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

Salinity as an Abiotic Stressor for Eliciting Bioactive Compounds in Marine Microalgae

This study investigated the impact of culture medium salinity (5-50 PSU) on the growth and maximum photochemical yield of photosystem II (Fv/Fm) and the composition of carotenoids, fatty acids, and bioactive substances in three marine microalgae (Chrysochromulina rotalis, Amphidinium carterae, and Heterosigma akashiwo). The microalgae were photoautotrophically cultured in discontinuous mode in a single stage (S1) and a two-stage culture with salt shock (S2). A growth model was developed to link biomass productivity with salinity for each species. C. rotalis achieved a maximum biomass productivity (Pmax) of 15.85 ± 0.32 mg·L-1·day-1 in S1 and 16.12 ± 0.13 mg·L-1·day-1 in S2. The salt shock in S2 notably enhanced carotenoid production, particularly in C. rotalis and H. akashiwo, where fucoxanthin was the main carotenoid, while peridinin dominated in A. carterae. H. akashiwo also exhibited increased fatty acid productivity in S2. Salinity changes affected the proportions of saturated, monounsaturated, and polyunsaturated fatty acids in all three species. Additionally, hyposaline conditions boosted the production of haemolytic substances in A. carterae and C. rotalis.

Horizontal gene transfer and symbiotic microorganisms regulate the adaptive evolution of intertidal algae, Porphyra sense lato

Intertidal algae may adapt to environmental challenges by acquiring genes from other organisms and relying on symbiotic microorganisms. Here, we obtained a symbiont-free and chromosome-level genome of Pyropia haitanensis (47.2 Mb), a type of intertidal algae, by using multiple symbiont screening methods. We identified 286 horizontal gene transfer (HGT) genes, 251 of which harbored transposable elements (TEs), reflecting the importance of TEs for facilitating the transfer of genes into P. haitanensis. Notably, the bulked segregant analysis revealed that two HGT genes, sirohydrochlorin ferrochelatase and peptide-methionine (R)-S-oxide reductase, play a significant role in the adaptation of P. haitanensis to heat stress. Besides, we found Pseudomonas, Actinobacteria, and Bacteroidetes are the major taxa among the symbiotic bacteria of P. haitanensis (nearly 50% of the HGT gene donors). Among of them, a heat-tolerant actinobacterial strain (Saccharothrix sp.) was isolated and revealed to be associated with the heat tolerance of P. haitanensis through its regulatory effects on the genes involved in proline synthesis (proC), redox homeostasis (ggt), and protein folding (HSP20). These findings contribute to our understanding of the adaptive evolution of intertidal algae, expanding our knowledge of the HGT genes and symbiotic microorganisms to enhance their resilience and survival in challenging intertidal environments.

Oil Palm AP2 Subfamily Gene EgAP2.25 Improves Salt Stress Tolerance in Transgenic Tobacco Plants

AP2/ERF transcription factor genes play an important role in regulating the responses of plants to various abiotic stresses, such as cold, drought, high salinity, and high temperature. However, less is known about the function of oil palm AP2/ERF genes. We previously obtained 172 AP2/ERF genes of oil palm and found that the expression of EgAP2.25 was significantly up-regulated under salinity, cold, or drought stress conditions. In the present study, the sequence characterization and expression analysis for EgAP2.25 were conducted, showing that it was transiently over-expressed in Nicotiana tabacum L. The results indicated that transgenic tobacco plants over-expressing EgAP2.25 could have a stronger tolerance to salinity stress than wild-type tobacco plants. Compared with wild-type plants, the over-expression lines showed a significantly higher germination rate, better plant growth, and less chlorophyll damage. In addition, the improved salinity tolerance of EgAP2.25 transgenic plants was mainly attributed to higher antioxidant enzyme activities, increased proline and soluble sugar content, reduced H2O2 production, and lower MDA accumulation. Furthermore, several stress-related marker genes, including NtSOD, NtPOD, NtCAT, NtERD10B, NtDREB2B, NtERD10C, and NtP5CS, were significantly up-regulated in EgAP2.25 transgenic tobacco plants subjected to salinity stress. Overall, over-expression of the EgAP2.25 gene significantly enhanced salinity stress tolerance in transgenic tobacco plants. This study lays a foundation for further exploration of the regulatory mechanism of the EgAP2.25 gene in conferring salinity tolerance in oil palm.

Mitigating salinity stress through interactions between microalgae and different forms (free-living & alginate gel-encapsulated) of bacteria isolated from estuarine environments

Salinity stress in estuarine environments poses a significant challenge for microalgal survival and proliferation. The interaction between microalgae and bacteria shows promise in alleviating the detrimental impacts of salinity stress on microalgae. Our study investigates this interaction by co-cultivating Chlorella sorokiniana, a freshwater microalga, with a marine growth-promoting bacterium Pseudomonas gessardii, both of which were isolated from estuary. In this study, bacteria were encapsulated using sodium alginate microspheres to establish an isolated co-culture system, preventing direct exposure between microalgae and bacteria. We evaluated microalgal responses to different salinities (5 PSU, 15 PSU) and interaction modes (free-living, gel-encapsulated), focusing on growth, photosynthesis, cellular metabolism, and extracellular polymeric substances (EPS) properties. High salinity inhibited microalgal proliferation, while gel-fixed interaction boosted Chlorella growth rate by 50.7 %. Both attached and free-living bacteria restored Chlorella's NPQ to normal levels under salt stress. Microalgae in the free-living interaction group exhibited a significantly lower respiratory rate compared to the pure algae group (-17.2 %). Increased salinity led to enhanced EPS polysaccharide secretion by microalgae, particularly in interaction groups (19.7 %). Both salt stress and interaction increased the proportion of aromatic proteins in microalgae's EPS, enhancing its stability by modulating EPS glycosidic bond C-O-C and protein vibrations. This alteration caused microalgal cells to aggregate, free-living bacteria co-culture group, and fixed co-culture group increasing by 427.5 %, 567.1 %, and 704.1 %, respectively. In gel-fixed bacteria groups, reduced neutral lipids don't accumulate starch instead, carbon redirects to cellular growth, aiding salt stress mitigation. These synergistic activities between salinity and bacterial interactions are vital in mitigating salinity stress, improving the resilience and growth of microalgae in saline conditions. Our research sheds light on the mechanisms of microalgal-bacterial interactions in coping with salt stress, offering insights into the response of estuarine microorganisms to global environmental changes and their ecological stability.

In vivo proteolytic profiling of the type I and type II metacaspases in Chlamydomonas reinhardtii exposed to salt stress

Metacaspases are cysteine proteases present in plants, fungi and protists. While the association of metacaspases with cell death is studied in a range of organisms, their native substrates are largely unknown. Here, we explored the in vivo proteolytic landscape of the two metacaspases, CrMCA-I and CrMCA-II, present in the green freshwater alga Chlamydomonas reinhardtii, using mass spectrometry-based degradomics approach, during control conditions and salt stress. Comparison between the cleavage events of CrMCA-I and CrMCA-II in metacaspase mutants revealed unique cleavage preferences and substrate specificity. Degradome analysis demonstrated the relevance of the predicted metacaspase substrates to the physiology of C. reinhardtii cells and its adaptation during salt stress. Functional enrichment analysis indicated an involvement of CrMCA-I in the catabolism of carboxylic acids, while CrMCA-II plays an important role in photosynthesis and translation. Altogether, our findings suggest distinct cellular functions of the two metacaspases in C. reinhardtii during salt stress response.

Different biological responses of Skeletonema costatum and Prorocentrum donghaiense to polymetallic nodules from seawaters

The negative impacts of polymetallic nodules mining on deep-sea benthic organisms have been widely established, but there is still a lack of understanding of the environmental impact on the surface ocean scenario. Phytoplankton growth experiment was conducted to determine the biological effect of polymetallic nodules on Prorocentrum donghaiense and Skeletonema costatum. The results showed that regardless of concentration and particle size, polymetallic nodules show a promoting effect on P. donghaiense (p < 0.05), the cell density in the experimental group increased by 35.2%-46.5% compared to the control at the end of the experiment. While fine particles significantly inhibited the growth of S. costatum (p < 0.05), the maximum inhibition rate on cell density reached 63.1%. Polymetallic nodules significantly enhance the Fv/Fm and the maximum electron transport rate of photosystem II in P. donghaiense, thereby increasing its growth rate. However, polymetallic nodules particles stimulated the antioxidant activity and extracellular polymeric substances secretion of S. costatum, resulting in phytoplankton flocculation and sedimentation, which inhibits its growth. Thus, these discriminatory impacts may cause alterations in biomass and community structure, ultimately affecting the ecological function.

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

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

Fatty acid production and associated gene pathways are altered by increased salinity and dimethyl sulfoxide treatments during cryopreservation of Symbiodinium pilosum (Symbiodiniaceae)

The Symbiodinium genus is ancestral among other Symbiodiniaceae lineages with species that are both symbiotic and free living. Changes in marine ecosystems threaten their existence and crucial ecological roles. Cryopreservation offers an avenue for their long-term storage for future habitat restoration after coral bleaching. In our previous study we demonstrated that high salinity treatments of Symbiodiniaceae isolates led to changes in their fatty acid (FA) profiles and higher cell viabilities after cryopreservation. In this study, we investigated the role of increased salinity on FA production and the genes involved in FA biosynthesis and degradation pathways during the cryopreservation of Symbiodinium pilosum. Overall, there was a twofold increase in mass of FAs produced by S. pilosum after being cultured in medium with increased salinity (54 parts per thousand; ppt). Dimethyl sulfoxide (Me2SO) led to a ninefold increase of FAs in standard salinity (SS) treatment, compared to a fivefold increase in increased salinity (IS) treatments. The mass of the FA classes returned to baseline during recovery. Transcriptomic analyses showed an acyl carrier protein gene was significantly upregulated after Me2SO treatment in the SS cultures. Cytochrome P450 reductase genes were significantly down regulated after Me2SO addition in SS treatment preventing FA degradation. These changes in the expression of FA biosynthesis and degradation genes contributed to more FAs in SS treated isolates. Understanding how increased salinity changes FA production and the roles of specific genes in regulating FA pathways will help improve current freezing protocols for Symbiodiniaceae and other marine microalgae.

Evaluation of short-term copper toxicity in a co-culture of Synechococcus sp., Chaetoceros gracilis and Pleurochrisys cf. roscoffensis exposed to changes in temperature and salinity levels

Metals such as copper (Cu) enter marine environments from natural and anthropogenic sources, causing changes in the biodiversity of marine microalgae and cyanobacteria. Cu plays a dual role as either a micronutrient or toxicant depending on the environmental concentration. Many studies have summarized the potential of Cu to become more toxic to microalgae under environmental stress (for instance climate change). Most of the data available on Cu toxicity concerning microalgae and cyanobacteria have been produced using single-species laboratory tests, and there is still a significant gap in the information concerning the behavior of a group of algae exposed to environmental stressors. Thus, the objective of this study was to evaluate the toxicity of Cu at two concentrations (C1 = 2 μg L-1 and C2 = 5 μg L-1) in multispecies bioassays using three phytoplankton species (one cyanobacteria, Synechococcus sp., and two microalgae, Chaetoceros gracilis and Pleurochrisys cf. roscoffensis). Combinations of two temperatures (20 and 23 °C) and two salinities (33 and 36 PSU), were applied in a 96 h study using flow cytometry analysis (FCM). Algal growth and reactive oxygen species (ROS) production by 2'7'-dichlorofluorescein (DCFH) were monitored by FCM. The results indicated that Synechococcus sp. was more sensitive than C. gracilis and P. roscoffensis to Cu stress at a temperature 23 °C and salinity of 36 PSU under both concentrations of Cu. Chlorophyll a fluorescence showed a significant decrease (p < 0.05) in Synechococcus sp. under 5 μg L-1 of Cu in the combined treatment of 20 °C and 33 PSU; however, there was a significant increase in P. roscoffensis in all combinations at C2 = 5 μg L-1 compared to the control with no Cu, indicating a potential hormetic response to Cu for P. roscoffensis. ROS levels were triggered in a combination of 23 °C and 33 PSU and 5 μg L-1 of Cu, which was higher than all the other combinations studied. Our study resulted in data concerning the potential impacts caused by possible future climate change scenarios in aquatic habitats chronically exposed to metals.

Exploitation of inland salt lake water by dilution and nutrient enrichment to cultivate Vischeria sp. WL1 (Eustigmatophyceae) for biomass and oil production

Salt lakes are significant components of global inland waters. Salt lake (SL) water can provide precious mineral resource for microbial growth. The prospect of utilizing diluted SL water for cultivation of a terrestrial oil-producing microalga Vischeria sp. WL1 was evaluated under laboratory conditions. Based on the detected mineral element composition, the water from Gouchi Salt Lake was diluted 2, 4, 6 and 8 folds and used with supplementation of additional nitrogen, phosphorus and iron (SL+ water). It was found that 4 folds diluted SL+ water was most favorable for biomass and oil production. When cultivated in this condition, Vischeria sp. WL1 gained a biomass yield of 0.82 g L-1 and an oil yield of 0.56 g L-1 after 24 days of cultivation, which is comparable to the optimum productivity we previously established. In addition, total monounsaturated fatty acid contents (64.4∼68.1 %) of the oils resulted from cultures in diluted SL+waters were higher than that in the control (55.5 %). It was also noteworthy that in all these cultures the oil contents (652.0∼681.0 mg g-1) accounted for the most of the biomass, which are far more than the protein and starch contents. This study demonstrates the feasibility of using SL water as a cost-effective mineral resource to cultivate microalgae for biomass and oil production.

Impact of macronutrients and salinity stress on biomass and biochemical constituents in Monoraphidium braunii to enhance biodiesel production

Microalgal lipids are precursors to the production of biodiesel, as well as a source of valuable dietary components in the biotechnological industries. So, this study aimed to assess the effects of nutritional (nitrogen, and phosphorus) starvations and salinity stress (NaCl) on the biomass, lipid content, fatty acids profile, and predicted biodiesel properties of green microalga Monoraphidium braunii. The results showed that biomass, biomass productivity, and photosynthetic pigment contents (Chl. a, b, and carotenoids) of M. braunii were markedly decreased by nitrogen and phosphorus depletion and recorded the maximum values in cultures treated with full of N and P concentrations (control, 100%). These parameters were considerably increased at the low salinity level (up to 150 mM NaCl), while an increasing salinity level (up to 250 mM NaCl) reduces the biomass, its productivity, and pigment contents. Nutritional limitations and salt stress (NaCl) resulted in significantly enhanced accumulation of lipid and productivity of M. braunii, which represented more than twofold of the control. Furthermore, these conditions have enhanced the profile of fatty acid and biodiesel quality-related parameters. The current study exposed strategies to improve M. braunii lipid productivity for biodiesel production on a small scale in vitro in terms of fuel quality under low nutrients and salinity stress.

A study on the mechanism of the impact of phenthoate exposure on Prorocentrum lima

Extensive application of organophosphorus pesticides such as phenthoate results in its abundance in ecosystems, particularly in waterbodies, thereby providing the impetus to assess its role in aquatic organisms. However, the impact of phenthoate on marine algal physiological and proteomic response is yet to be explored despite its biological significance. In this study, we thus ought to investigate the impact of phenthoate in the marine dinoflagellate Prorocentrum lima, which is known for synthesizing okadaic acid (OA), the toxin responsible for diarrhetic shellfish poisoning (DSP). Our results showed that P. lima effectively absorbed phenthoate in seawater, with a reduction efficiency of 90.31% after 48 h. Surprisingly, the provision of phenthoate (100 and 1000 µg/L) substantially reduced the OA content of P. lima by 35.08% and 60.28% after 48 h, respectively. Meanwhile, phenthoate treatment significantly reduced the oxidative stress in P. lima. Proteomic analysis revealed that the expression level of seven crucial proteins involved in endocytosis was upregulated, suggesting that P. lima could absorb phenthoate via the endocytic signaling pathway. Importantly, phenthoate treatment resulted in the downregulation of proteins such as polyketide synthase (PKS)- 2, Cytochrome P450 (CYP450)- 1, and CYP450-2, involved in OA synthesis, thereby decreasing the OA biosynthesis by P. lima. Our results demonstrated the potential role of P. lima in the removal of phenthoate in water and exemplified the crucial proteins and their possible molecular mechanisms underpinning the phenthoate remediation by P. lima and also the regulatory role of phenthoate in restricting the OA metabolism. Collectively, these findings uncovered the synergistic mechanisms of phenthoate and P. lima in remediating phenthoate and reducing the toxic impact of P. lima.

H2O2-dependent oxidation of the transcription factor GmNTL1 promotes salt tolerance in soybean

Reactive oxygen species (ROS) play an essential role in plant growth and responses to environmental stresses. Plant cells sense and transduce ROS signaling directly via hydrogen peroxide (H2O2)-mediated posttranslational modifications (PTMs) on protein cysteine residues. Here, we show that the H2O2-mediated cysteine oxidation of NAC WITH TRANS-MEMBRANE MOTIF1-LIKE 1 (GmNTL1) in soybean (Glycine max) during salt stress promotes its release from the endoplasmic reticulum (ER) membrane and translocation to the nucleus. We further show that an oxidative posttranslational modification on GmNTL1 residue Cys-247 steers downstream amplification of ROS production by binding to and activating the promoters of RESPIRATORY BURST OXIDASE HOMOLOG B (GmRbohB) genes, thereby creating a feed-forward loop to fine-tune GmNTL1 activity. In addition, oxidation of GmNTL1 Cys-247 directly promotes the expression of CATION H+ EXCHANGER 1 (GmCHX1)/SALT TOLERANCE-ASSOCIATED GENE ON CHROMOSOME 3 (GmSALT3) and Na+/H+ Antiporter 1 (GmNHX1). Accordingly, transgenic overexpression of GmNTL1 in soybean increases the H2O2 levels and K+/Na+ ratio in the cell, promotes salt tolerance, and increases yield under salt stress, while an RNA interference-mediated knockdown of GmNTL1 elicits the opposite effects. Our results reveal that the salt-induced oxidation of GmNTL1 promotes its relocation and transcriptional activity through an H2O2-mediated posttranslational modification on cysteine that improves resilience of soybean against salt stress.

Crosstalk of AsA/DHA and GSH/GSSG ratios' role in growth-phase dependent antioxidative defense in euryhaline and freshwater microalgae: explored for the first time

The cooperative role of vital components of the antioxidative defense pathway in addition to redox couples was studied in a growth-phase dependent manner at 20, 30, and 40 days after subculturing (DAS) in five different euryhaline microalgal strains (EMS) Scenedesmus MKB (B-S), Spirulina subsalsa (B-6), Anabaena sp. (B-7), Chlorella sp. (B-8), and Chlorosarcinopsis eremi (B-18) collected from waterlogged areas of Punjab, India and in two freshwater microalgal strains (FMS). EMS surpasses to maintain a high redox couple's ratio ascorbic acid/dehydroascorbate (AsA/DHA), and reduced glutathione/oxidized glutathione (GSH/GSSG) through a close-knit pattern of antioxidative enzymes including high specific activities of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), glutathione reductase (GR), dehydroascorbate reductase (DHAR) and less specific activity of glutathione peroxidase (GPX). While FMS struggled for the same irrespective of near similar total glutathione and higher specific activity of GPX might be answerable for the lesser redox ratio than EMS. However, high specific activity of catalase (CAT) might be sought to compensate for the less increase of APX in FMS. The fact significantly less H2O2, and malondialdehyde (MDA) with DAS in EMS than in FMS and higher redox ratios exquisitely elevate their tolerance ability making EMS a captivating prospect for cultivation in waterlogged areas. Additionally, their abundance of potent antioxidants further highlights the potential of EMS as an excellent source of these beneficial compounds.

Graphical Abstract

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.

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.

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.

Role of secondary metabolites in distressed microalgae

Proficient photosynthetic microalgae/cyanobacteria produce a remarkable amount of various biomolecules. Secondary metabolites (SM) represent high value products for global biotrend application. Production improvement can be achieved by nutritional, environmental, and physiological stress as a first line tools for their stimulation. In recent decade, an increasing interest in algal stress biology and omics techniques have deepened knowledge in this area. However, deep understanding and connection of specific stress elucidator are missing. Hence, the present review summarizes recent evidence with an emphasis on the carotenoids, phenolic, and less-discussed compounds (glycerol, proline, mycosporins-like amino acids). Even when they are synthesized at very low concentrations, it highlights the need to expand knowledge in this area using genome-editing tools and omics approaches.

Influence of salinity and temperature on the growth, productivity, photosynthetic activity and intracellular ROS of two marine microalgae and cyanobacteria

Global Climate Change could change physical parameters in oceans, such as salinity and temperature. The impact of such changes in phytoplankton has not been well stated yet. In this study the effect of combination of three levels of temperature (20, 23, and 26 °C), and three levels of salinity (33, 36, and 39) on growth of a mixture co-cultivation of three common species from phytoplankton (one cyanobacteria, Synechococcus sp., and two microalgae, Chaetoceros gracilis, and Rhodomonas baltica), is monitored by flow cytometry under controlled cultivation conditions in a 96 h study. Chlorophyll content, enzymes activities and oxidative stress were also measured. Results demonstrate that cultures of Synechococcus sp. Exhibited a high growth at the highest temperature chosen in this study (26 °C) combined with the three selected salinity levels 33, 36, and 39. Nevertheless, Chaetoceros gracilis grew very slowly with the combination of high temperature (39 °C) and all salinities, while Rhodomonas baltica did not grow at temperatures higher than 23 °C. Maximum dry biomass and ash-free dry weight for the microalgal mixture were reached at salinity of 39 and temperature of 20 °C, the but highest chlorophyll fluorescence values were found at 30 salinity and 20 °C, decreasing as salinity and temperature increased.

Application of ionizing radiation as an elicitor to enhance the growth and metabolic activities in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii is a eukaryotic, unicellular photosynthetic organism and a potential algal platform for producing biomass and recombinant proteins for industrial use. Ionizing radiation is a potent genotoxic and mutagenic agent used for algal mutation breeding that induces various DNA damage and repair responses. In this study, however, we explored the counterintuitive bioeffects of ionizing radiation, such as X- and γ-rays, and its potential as an elicitor to facilitate batch or fed-batch cultivation of Chlamydomonas cells. A certain dose range of X- and γ-rays was shown to stimulate the growth and metabolite production of Chlamydomonas cells. X- or γ-irradiation with relatively low doses below 10 Gy substantially increased chlorophyll, protein, starch, and lipid content as well as growth and photosynthetic activity in Chlamydomonas cells without inducing apoptotic cell death. Transcriptome analysis demonstrated the radiation-induced changes in DNA damage response (DDR) and various metabolic pathways with the dose-dependent expression of some DDR genes, such as CrRPA30, CrFEN1, CrKU, CrRAD51, CrOASTL2, CrGST2, and CrRPA70A. However, the overall transcriptomic changes were not causally associated with growth stimulation and/or enhanced metabolic activities. Nevertheless, the radiation-induced growth stimulation was strongly enhanced by repetitive X-irradiation and/or subsequent cultivation with an inorganic carbon source, i.e., NaHCO3, but was significantly inhibited by treatment of ascorbic acid, a scavenger of reactive oxygen species (ROS). The optimal dose range of X-irradiation for growth stimulation differed by genotype and radiation sensitivity. Here, we suggest that ionizing radiation within a certain dose range determined by genotype-dependent radiation sensitivity could induce growth stimulation and enhance metabolic activities, including photosynthesis, chlorophyll, protein, starch, and lipid synthesis in Chlamydomonas cells via ROS signaling. The counterintuitive benefits of a genotoxic and abiotic stress factor, i.e., ionizing radiation, in a unicellular algal organism, i.e., Chlamydomonas, may be explained by epigenetic stress memory or priming effects associated with ROS-mediated metabolic remodeling.

Production of Arthrospira platensis: Effects on Growth and Biochemical Composition of Long-Term Acclimatization at Different Salinities

Arthrospira platensis is an edible cyanobacterium with high nutritional value. Even though A. platensis is not a marine species, it can be adapted to higher salinities, a strategy that could allow mass cultivation using brackish or saline water. In this work A. platensis was long-term adapted at different salinities (5-60 g/L NaCl added as natural sea salt) to evaluate the growth and biochemical composition of the biomass produced. Biomass production was enhanced in salinity up to 40 g/L NaCl, while at 60 g/L NaCl biomass production slightly decreased. However, it displayed higher values compared to the conventional Zarrouk growth medium. By increasing the salinity, carbohydrate content increases, while proteins, phycocyanin, carotenoids, and total phenolics decreased. Biomass content in lipids, and chlorophyll along with the antioxidant capacity of extracts, was not significantly affected. A. platensis tended to increase the unsaturated fatty acids, while amino acid composition was not significantly affected by the increased salinity. However, in vitro protein digestibility was negatively affected when salinity was above 20 g/L NaCl. It was macroscopically observed that trichomes were longer at higher salinities, and especially at 40 g/L NaCl. The results suggest that A. platensis when acclimated in long-term can be grown successfully at various salinities.

Co-Expression of Lipid Transporters Simultaneously Enhances Oil and Starch Accumulation in the Green Microalga Chlamydomonas reinhardtii under Nitrogen Starvation

Lipid transporters synergistically contribute to oil accumulation under normal conditions in microalgae; however, their effects on lipid metabolism under stress conditions are unknown. Here, we examined the effect of the co-expression of lipid transporters, fatty acid transporters, (FAX1 and FAX2) and ABC transporter (ABCA2) on lipid metabolism and physiological changes in the green microalga Chlamydomonas under nitrogen (N) starvation. The results showed that the TAG content in FAX1-FAX2-ABCA2 over-expressor (OE) was 2.4-fold greater than in the parental line. Notably, in FAX1-FAX2-ABCA2-OE, the major membrane lipids and the starch and cellular biomass content also significantly increased compared with the control lines. Moreover, the expression levels of genes directly involved in TAG, fatty acid, and starch biosynthesis were upregulated. FAX1-FAX2-ABCA2-OE showed altered photosynthesis activity and increased ROS levels during nitrogen (N) deprivation. Our results indicated that FAX1-FAX2-ABCA2 overexpression not only enhanced cellular lipids but also improved starch and biomass contents under N starvation through modulation of lipid and starch metabolism and changes in photosynthesis activity. The strategy developed here could also be applied to other microalgae to produce FA-derived energy-rich and value-added compounds.

Effects of Temperature, pH, and NaCl Concentration on Biomass and Bioactive Compound Production by Synechocystis salina

Synechocystis salina is a cyanobacterium that has biotechnological potential thanks to its ability to synthesize several bioactive compounds of interest. Therefore, this study aimed to find optimal conditions, in terms of temperature (15-25 °C), pH (6.5-9.5), and NaCl concentration (10-40 g·L^-1), using as objective functions the productivities of biomass, total carotenoids, total PBPs, phycocyanin (PC), allophycocyanin (APC), phycoerythrin (PE), and antioxidants (AOXs) capacity of Synechocystis salina (S. salina) strain LEGE 06155, based in factorial design resorting to Box-Behnken. The model predicted higher biomass productivities under a temperature of 25 °C, a pH of 7.5, and low NaCl concentrations (10 g·L^-1). Maximum productivities in terms of bioactive compounds were attained at lower NaCl concentrations (10 g·L^-1) (except for PE), with the best temperature and pH in terms of carotenoids and total and individual PBPs ranging from 23-25 °C to 7.5-9.5, respectively. PE was the only pigment for which the best productivity was reached at a lower temperature (15 °C) and pH (6.5) and a higher concentration of NaCl (≈25 g·L^-1). AOX productivities, determined in both ethanolic and aqueous extracts, were positively influenced by lower temperatures (15-19 °C) and higher salinities (≈15-25 g·L^-1). However, ethanolic AOXs were better recovered at a higher pH (pH ≈ 9.5), while aqueous AOXs were favored by a pH of 8. The model showed that biomass production can be enhanced by 175% (compared to non-optimized conditions), total carotenoids by 91%, PC by 13%, APC by 50%, PE by 130%, and total PBPs by 39%; for AOX productivities, only water extracts exhibited a (marginal) improvement of 1.4%. This study provided insightful information for the eventual upgrading of Synechocystis salina biomass in the biotechnological market.

Biodesalination using halophytic cyanobacterium Phormidium keutzingianum from brackish to the hypersaline water

The biodesalination potential at different levels of salinity of Phormidium keutzingianum (P. keutzingianum) was investigated. A wide range of salinity from brackish to hypersaline water was explored in this study to ensure the adaptability of P. keutzingianum in extreme stress conditions. Brackish to hypersaline salt solutions were tested at selected NaCl concentrations 10, 30, 50, and 70 g.L-1. Chloride, pH, nitrate, and phosphate were the main parameters measured throughout the duration of the experiment. Biomass growth estimation revealed that the studied strain is adaptable to all the salinities inoculated. During the first growth phase (till day 20), chloride ion was removed up to 43.52% and 45.69% in 10 and 30 g.L-1 of salinity, respectively. Fourier transform infrared spectrometry analysis performed on P. keutzingianum showed the presence of active functional groups at all salinity levels, which resulted in biosorption leading to the bioaccumulation process. Samples for scanning electron microscopy (SEM) analysis supported with electron dispersive X-ray spectroscopy analysis (EDS) showed NaCl on samples already on day 0. This ensures the occurrence of the biosorption process. SEM-EDS results on 10th d showed evidence of additional ions deposited on the outer surface of P. keutzingianum. Calcium, magnesium, potassium, sodium, chloride, phosphorus, and iron were indicated in SEM-EDS analysis proving the occurrence of the biomineralization process. These findings confirmed that P. keutzingianum showed biomass production, biosorption, bioaccumulation, and biomineralization in all salinities; hence, the strain affirms the biodesalination process.