7-91.7%, and 54.6-93.5% for the MPBRs with HRT of 4 d, 2 d, and 1 d, respectively. Chlorella sp. Was found to tolerate higher concentrations of SDZ in the MPBR system, and the resulting stress from high concentrations of SDZ effectively increased the lipid content of microalgae for potential biodiesel production. With the increase of influent SDZ concentration from 0 to 100.0 mg L-1, the lipid content of microalgae increased by 43.5%. Chlorophyll content, superoxide dismutase activity, and malondialdehyde content of microalgae were also evaluated to explore the mechanism of microalgae tolerance to SDZ stress in MPBR.

Transcriptome analysis of changes in M. aeruginosa growth and microcystin production under low concentrations of ethinyl estradiol

Ethinyl estradiol (EE2) is a synthetic environmental estrogen with considerable estrogenic activity. It has been found to consequently pose a significant threat to the aquatic ecosystem. Harmful algal blooms are a major aquatic ecological issue. However, the relationship between EE2 and cyanobacterial bloom is mainly unknown. In this study, the physiological and molecular responses of Microcystis aeruginosa to EE2 exposure were investigated. A low level of EE2 (0.02 μg/L) significantly enhanced the growth of algal cells (P < 0.05), whereas higher concentrations of EE2 (0.2-200 μg/L) inhibited it. EE2 at doses ranging from 0.02 to 200 μg/L promoted the production of microcystins (MCs), with genes mcyABD playing a key role in the regulation of MC synthesis. The alterations of chlorophyll-a, carotenoid, and phycocyanin contents caused by EE2 showed the same trend as cell growth. At the molecular level, 200 μg/L EE2 significantly down-regulated genes in photosynthetic pigment synthesis, light harvesting, electron transfer, NADPH, and ATP generation. High concentrations of EE2 caused oxidative damage to algal cells on the 4th d. After 12d exposure, although there was no significant change in superoxide dismutase (SOD) content and no damage observed in membrane lipids, genes related to SOD and glutathione were changed. In addition, due to the down-regulation of pckA, PK, gltA, nrtA, pstS, etc., carbon fixation, glycolysis, TCA cycle, nitrogen and phosphorus metabolism were hindered by EE2 (200 μg/L). Gene fabG in fatty acid biosynthesis was significantly up-regulated, promoting energy storage in cells. These findings provide important clues to elucidate the effects and mechanisms of cyanobacterial blooms triggered by EE2 and help to effectively prevent and control cyanobacterial blooms.

Bioprocess development to enhance biomass and lutein production from Chlorella sorokiniana Kh12

Present study focused on optimizing bioprocess condition for microalgal lutein production. From previous baseline yields of biomass (3.46 g/L) and lutein (13.7 mg/g), this study examined few key parameters. The 3X:3X ratio macro- and micronutrients was the most affecting parameter with highest biomass and lutein yields of 4.61 g/L and 14.3 mg/g. Temperature 30 °C enhanced the lutein up to 17.3 mg/g but reduced the biomass to 3 g/L. The light effects study showed 10 k lux was most effective for lutein up to 14 mg/g, and effect of increasing salinity (25-75 %) was detrimental. All the above parameters' optimization resulted in a lipid content of 22.5-26.5 %. A maximum lutein productivity and yield of 0.451 mg/L/d and 65.74 mg/L with a 3X:3X macro- and micronutrient ratio was achieved. The Chlorella sorokiniana Kh12 strain exhibited one of the highest yields among recent reports; hence it could be a source for commercial lutein production.

From lab to application: Cultivating limnetic microalgae in seawater coupled with wastewater for biodiesel production on a pilot scale

The technology of cultivating salt-tolerant limnetic microalgae in seawater reduces the freshwater demand and costs of biodiesel production. However, all current trials still occur on the bench scale, and efforts for pilot-scale operation are urgently needed. This study firstly optimised the diameter of the photobioreactors (PBRs) to 0.2 m, as the single module to produce lipid-rich Golenkinia sp. SDEC-16 because of the better algal growth and light attenuation in the PBRs, and then established a 1000 L algal cultivation system. In a medium of seawater supplemented with monosodium glutamate wastewater at a ratio of 1:1000 (S-MSGW), the biomass productivity was 0.26 g/L/d, which was approaching the 0.30 g/L/d obtained in BG11, and the lipid productivity (98.99 mg/L/d) was doubled in comparison to growth in BG11. C16-C18 accounted for more than 98% of the total fatty acid in S-MSGW, and the biodiesel properties also met the biodiesel standards. The input cost of the biodiesel in this pilot-scale system was estimated to be 2.2 $/kg. When considering the carbon reduction and diversified application of the biomass, Golenkinia sp. would annually capture 186.77 kg/m³ PBR of CO₂, and yield an output-to-input ratio (OIR) of 3.4 in S-MSGW, higher than the 2.8 in BG11.

Physiological and morphological responses of Chlorella pyrenoidosa to different exposure methods of graphene oxide quantum dots

Graphene oxide quantum dots (GOQDs) can convert the ultraviolet (200- 380 nm) into available wavelength (400- 700 nm) for microalgae cultivation. However, it has not been applied in large-scale microalgae culture due to its high cost and difficulties in recovery. This study proposed a new strategy for the sustainable use of GOQDs, namely, GOQDs solution was added to the outer sandwich of the reactor. Herein, the effects of direct and indirect exposure of different GOQDs concentrations (0, 100, and 1000 mg/L) on the microalgae culture were compared. When microalgae were directly exposed to the GOQDs, 100 mg/L of GOQDs increased the biomass production of microalgae by 24.0 %, while 1000 mg/L of GOQDs decreased biomass production by 31 %. High concentration of GOQDs (direct exposure) could cause extra oxidative stress in the microalgae cells and result in a significant reduction of pigment content. When microalgae were indirectly exposed to the GOQDs, the increased concentration of GOQDs enhanced the growth of microalgae. Compared to the blank group, 1000 mg/L of GOQDs increased the microalgae biomass production and bioenergy by 14.1 % and 40.17 %, respectively. The indirect exposure of GOQDs can effectively avoid photo-oxidation and organelle damage to the microalgae cells. Overall, the indirect exposure of GOQDs is a sustainable way for effectively promoting microalgae growth and reducing the application cost.

Nickel toxicity pretreatment attenuates salt stress by activating antioxidative system and ion homeostasis in tomato (Solanum lycopersicon L.): an interplay from mild to severe stress

Plants antioxidative system is the first line of defense against oxidative stress caused secondarily by toxic ions under salinity. Plants with pre-activated antioxidative system can better adapt to salinity and can result in higher growth and yield. The current experiment was conducted to assess the adaptation of two tomato genotypes (Riogrande and Green Gold) with pre-activated antioxidative enzymes against salt stress. Tomato seedlings were exposed to mild stress (Ni: 0, 15 and 30 mg L^-1) for three weeks to activate the antioxidative enzymes. The seedlings with pre-activated antioxidative enzymes were then grown under severe stress in hydroponics (0, 75 and 150 mM NaCl) and soil (control, 7.5 and 15 dS m^-1) to check the adaptation, growth and yield. The results showed that Ni toxicity significantly enhanced activities of antioxidant enzymes (SOD, CAT, APX and POX) in both the genotypes and reduced growth with higher values in genotype Riogrande than Green Gold. The seedlings with pre-activated antioxidant enzymes showed better growth, low Na⁺ and high K⁺ uptake and maintained higher antioxidative enzymes activity than non-treated seedlings after four weeks of salt stress treatment in hydroponics. Similarly, the results in soil salinity treatment of the Ni pretreated seedlings showed higher yield characteristics (fruit yield per plant, average fruit weight and fruit diameter) than non-treated seedlings. However, Ni pretreatment had nonsignificant effect on tomato fruit quality characteristics like fruit dry matter percentage, total soluble solids, fruit juice pH and titratable acidity. The genotype Riogrande showed better growth, yield and fruit quality than Green Gold due to higher activity of antioxidant enzymes and better ion homeostasis as a result of Ni pretreatment. The results suggest that pre-activation antioxidant enzymes by Ni treatment proved to be an effective strategy to attenuate salt stress for better growth and yield of tomato plants.

Cold stress combined with salt or abscisic acid supplementation enhances lipogenesis and carotenogenesis in Phaeodactylum tricornutum (Bacillariophyceae)

Compounds from microalgae such as ω3-fatty acids or carotenoid are commercially exploited within the pharmacology, nutraceutical, or cosmetic sectors. The co-stimulation of several compounds of interest may improve the cost-effectiveness of microalgal biorefinery pipelines. This study focussed on Phaeodactylum tricornutum to investigate the effects on lipogenesis and carotenogenesis of combined stressors, here cold temperature and addition of NaCl salt or the phytohormone abscisic acid, using a two-stage cultivation strategy. Cold stress with NaCl or phytohormone addition increased the neutral lipid content of the biomass (20 to 35%). These treatments also enhanced the proportions of EPA (22% greater than control) in the fatty acid profile. Also, these treatments had a stimulatory effect on carotenogenesis, especially the combination of cold stress with NaCl addition, which returned the highest production of fucoxanthin (33% increase). The gene expression of diacylglycerol acyltransferase (DGAT) and the ω-3 desaturase precursor (PTD15) were enhanced 4- and 16-fold relative to the control, respectively. In addition, zeaxanthin epoxidase 3 (ZEP3), was downregulated at low temperature when combined with abscisic acid. These results highlight the benefits of applying a combination of low temperature and salinity stress, to simultaneously enhance the yields of the valuable metabolites EPA and fucoxanthin in Phaeodactylum tricornutum.

Improving the content of high value compounds in Nordic Desmodesmus microalgal strains

Nordic Desmodesmus microalgal strains (2-6) and (RUC-2) were exposed to abiotic stress (light and salt) to enhance lipids and carotenoids. The biomass output of both strains increased by more than 50% during light stress of 800 μmol m^-2 s^-1 compared to control light. The biomass of Desmodesmus sp. (2-6) contained most lipids (15% of dry weight) and total carotenoids (16.6 mg g^-1) when grown at moderate light stress (400 μmol m^-2 s^-1), which further could be enhanced up to 2.5-fold by salinity stress. Desmodesmus sp. (RUC-2) exhibited maximal lipid (26.5%) and carotenoid (43.8 mg L^-1) content at light intensities of 400 and 100 μmol m^-2 s^-1, respectively. Salinity stress stimulated lipid accumulation by 39%. Nordic Desmodesmus strains therefore are not only able to tolerate stress conditions, but their biomass considerably improves under stress. These strains have high potential to be used in algal bio-factories on low-cost medium like Baltic seawater.

Progress in microalgal mediated bioremediation systems for the removal of antibiotics and pharmaceuticals from wastewater

Worldwide demand for antibiotics and pharmaceutical products is continuously increasing for the control of disease and improvement of human health. Poor management and partial metabolism of these compounds result in the pollution of aquatic systems, leading to hazardous effects on flora, fauna, and ecosystems. In the past decade, the importance of microalgae in micropollutant removal has been widely reported. Microalgal systems are advantageous as their cultivation does not require additional nutrients: they can recover resources from wastewater and degrade antibiotics and pharmaceutical pollutants simultaneously. Bioadsorption, degradation, and accumulation are the main mechanisms involved in pollutant removal by microalgae. Integration of microalgae-mediated pollutant removal with other technologies, such as biodiesel, biochemical, and bioelectricity production, can make this technology more economical and efficient. This article summarizes the current scenario of antibiotic and pharmaceutical removal from wastewater using microalgae-mediated technologies.

Anaerobic biodegradation and detoxification of chloroacetamide herbicides by a novel Proteiniclasticum sediminis BAD-10T

Chloroacetamide herbicides (CAAHs) are important herbicides that were widely used to control agricultural weeds. However, their mass applications have seriously contaminated environment, and they are toxic to living beings. CAAHs are easy to enter anoxic environments such as subsoil, wetland sediment, and groundwater, where CAAHs are mainly degraded by anaerobic organisms. To date, there are no research on the anaerobic degradation of CAAHs by pure isolate and toxicity of anaerobic metabolites of CAAHs. In this study, the anaerobic degradation kinetics and metabolites of CAAHs by an anaerobic isolate BAD-10^T and the toxicity of anaerobic metabolites were studied. Isolate BAD-10^T could degrade alachlor, acetochlor, propisochlor, butachlor, pretilachlor and metolachlor with the degradation kinetics fitting the pseudo-first-order kinetics equation. The degradation rates of CAAHs were significantly affected by the length of N-alkoxyalkyl groups, the shorter the N-alkoxyalkyl groups, the higher the degradation rates. Four metabolites 2-ethyl-6-methyl-N-(ethoxymethyl)-acetanilide (EMEMA), N-(2-methyl-6-ethylphenyl)-acetamide (MEPA), N-2-ethylphenyl acetamide and 2-ethyl-N-carboxyl aniline were identified during acetochlor degradation, and an anaerobic catabolic pathway of acetochlor was proposed. The toxicity of EMEMA and EMPA for zebrafish, Arabidopsis and Chlorella ellipsoidea were obviously lower than that of acetochlor, indicating that the anaerobic degradation of acetochlor by isolate BAD-10^T is a detoxification process. The work reveals the anaerobic degradation kinetics and catabolic pathway of CAAHs and highlights a potential application of Proteiniclasticum sediminis BAD-10^T for bioremediation of CAAHs residue-contaminated environment.

Novel microalgae strains from selected lower Himalayan aquatic habitats as potential sources of green products

Microalgal biomass provides a renewable source of biofuels and other green products. However, in order to realize economically viable microalgal biorefinery, strategic identification and utilization of suitable microalgal feedstock is fundamental. Here, a multi-step suboptimal screening strategy was used to target promising microalgae strains from selected freshwaters of the study area. The resulting strains were found to be affiliated to seven closely-related genera of the family Scenedesmaceae, as revealed by both morphologic and molecular characterization. Following initial screening under upper psychrophilic to optimum mesophilic (irregular temperature of 14.1 to 35.9°C) cultivation conditions, superior strains were chosen for further studies. Further cultivation of the selected strains under moderate to extreme mesophilic cultivation conditions (irregular temperature of 25.7 to 42.2°C), yielded up to 74.12 mgL^-1day^-1, 19.96 mgL^-1day^-1, 48.56%, 3.34 μg/mL and 1.20 μg/mL, for biomass productivity, lipid productivity, carbohydrate content, pigments content and carotenoids content respectively. These performances were deemed promising compared with some previous, optimum conditions-based reports. Interestingly, the fatty acids profile and the high carotenoids content of the studied strains revealed possible tolerance to the stress caused by the changing suboptimal cultivation conditions. Overall, strains AY1, CM6, LY2 and KL10 were exceptional and may present sustainable, promising feedstock for utilization in large-scale generation of green products, including biodiesel, bioethanol, pigments and dietary supplements. The findings of this study, which exposed promising, eurythermal strains, would expand the current knowledge on the search for promising microalgae strains capable of performing under the largely uncontrolled large-scale cultivation settings.

Photosynthetic Toxicity of Enrofloxacin on Scenedesmus obliquus in an Aquatic Environment

Aquaculture facilities are a potential source of antibiotics in aquatic environments, having adverse effects on the algae species. In this study, the toxicity induced by enrofloxacin (ENR) on the algae Scenedesmus obliquus was evaluated. The uptake of ENR and the change in the growth and photosynthesis of algae were analyzed. At the exposure doses of 10-300 μg/L, the accumulated levels of ENR in algae were 10.61-18.22 μg/g and 12.09-18.34 μg/g after 48 h and 96 h of treatment, respectively. ENR inhibited the growth of algae, with a concentration for 50% effect of 119.74 μg/L, 53.09 μg/L, 64.37 μg/L, and 52.64 μg/L after 24 h, 48 h, 72 h and 96 h of treatment, respectively, indicating the self-protection and repair ability of algae in a short period of time. Furthermore, the chlorophyll contents decreased in all treatment groups, and the photosynthetic system Ⅱ parameters decreased in a dose-dependent manner under ENR stress, suggesting that ENR caused a disorder in the electron transport of the photosynthesis of algae, and the carbon fixation and assimilation processes were thus damaged. These results indicate that ENR poses a considerable risk to aquatic environments, affects the carbon sinks, and even has an adverse effect on human health.

Integrated comparison of growth and oxidative stress induced by tylosin in two freshwater algae Chlorella vulgaris and Raphidocelis subcapitata

Two model algae, Chlorella vulgaris (C. vulgaris) and Raphidocelis subcapitata (R. subcapitata), are commonly used in registration procedures to evaluate compounds with antimicrobial capacity. However, it has been found that these two algae show considerable differences in sensitivity when exposed to antibiotics. The selection of a suitable test species plays a crucial role in assessing the environmental hazards and risks of a compound, as the balance between oxidative stress and antioxidants is a key factor for alga growth. This study was conducted to investigate the status of oxidative stress and mechanism of antioxidant defense system of algae under antibiotic stress. Different tylosin (TYN) exposure-concentrations were used for the tests in this study. Oxidative stress biomarkers (malondialdehyde (MDA)), non-enzymatic antioxidants (reduced glutathione (GSH)), antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GP), glutathione S-transferase (GST)) and photosynthetic pigments were measured to determine the status of the antioxidant defense system. With increasing TYN concentration, the growth of R. subcapitata was significantly inhibited, while there was no effect on C. vulgaris. When the growth of R. subcapitata was inhibited, the content of MDA was significantly increased and the antioxidant system was activated, which indicated a significant increase in the activity of SOD and CAT.

Bright as day and dark as night: light-dependant energy for lipid biosynthesis and production in microalgae

Microalgae are photosynthetic organisms functioning as the green bio-factories for various pharmaceutical and biofuel products. To date, numerous attempts have been carried out to manipulate culture conditions to maximize the production of the desired metabolites. Because light is the energy source of microalgae for their growth and metabolites biosynthesis, it has been one of the most investigated variables emphasized on the deep understanding of how microalgae respond towards light changes as an external stimulus. This review discusses the effects of different light sources, light intensities, light wavelengths and length of photoperiod on various microalgae species, especially in terms of biomass and lipid productivity. Additionally, the relationship between photoregulation processes and lipid productivity of microalgae are also deliberated. The current available approaches of microalgae mass cultivation, including different types of open and closed systems are recapitulated with the intention to highlight the significant insights for the design of future photoreactors.

Enhancement of growth and biomolecules (carbohydrates, proteins, and chlorophylls) of isolated Chlorella thermophila using optimization tools

The production of multiple products from microalgae is essential for economic sustainability and the knowledge of optimum cultivation conditions for high growth and biomolecule synthesis of a microalgal strain is the prerequisite for its commercial production. In this work, optimization of nutrient concentrations for the cultivation of isolated Chlorella thermophila was performed by manipulating nine nutrients with the objectives of maximization of growth, carbohydrate, protein, and chlorophyll contents. Experiments were designed and effects of the parameters were studied using Taguchi orthogonal array (TOA). Experimental results of TOA were used for modeling artificial neural networks (ANN) followed by the optimization using genetic algorithm (GA) to find global optimal solutions. Results showed an increase of 36, 88, 36, and 88% for growth, carbohydrates, proteins, and chlorophylls, respectively, at optimal combinations of parameters given by TOA. Results obtained through the ANN-GA optimization were 9, 10, and 3% more compared to the TOA for biomass, carbohydrates, and chlorophylls, respectively with experimental verification. Nitrates and bicarbonate were found to play the most pivotal role in biomass and biomolecule synthesis of the isolated microalgal strain. Results of the current investigation can be used in the industrial scale-up for the production of multiple products using the biorefinery approach.

Ameliorating Effect of Bicarbonate on Salinity Induced Changes in the Growth, Nutrient Status, Cell Constituents and Photosynthetic Attributes of Microalga Chlorella vulgaris

The cells of Chlorella vulgaris exhibited NaCl (0-400 mM) induced decrease in the growth, protein, chlorophyll, carbohydrate and total organic carbon, whereas total lipid and proline content increased with rising level of NaCl. Addition of NaHCO₃ (20 mM) exhibited antagonistic effect against the adverse effect of salinity on the growth, level of macromolecules except proline. The SEM-EDS analysis of NaCl treated cells exhibited morphological variations as well as reduced accumulation of Na and Cl due to the presence of NaHCO₃. The results on chlorophyll fluorescence induction kinetics revealed NaCl induced decline in the photosynthetic performance and quantum yield, while non-photochemical quenching of chlorophyll was enhanced, particularly at lower concentrations of NaCl. Addition of NaHCO₃ to NaCl treated cells exhibited further increase in the non-photochemical quenching values. Thus, these results demonstrated that adverse impact of NaCl on the C. vulgaris cells was significantly mitigated in the presence of bicarbonate.

Interactions between antibiotics and heavy metals determine their combined toxicity to Synechocystis sp

Co-pollution of antibiotics and metals is prevailing in aquatic environments. However, risks of coexisted antibiotics and metals on aquatic organisms is unclear. This study investigated the combined toxicity of antibiotics and metals towards Synechocystis sp. PCC 6803, a cyanobacterium. We found that the joint toxicity of antibiotics and metals is dependent on their interplays. The complexation between chlortetracycline (CTC) and copper/cadmium (Cu(II)/Cd(II)) resulted in their antagonistic toxicity. Contrarily, an additive toxicity was found between florfenicol (FLO) and Cu(II)/Cd(II) due to lack of interactions between them. CTC facilitated the intracellular uptake of Cu(II) and Cd(II) by increasing the membrane permeability. However, FLO had no obvious effects on the internalization of metals in Synechocystis sp. Proteomic analysis revealed that the photosynthetic proteins was down-regulated by CTC and FLO, and ribosome was the primary target of FLO. These results were verified by parallel reaction monitoring (PRM). Cu(II) induced the up-regulation of iron-sulfur assembly, while Cd(II) disturbed the cyclic electron transport in Synechocystis sp. The co-exposure of CTC and metals markedly alleviated the dysregulation of proteins, while the co-exposure of FLO and metals down-regulated biological functions such as ATP synthesis, photosynthesis, and carbon fixation of Synechocystis sp., compared with their individuals. This supports their joint toxicity effects. Our findings provide better understanding of combined toxicity between multiple pollutants in aquatic environments.

The response of three typical freshwater algae to acute acid stress in water

The effect of acidic pH conditions on the physiological response of three typical freshwater algae, Chlorella vulgaris, Microcystis aeruginosa and Scenedesmus quadricauda, was investigated in this study. The results of the cultivation experiment indicated that the mortality of the three algae in the logarithmic growth phase increased with increasing exposure time and acidity under acute acidic conditions. The tolerance of S. quadricauda was stronger than that of the other two species under long-term (6 h) exposure to the same acidity; in contrast, C. vulgaris exhibited the greatest tolerance under short-term exposure conditions. The decrease in chlorophyll a (Chl a) content indicated that the photosynthesis of algae was inhibited under acid stress and that the algae could not continue to grow normally. This was consistent with the changes in lipid peroxidation and antioxidant enzyme activity, which were reflected by the malondialdehyde (MDA) content and superoxide dismutase (SOD) activity under acid stress, respectively. The results of this study demonstrated that when exposed to acute acidic conditions, the tolerance of three typical freshwater algae to acidity was significantly different. These findings provide valuable information for poorly mixed acidification operations designed to adjust the pH in lakes, reservoirs, or intake pipes of purification plants.

Double-dose responses of Scenedesmus capricornus microalgae exposed to humic acid

Dissolved organic matter (DOM) has been found to attenuate the ecotoxicity of various environmental pollutants, but research on its own toxic effects in aquatic ecosystems has been very limited. Herein, the toxic effects of humic acid (HA), a represent DOM typically found in natural waters, on the freshwater alga Scenedesmus capricornus were investigated. As result, HA exerted a double-dose effect on the growth of Scenedesmus capricornus. At HA concentrations below 2.0 mgC/L, the growth of Scenedesmus capricornus was slightly promoted, as was the synthesis of chlorophyll and macromolecules in the algae. Moreover, S. capricornus can maintain its growth by secreting fulvic acid as a nutrient carbon source. However, the growth of Scenedesmus capricornus was significantly inhibited when HA was beyond 2.0 mgC/L. The main mechanisms of humic acid's toxicity were membrane damage and oxidative stress. Particularly, when the oxidative stress exceeds the algae's carrying capacity, the synthesis of EPS is greatly inhibited and HA damage results. Taken together, DOM may have both positive and negative effects on aquatic ecosystems.

Innovative application of brackish groundwater without the addition of nutrients in the cultivation of Spirulina and Chlorella for carbohydrate and lipid production

Brackish groundwater is promising for the cultivation of economically important microalgae; however, its effects have been evaluated only after nutrient supplementation. In this study, 100% brackish groundwater was evaluated as a culture medium for Spirulina sp. (BGWS) and Chlorella fusca (BGWC). In addition, the effects of supplementation with 25% of the nutrients from Zarrouk (BGWS25) and BG-11 (BGWC25) culture media were evaluated. BGWS and BGWC increased the concentration (68.1% w w^-1) and productivity of carbohydrate (35.3 mg L^-1 d^-1) in Spirulina sp. and increased the concentration (56.4% w w^-1) and productivity (13.5 mg L^-1 d^-1) of lipids in C. fusca biomass, when compared to that in the respective controls. The use of brackish groundwater as the sole culture medium is an innovative alternative for the economic production of biomass rich in carbohydrates and lipids. This has potential applications for biofuel production.

Utilization of the microalga Scenedesmus quadricauda for hexavalent chromium bioremediation and biodiesel production

The purpose of this study was to evaluate the bioremediation potential of the microalga Scenedesmus quadricauda in removing hexavalent chromium (Cr (VI)) from synthetic wastewater, under autotrophic and heterotrophic conditions and different inoculum concentrations. In both cultivation modes, the highest inoculum density of 0.8 g L^-1 led to the highest Cr (VI) removal efficiency. In addition, Cr (VI) stress was more severe in 10 ppm compared to 5 ppm, while heavy metal effects were alleviated under heterotrophic conditions. Concurrently, Cr (VI) stress affected biomass quality, resulting in an increase in lipid and carbohydrate production and decreased proteins. Furthermore, under higher Cr (VI) concentration more saturated and monounsaturated fatty acids were produced, while monounsaturated fatty acids content was also greater under heterotrophic conditions. In total, the findings of this study highlight the advantages of heterotrophic cultivation of microalgae for concomitant industrial wastewater treatment and biofuel production.

Recycling spent water from microalgae harvesting by fungal pellets to re-cultivate Chlorella vulgaris under different nutrient loads for biodiesel production

Fungal pellet is an emerging material to collect oleaginous microalgae, but rare studies have noticed that harvested water is available resource for the next round of cultivation. To systematically optimize regrowth performances of microalgae Chlorella vulgaris, separated water after harvesting by fungi Aspergillus oryzae was prepared under different N/P ratios. The results showed that chlorophylls and enzymes were significantly affected by the proportion of N and P. Although nutrient deficiency was functioned as a stress factor to restrict carbohydrate and protein synthesis, lipid content was obviously increased by 12.69%. The percentage of saturated fatty acids associated with oxidation stability increased, while this part in fresh wastewater accounted for only 36.96%. The favorable biomass concentration (1.37 g/L) with the highest lipid yield (0.42 g/L) appeared in N/P of 6:1. More strikingly, suitable conditions could save 52.4% of cultivation costs. These experiments confirmed that reusing bioflocculated water could be effectively utilized for biodiesel production.

Salt induced oxidative stress alters physiological, biochemical and metabolomic responses of green microalga Chlamydomonas reinhardtii

Salinity is one of the most significant environmental factors limiting microalgal biomass productivity. In the present study, the model microalga Chlamydomonas reinhardtii (C. reinhardtii) was exposed to 200 mM NaCl for eight days to explore the physiological, biochemical and metabolomic changes. C. reinhradtii exhibited a significant decrease in growth rate, and Chl a and Chl b levels. 200 mM NaCl induced ROS generation in C. reinhardtii with increase in H2O2 content. This caused lipid peroxidation with increase in MDA levels. C. reinhardtii also exhibited an increase in carbohydrate and lipid accumulation under 200 mM NaCl conditions as storage molecules in cells to maintain microalgal survival. In addition, NaCl stress increased the content of carotenoids, polyphenols and osmoprotectant molecules such as proline. SOD and APX activities decreased, while ROS-scavenger enzymes (POD and CAT) decreased. Metabolomic response showed an accumulation of the major molecules implicated in membrane remodelling and stress resistance such oleic acid (40.29%), linolenic acid (19.29%), alkanes, alkenes and phytosterols. The present study indicates the physiological, biochemical and metabolomic responses of C. reinhardtii to salt stress.

The Active Phytohormone in Microalgae: The Characteristics, Efficient Detection, and Their Adversity Resistance Applications

Phytohormones are a class of small organic molecules that are widely used in higher plants and microalgae as chemical messengers. Phytohormones play a regulatory role in the physiological metabolism of cells, including promoting cell division, increasing stress tolerance, and improving photosynthetic efficiency, and thereby increasing biomass, oil, chlorophyll, and protein content. However, traditional abiotic stress methods for inducing the accumulation of energy storage substances in microalgae, such as high light intensity, high salinity, and heavy metals, will affect the growth of microalgae and will ultimately limit the efficient accumulation of energy storage substances. Therefore, the addition of phytohormones not only helps to reduce production costs but also improves the efficiency of biofuel utilization. However, accurate and sensitive phytohormones determination and analytical methods are the basis for plant hormone research. In this study, the characteristics of phytohormones in microalgae and research progress for regulating the accumulation of energy storage substances in microalgae by exogenous phytohormones, combined with abiotic stress conditions at home and abroad, are summarized. The possible metabolic mechanism of phytohormones in microalgae is discussed, and possible future research directions are put forward, which provide a theoretical basis for the application of phytohormones in microalgae.

Unraveling individual and combined toxicity of nano/microplastics and ciprofloxacin to Synechocystis sp. at the cellular and molecular levels

Although nanoplastics/microplastics (NPs/MPs) may interact with co-contaminants (e.g. antibiotics) in aquatic systems, little is known about their combined toxicity. Here, we compared the individual toxicity of NPs/MPs or ciprofloxacin (CIP, a very commonly detected antibiotic) and their combined toxicity toward a unicellular cyanobacterium Synechocystis sp. in terms of the cellular responses and metabolomic analysis. We found that CIP exhibited an antagonistic effect with NPs/MPs due to its adsorption onto the surface of NPs/MPs. Particle size-dependent toxic effects of NPs/MPs were observed. Reactive oxygen species (ROS) was verified as an important factor for NPs/MPs to inhibit cell growth, other than for CIP. Metabolomics further revealed that Synechocystis sp. up-regulated glycerophospholipids, amino acids, nucleotides, and carbohydrates to tolerate CIP pressure. NPs/MPs downregulated the TCA cycle and glycerophospholipids metabolism and impaired the primary production and membrane integrity via adhesion with Synechocystis sp.. Additionally, the toxicity of NPs/MPs throughout ten growth cycles at a sublethal concentration unveiled its potential risks in interfering with metabolism. Collectively, our findings provide insights into the joint ecotoxicity of NPs/MPs and antibiotics, and highlight the potential risks of co-pollutants at environmental relevant concentrations.