Potential of phytohormones as a strategy to improve microalgae productivity for biotechnological applications

Biogas slurry treatment and biogas upgrading by microalgae-based systems under the induction of different phytohormones

The low grade of biogas and the difficulty of treating biogas slurry are the two major bottlenecks limiting the sustainable development of the fermentation engineering. This study investigates the potential role of microalgae-microbial symbiosis and phytohormones in solving this challenge. Chlorella microalgae were combined with endophytic bacteria (S395-2) and Clonostachys fungus to construct symbiotic systems. Growth, photosynthetic activity, and carbon dioxide and pollutant removal out of biogas slurry and biogas were analyzed under treatment with three different phytohormones (cytokinin, synthetic strigolactones (GR24), natural strigolactones). The Chlorella-S395-2-Clonostachys symbiont achieved the highest purification efficiency under GR24 induction, with removal efficiency exceeding 86% for chemical oxygen demand, total phosphorous, and total nitrogen, as well as over 76% for CO2. Economic efficiency can be increased by about 150%. The positive correlation between treatment effectiveness and co-culture performance suggests a promising avenue for developing symbiotic systems for biogas slurry treatment and biogas upgrading.

Effect of phytohormone on proliferation and accumulation of cellular metabolites of microalgae Isochrysis zhanjiangensis

Phytohormones play a role in regulating microalgae cells tolerance to adversity. This paper examines the effects of different temperatures (20 °C, 25 °C, 30 °C and 35 °C) on the physiological characteristics and endogenous phytohormones of the Isochrysis Zhanjiangensis (IZ) and its mutagenic strain (3005). The results showed that the endogenous phytohormones indole acetic acid (IAA) and jasmonic acid (JA) exhibited significant differences (P<0.05) between the two strains. The addition of 0.5 mg·L-1 exogenous JA inhibitor ibuprofen (IBU) improved cell growth of IZ, and was extremely effective in the accumulation of polysaccharides, which accounted for 33.25 %. Transcriptomic analyses revealed that genes involved in photosynthesis, such as PetC and PsbO, exhibited significantly elevated expression of the strain IZ, while the pathways related to JA synthesis may be the factor affecting microalgae temperature tolerance. This study provides a theoretical foundation for elucidating the underlying mechanisms and potential applications for high temperature tolerance in IZ.

Phytohormone-induced changes in growth, physiology, and biochemistry of Aurantiochytrium sp. for sustainable bioproduction

The study aimed to assess the effects of nine combinations of phytohormones, salicylic acid (SA), gibberellic acid (GA), and jasmonic acid (JA) on the growth, physiology, and biochemistry of Aurantiochytrium sp. Parameters like optical density (OD), biomass, protein content, hydrogen peroxide (H2O2), malondialdehyde (MDA), catalase activity (CAT), and gene expression (malic enzyme (ME) and acetyl-CoA carboxylase (ACCase)) were assessed at various cultivation stages (24, 48, 72, and 96 h). The research also analyzed fatty acid composition, unsaturated fatty acids (UFA), saturated fatty acids (SFA), and the UFA to SFA ratio (USS) to understand the biochemical changes induced by phytohormones. Results demonstrated that modifying phytohormone concentrations significantly affected the characteristics of the microalgae, particularly in correlation with different growth stages, emphasizing the necessity of precise control of phytohormone levels for optimizing cultivation conditions and enhancing bioactive compound production in Aurantiochytrium sp.

Stress-Induced Production of Bioactive Oxylipins in Marine Microalgae

Microalgae, stemming from a complex evolutionary lineage, possess a metabolic composition influenced by their evolutionary journey. They have the capacity to generate diverse polyunsaturated fatty acids (PUFAs), akin to those found in terrestrial plants and oily fish. Also, because of their numerous double bonds, these metabolic compounds are prone to oxidation processes, leading to the creation of valuable bioactive molecules called oxylipins. Moreover, owing to their adaptability across various environments, microalgae offer an intriguing avenue for biosynthesizing these compounds. Thus, modifying the culture conditions could potentially impact the profiles of oxylipins. Indeed, the accumulation of oxylipins in microalgae is subject to the influence of growth conditions, nutrient availability, and stressors, and adjusting these factors can enhance their production in microalgae culture. Consequently, the present study scrutinized the LC-MS/MS profiles of oxylipins from three marine microalgae species (two Haptagophytes and one Chlorophyte) cultivated in 1 L of photobioreactors under varying stress-inducing conditions, such as the introduction of H2O2, EtOAc, and NaCl, during their exponential growth phase. Approximately 50 oxylipins were identified, exhibiting different concentrations depending on the species and growth circumstances. This research suggests that microalgae metabolisms can be steered toward the production of bioactive oxylipins through modifications in the culture conditions. In this instance, the application of a low dose of hydrogen peroxide to Mi 124 appears to stimulate the production of nonenzymatic oxylipins. For Mi136, it is the application of salt stress that seems to increase the overall production of oxylipins. In the case of Mi 168, either a low concentration of H2O2 or a high concentration of AcOEt appears to have this effect.

Resource recovery and contaminants of emerging concern mitigation by microalgae treating wastewater

This study aimed to investigate the recovery of agricultural biostimulants and biogas from microalgae treating wastewater, in the framework of a circular bioeconomy. To this end, municipal wastewater was treated in demonstrative raceway ponds, and microalgal biomass (Scenedesmus sp.) was then harvested and downstream processed to recover biostimulants and biogas in a biorefinery approach. The effect of microalgal biostimulants on plants was evaluated by means of bioassays, while the biogas produced was quantified in biochemical methane potential (BMP) tests. Furthermore, the fate of contaminants of emerging concern (CECs) over the process was also assessed. Bioassays confirmed the biostimulant effect of microalgae, which showed gibberellin-, auxin- and cytokinin-like activity in watercress seed germination, mung bean rooting, and wheat leaf chlorophyll retention. In addition, the downstream process applied to raw biomass acted as a pre-treatment to enhance anaerobic digestion performance. After biostimulant extraction, the residual biomass represented 91% of the methane yield from the raw biomass (276 mLCH4·g-1VS). The kinetic profile of the residual biomass was 43% higher than that of the unprocessed biomass. Co-digestion with primary sludge further increased biogas production by 24%. Finally, the concentration of CECs in wastewater was reduced by more than 80%, and only 6 out of 22 CECs analyzed were present in the biostimulant obtained. Most importantly, the concentration of those contaminants was lower than in biosolids that are commonly used in agriculture, ensuring environmental safety.

A Holistic Approach to Producing Anti-Vibrio Metabolites by an Endosymbiotic Dinoflagellate Using Wastewater from Shrimp Rearing

The aquaculture industry requires green solutions to solve several environmental challenges, including adequate wastewater remediation and natural drug applications to treat bacteria- and virus-related diseases. This study investigated the feasibility of cultivating the dinoflagellate Durusdinium glynnii in aquaculture wastewater from shrimp rearing in a synbiotic system (AWW-SS), with different dilutions of f/2 medium (FM). Interestingly, D. glynnii demonstrated enhanced growth in all AWW-SS treatments compared to the control (FM). The highest growth rates were achieved at AWW-SS:FM dilutions of 75:25 and 50:50. The removal of total nitrogen and total phosphorus reached 50.1 and 71.7%, respectively, of the crude AWW-SS. Biomass extracts of D. glynnii grown with AWW-SS were able to inhibit the growth of the bacteria Vibrio parahaemolyticus (inhibition zone of 10.0 ± 1.7 mm) and V. vulnificus (inhibition zone of 11.7 ± 1.5 mm). The presented results demonstrate that the dinoflagellate D. glynnii is a potential candidate for the development of circularity for sustainable aquaculture production, particularly by producing anti-Vibrio compounds at a near-zero cost.

Enhancing strategies of photosynthetic hydrogen production from microalgae: Differences in hydrogen production between prokaryotic and eukaryotic algae

Hydrogen production through the metabolic bypass of microalgae photosynthesis is an environmentally friendly method. This review examines the genetic differences in hydrogen production between prokaryotic and eukaryotic microalgae. Additionally, the pathways for enhancing microalgae-based photosynthetic hydrogen generation are summarized. The main strategies for enhancing microalgal hydrogen production involve inhibiting the oxygen-generating process of photosynthesis and promoting the oxygen tolerance of hydrogenase. Future research is needed to explore the regulation of physiological metabolism through quorum sensing in microalgae to enhance photosynthetic hydrogen production. Moreover, effective evaluation of carbon emissions and sequestration across the entire photosynthetic hydrogen production process is crucial for determining the sustainability of microalgae-based production approaches through comprehensive lifecycle assessment. This review elucidates the prospects and challenges associated with photosynthetic hydrogen production by microalgae.

A systematic review of antibiotics and antibiotic resistance genes (ARGs) in mariculture wastewater: Antibiotics removal by microalgal-bacterial symbiotic system (MBSS), ARGs characterization on the metagenomic

Antibiotic residues in mariculture wastewater seriously affect the aquatic environment. Antibiotic Resistance Genes (ARGs) produced under antibiotic stress flow through the environment and eventually enter the human body, seriously affecting human health. Microalgal-bacterial symbiotic system (MBSS) can remove antibiotics from mariculture and reduce the flow of ARGs into the environment. This review encapsulates the present scenario of mariculture wastewater, the removal mechanism of MBSS for antibiotics, and the biomolecular information under metagenomic assay. When confronted with antibiotics, there was a notable augmentation in the extracellular polymeric substances (EPS) content within MBSS, along with a concurrent elevation in the proportion of protein (PN) constituents within the EPS, which limits the entry of antibiotics into the cellular interior. Quorum sensing stimulates the microorganisms to produce biological responses (DNA synthesis - for adhesion) through signaling. Oxidative stress promotes gene expression (coupling, conjugation) to enhance horizontal gene transfer (HGT) in MBSS. The microbial community under metagenomic detection is dominated by aerobic bacteria in the bacterial-microalgal system. Compared to aerobic bacteria, anaerobic bacteria had the significant advantage of decreasing the distribution of ARGs. Overall, MBSS exhibits remarkable efficacy in mitigating the challenges posed by antibiotics and resistant genes from mariculture wastewater.

Synergistic and stepwise treatment of resveratrol and catechol in Haematococcus pluvialis for the overproduction of biomass and astaxanthin

To increase the production of biomass and astaxanthin from Haematococcus pluvialis to meet the high market demand for astaxanthin, this study recruited two typical and negligible phytohormones (namely resveratrol and catechol) for the stepwise treatments of H. pluvialis. It was found that the hybrid and sequential treatments of resveratrol (200 μmol) and catechol (100 μmol) had achieved the maximum astaxanthin content at 33.96 mg/L and 42.99 mg/L, respectively. Compared with the hybrid treatment, the physiological data of H. pluvialis using the sequential strategy revealed that the enhanced photosynthetic performance via the Calvin cycle by RuBisCO improved the biomass accumulation during the macrozooid stage; meanwhile, the excessive ROS production had occurred to enhance astaxanthin production with the help of NADPH overproduction during the hematocyst stage. Overall, this study provides improved knowledge of the impacts of phytohormones in improving biomass and astaxanthin of H. pluvialis, which shed valuable insights for advancing microalgae-based biorefinery.

Phytohormone profiling in an evolutionary framework

The genomes of charophyte green algae, close relatives of land plants, typically do not show signs of developmental regulation by phytohormones. However, scattered reports of endogenous phytohormone production in these organisms exist. We performed a comprehensive analysis of multiple phytohormones in Viridiplantae, focusing mainly on charophytes. We show that auxin, salicylic acid, ethylene and tRNA-derived cytokinins including cis-zeatin are found ubiquitously in Viridiplantae. By contrast, land plants but not green algae contain the trans-zeatin type cytokinins as well as auxin and cytokinin conjugates. Charophytes occasionally produce jasmonates and abscisic acid, whereas the latter is detected consistently in land plants. Several phytohormones are excreted into the culture medium, including auxin by charophytes and cytokinins and salicylic acid by Viridiplantae in general. We note that the conservation of phytohormone biosynthesis and signaling pathways known from angiosperms does not match the capacity for phytohormone biosynthesis in Viridiplantae. Our phylogenetically guided analysis of established algal cultures provides an important insight into phytohormone biosynthesis and metabolism across Streptophyta.

Removal of antibiotics by four microalgae-based systems for swine wastewater treatment under different phytohormone treatment

This study examined the removal of typical antibiotics from simulated swine wastewater. Microalgae-bacteria/fungi symbioses were constructed using Chlorella ellipsoidea, endophytic bacteria (S395-2), and Clonostachys rosea as biomaterials. The growth, photosynthetic performance, and removal of three types of antibiotics (tetracyclines, sulfonamides, and quinolones) induced by four phytohormones were analyzed in each system. The results showed that all four phytohormones effectively improved the tolerance of symbiotic strains against antibiotic stress; strigolactones (GR24) achieved the best performance. At 10-9 M, GR24 achieved the best removal of antibiotics by C. elliptica + S395-2 + C. rosea symbiosis. The average removals of tetracycline, sulfonamide, and quinolone by this system reached 96.2-99.4 %, 75.2-81.1 %, and 66.8-69.9 %, respectively. The results of this study help to develop appropriate bio enhancement strategies as well as design and operate algal-bacterial-fungal symbiotic processes for the treatment of antibiotics-containing wastewater.

Dialysis bag-microalgae photobioreactor: Novel strategy for enhanced bioresource production and wastewater purification

Cultivating microalgae in wastewater offers various advantages, but it still faces limitations such as bacteria and other impurities in wastewater affecting the growth and purity of microalgae, difficulty in microalgae harvesting, and extracellular products of microalgae affecting effluent quality. In this study, a novel dialysis bag-microalgae photobioreactor (Db-PBR) was developed to achieve wastewater purification and purer bioresource recovery by culturing microalgae in a dialysis bag. The dialysis bag in the Db-PBR effectively captured the microalgae cells and promoted their lipid accumulation, leading to higher biomass (1.53 times of the control) and lipid production (2.50 times of the control). During the stable operation stage of Db-PBR, the average soluble microbial products (SMP) content outside the dialysis bag was 25.83 mg L-1, which was significantly lower than that inside the dialysis bag (185.63 mg L-1), indicating that the dialysis bag effectively intercepted the SMP secreted by microalgae. As a result, the concentration of dissolved organic carbon (DOC) in Db-PBR effluent was significantly lower than that of traditional photobioreactor. Furthermore, benefiting from the dialysis bag in the reactor effectively intercepted the microorganisms in wastewater, significantly improving the purity of the cultured microalgae biomass, which is beneficial for the development of high-value microalgae products.

Enhancing lipid production and sedimentation of Chlorella pyrenoidosa in saline wastewater through the addition of agricultural phytohormones

In this study, the effect of external agricultural phytohormones (mixed phytohormones) addition (1.0, 5.0, 10.0, and 20.0 mg L-1) on the growth performance, lipid productivity, and sedimentation efficiency of Chlorella pyrenoidosa cultivated in saline wastewater was investigated. Among the different concentrations evaluated, the highest biomass (1.00 g L-1) and lipid productivity (11.11 mg L-1 d-1) of microalgae were obtained at 10.0 mg L-1 agricultural phytohormones addition. Moreover, exogenous agricultural phytohormones also improved the sedimentation performance of C. pyrenoidosa, which was conducive to the harvest of microalgae resources, and the improvement of sedimentation performance was positively correlated with the amount of agricultural phytohormones used. The promotion of extracellular polymeric substances synthesis by phytohormones in microalgal cells could be considered as the reason for its promotion of microalgal sedimentation. Transcriptome analysis revealed that the addition of phytohormones upregulated the expression of genes related to the mitogen-activated protein kinase (MAPK)-mediated phytohormone signaling pathway and lipid synthesis, thereby improving salinity tolerance and lipid production in C. pyrenoidosa. Overall, agricultural phytohormones provide an effective and inexpensive strategy for increasing the lipid productivity and sedimentation efficiency of microalgae cultured in saline wastewater.

Comparison of plant biostimulating properties of Chlorella sorokiniana biomass produced in batch and semi-continuous systems supplemented with pig manure or acetate

Microalgae-derived biostimulants provide an eco-friendly biotechnology for improving crop productivity. The strategy of circular economy includes reducing biomass production costs of new and robust microalgae strains grown in nutrient-rich wastewater and mixotrophic culture where media is enriched with organic carbon. In this study, Chlorella sorokiniana was grown in 100 l bioreactors under sub-optimal conditions in a greenhouse. A combination of batch and semi-continuous cultivation was used to investigate the growth, plant hormone and biostimulating effect of biomass grown in diluted pig manure and in nutrient medium supplemented with Na-acetate. C. sorokiniana tolerated the low light (sum of PAR 0.99 ± 0.18 mol/photons/(m2/day)) and temperature (3.7-23.7° C) conditions to maintain a positive growth rate and daily biomass productivity (up to 149 mg/l/day and 69 mg/l/day dry matter production in pig manure and Na-acetate supplemented cultures respectively). The protein and lipid content was significantly higher in the biomass generated in batch culture and dilute pig manure (1.4x higher protein and 2x higher lipid) compared to the Na-acetate enriched culture. Auxins indole-3-acetic acid (IAA) and 2-oxindole-3-acetic acid (oxIAA) and salicylic acid (SA) were present in the biomass with significantly higher auxin content in the biomass generated using pig manure (> 350 pmol/g DW IAA and > 84 pmol/g DW oxIAA) compared to cultures enriched with Na-acetate and batch cultures (< 200 pmol/g DW IAA and < 27 pmol/g DW oxIAA). No abscisic acid and jasmonates were detected. All samples had plant biostimulating activity measured in the mungbean rooting bioassay with the Na-acetate supplemented biomass eliciting higher rooting activity (equivalent to 1-2 mg/l IBA) compared to the pig manure (equivalent to 0.5-1 mg/l IBA) and batch culture (equivalent to water control) generated biomass. Thus C. sorokiniana MACC-728 is a robust new strain for biotechnology, tolerating low light and temperature conditions. The strain can adapt to alternative nutrient (pig manure) and carbon (acetate) sources with the generated biomass having a high auxin concentration and plant biostimulating activity detected with the mungbean rooting bioassay.

Insight into the roles of hematite iron oxide nanoparticles on microalgae growth, urban wastewater treatment and bioproducts generation: Gompertz simulation, nutrient mass balance and gene expression

In this study, the effect of α-Fe2O3 nanoparticles spiking in urban wastewater (UWW) on growth rate, wastewater treatment ability and bioproducts generation of C. vulgaris and Spirulina was investigated and compared with pure cultivation system. The biomass concentration of C. vulgaris and Spirulina improved by 20 % and 39 % at 10 and 15 mg/L α-Fe2O3, respectively while the both microalgae growth pattern fitted better with Gompertz simulation after treatment with α-Fe2O3. The nutrients mass balance revealed that 1 g of treated C. vulgaris and Spirulina could uptake more COD, TN and TP in comparison to the untreated cells. The lipid generation increased remarkably (C. vulgaris: 45 % and Spirulina: 72 %) after α-Fe2O3 treatment. While, the addition of α-Fe2O3 showed no significant impact on the protein and carbohydrate productivity. Overall, this study evangelize the role of nanoparticles on promoting microalgae applications as a sustainable approach for UWW treatment and promising feedstock for biofuel production.

Effects of staged multiple phytohormones application on capillary-driven attached Chlorella sp. biofilm

Comparing with single phytohormone application, applying multiple phytohormones to microalgae-based wastewater treatment systems can offer more extensive growth-promoting and stress-protecting effects for microalgae, yet the advantage of stress-relieving salicylic acid (SA) under combined phytohormones application scenario has not been exploited. Employing the improved capillary-driven attached microalgae culturing device (CD-PBR) previously used for single phytohormone application, this study compared the effects of mixed and single phytohormone(s) addition under as low as 10-7 M dosage. In order to make the best of SA for its stress-relieving property, postponed SA addition combined with applying other phytohormone(s) at the beginning of microalgae cultivation was also investigated. Combination of 10-6 M 6-benzylaminopurine (6-BA) with 10-7 M SA was sufficient for enhancing growth-promoting effects and anti-oxidative responses for attached Chlorella sp., while indole-3-acetic acid (IAA) addition was unnecessary. Combination of 6-BA addition at the beginning while postponed SA addition on Day 4 could further sustain such beneficial effects, while removing up to 99.7% total nitrogen (TN) and 97.9% total phosphorus (TP) from the bulk liquid. These results provided innovative strategies on mixed phytohormones addition for microalgae.

Melatonin, a phytohormone for enhancing the accumulation of high-value metabolites and stress tolerance in microalgae: Applications, mechanisms, and challenges

High-value metabolites, such as carotenoids, lipids, and proteins, are synthesized by microalgae and find applications in various fields, including food, health supplements, and cosmetics. However, the potential of the microalgal industry to serve these sectors is constrained by low productivity and high energy consumption. Environmental stressors can not only stimulate the accumulation of secondary metabolites in microalgae but also induce oxidative stress, suppressing cell growth and activity, thereby resulting in a decrease in overall productivity. Using melatonin (MT) under stressful conditions is an effective approach to enhance the productivity of microalgal metabolites. This review underscores the role of MT in promoting the accumulation of high-value metabolites and enhancing stress resistance in microalgae under stressful and wastewater conditions. It discusses the underlying mechanisms whereby MT enhances metabolite synthesis and improves stress resistance. The review also offers new perspectives on utilizing MT to improve microalgal productivity and stress resistance in challenging environments.

Synergic association of the consortium Arthrospira maxima with the microalga growth-promoting bacterium Azospirillum cultured under the stressful biogas composition

The present study evaluates the association of the blue-green microalga Arthrospira maxima (Spirulina), which is known for its CO2 fixation, biomass, and high-value metabolite production, with the microalga growth-promoting bacterium Azospirillum brasilense under the stressful composition of biogas. The results demonstrated that A. maxima co-cultured with A. brasilense under the high CO2 (25%) and methane (CH4; 75%) concentrations of biogas recorded a CO2 fixation rate of 0.24 ± 0.03 g L-1 days-1, thereby attaining a biomass production of 1.8 ± 0.03 g L-1. Similarly, the biochemical composition quality of this microalga enhanced the attainment of higher contents of carbohydrates, proteins, and phycocyanin than cultured alone. However, metabolites other than tryptophan (Trp) and indole-3-acetic acid could have supported this beneficial interaction. Overall, the results demonstrate that this prokaryotic consortium of A. maxima-A. brasilense established a synergic association under biogas, which represents a sustainable strategy to improve the bio-refinery capacity of this microalga and increase the usefulness of A. brasilense in multiple economic sectors.

Metabolomic Response of Thalassiosira weissflogii to Erythromycin Stress: Detoxification Systems, Steroidal Metabolites, and Energy Metabolism

Erythromycin, a macrolide antibiotic, is a prioritized pollutant that poses a high risk to environmental health. It has been detected in different environmental matrices and can cause undesired effects in aquatic organisms, particularly freshwater algae, which are primary producers. However, the impact of erythromycin on marine algae remains largely unexplored. Erythromycin has been reported to induce hormetic effects in the marine diatom Thalassiosira weissflogii (T. weissflogii). These effects are associated with the molecular pathways and biological processes of ribosome assembly, protein translation, photosynthesis, and oxidative stress. However, the alterations in the global gene expression have yet to be validated at the metabolic level. The present study used non-targeted metabolomic analysis to reveal the altered metabolic profiles of T. weissflogii under erythromycin stress. The results showed that the increased cell density was possibly attributed to the accumulation of steroidal compounds with potential hormonic action at the metabolic level. Additionally, slight increases in the mitochondrial membrane potential (MMP) and viable cells were observed in the treatment of 0.001 mg/L of erythromycin (an environmentally realistic level). Contrarily, the 0.75 and 2.5 mg/L erythromycin treatments (corresponding to EC20 and EC50, respectively) showed decreases in the MMP, cell density, and viable algal cells, which were associated with modified metabolic pathways involving ATP-binding cassette (ABC) transporters, the metabolism of hydrocarbons and lipids, thiamine metabolism, and the metabolism of porphyrin and chlorophyll. These findings suggest that metabolomic analysis, as a complement to the measurement of apical endpoints, could provide novel insights into the molecular mechanisms of hormesis induced by antibiotic agents in algae.

Regulation effects of indoleacetic acid on lipid production and nutrient removal of Chlorella pyrenoidosa in seawater-containing wastewater

The utilization of seawater supplemented with wastewater nutrients for microalgae cultivation represents a promising and cost-effective approach that combines the benefits of wastewater treatment and microalgal resource recovery. However, the high salt content in seawater poses a significant challenge, hindering microalgal growth and reducing the removal of nitrogen and phosphorus on a large scale. The phytohormone indoleacetic acid (IAA) was used in this study to enhance stress resistance and lipid production of Chlorella pyrenoidosa grown in seawater-wastewater medium. Compared to the control groups involving regular wastewater and seawater-containing wastewater without IAA, Chlorella pyrenoidosa cultivated in the seawater-containing wastewater supplemented with IAA exhibited remarkable outcomes. Specifically, microalgae in IAA-enhanced seawater-containing wastewater achieved the highest lipid productivity (22.67 mg L-1 d-1) along with impressive nitrogen (99.3 %) and phosphorus (97.3 %) removal rates. Moreover, their cell sedimentation ratio reached 76.6 %, indicating enhanced settling properties. Additionally, the physiological mechanism changes after exposure to seawater stress and IAA were revealed based on the changes in antioxidant enzymes, endogenous hormones, and fatty acid saturation. Furthermore, the transcriptomic analysis elucidated the molecular mechanisms underlying microalgal lipid synthesis and their response to antioxidant stress when exposed to seawater. The supplementation of IAA under seawater stress stimulated energy metabolism and the antioxidant response in microalgal cells, effectively mitigating the adverse effects of seawater stress and promoting overall algal lipid productivity. Overall, this study unveiled the potential of exogenous plant hormones, particularly IAA, in enhancing stress resistance and lipid productivity of microalgae grown in seawater-wastewater medium, which significantly contributed towards the efficient use of seawater resources for microalgae cultivation and biofuel production.

Biochemical responses of freshwater microalgae Chlorella sorokiniana to combined exposure of Zn(Ⅱ) and estrone with simultaneous pollutants removal

With the development of livestock industry, contaminants such as divalent zinc ions (Zn (Ⅱ)) and estrone are often simultaneously detected in livestock wastewater. Nevertheless, the combined toxicity of these two pollutants on microalgae is still unclear. Moreover, microalgae have the potential for biosorption and bioaccumulation of heavy metals and organic compounds. Thus, this study investigated the joint effects of Zn (Ⅱ) and estrone on microalgae Chlorella sorokiniana, in terms of growth, photosynthetic activity and biomolecules, as well as pollutants removal by algae. Interestingly, a low Zn (Ⅱ) concentration promoted C. sorokiniana growth and photosynthetic activity, while the high concentration experienced inhibition. As the increase of estrone concentration, chlorophyll a content increased continuously to resist the environmental stress. Concurrently, the secretion of extracellular polysaccharides and proteins by algae increased with exposure to Zn (Ⅱ) and estrone, reducing toxicity of pollutants to microalgae. Reactive oxygen species and superoxide dismutase activity increased as the increase of pollutant concentration after 96 h cultivation, but high pollutant concentrations resulted in damage of cells, as proved by increased MDA content. Additionally, C. sorokiniana displayed remarkable removal efficiency for Zn (Ⅱ) and estrone, reaching up to 86.14% and 84.96% respectively. The study provides insights into the biochemical responses of microalgae to pollutants and highlights the potential of microalgae in pollutants removal.

Plant response to heavy metal stress toxicity: the role of metabolomics and other omics tools

Metabolomic investigations offers a significant foundation for improved comprehension of the adaptability of plants to reconfigure the key metabolic pathways and their response to changing climatic conditions. Their application to ecophysiology and ecotoxicology help to assess potential risks caused by the contaminants, their modes of action and the elucidation of metabolic pathways associated with stress responses. Heavy metal stress is one of the most significant environmental hazards affecting the physiological and biochemical processes in plants. Metabolomic tools have been widely utilised in the massive characterisation of the molecular structure of plants at various stages for understanding the diverse aspects of the cellular functioning underlying heavy metal stress-responsive mechanisms. This review emphasises on the recent progressions in metabolomics in plants subjected to heavy metal stresses. Also, it discusses the possibility of facilitating effective management strategies concerning metabolites for mitigating the negative impacts of heavy metal contaminants on the growth and productivity of plants.

Analysis of the mechanism of exogenous indole-3-acetic acid on the enrichment of d-glucose in Chlorococcum humicola cultured by sludge extracts

Addressing problems of high organic toxicity in the wastewater treatment process, microalgae have been used to reduce the toxicity in sludge and to synthesize non-toxic and recoverable biomass of resources. Phytohormone is a core regulator of plant growth and current research has generally focused on their promotion of cell division and cell expansion. Effects of phytohormone on the enrichment mechanism of microalgae directional polysaccharides accumulation remain poorly elucidated. This study was carried out to investigate the effects of exogenous indole-3-acetic acid (IAA) on growth characteristics, biomass accumulation, and photosynthesis capacity of Chlorococcum humicola cultured in sludge extract and further find the d-glucose enrichment mechanism of it through proteomic. The results indicated that the optimal culture conditions were the 75 % sludge extract and 25 % selenite enrichment (SE) medium with 5 × 10-6 mol/L indole-3-acetic acid. Polysaccharides increased significantly from day 20 and accumulated to (326.59 ± 13.06) mg/L on day 30, in which the d-glucose proportion increased to 61.53 %. Most notably, proteomic tests were performed and found that the photosynthesis-related proteins including the differential proteins of photosystem electron transport, ATP and NADPH catalytic synthesis were significantly up-regulated. At the end of the path, three pathways of d-glucose enrichment with α-d-Glucose-1P as a precursor were summarized through indole-3-acetic acid activation on amylase, endoglucanase and Beta-glucosidase, etc. These results provide insights to explore the directed enrichment of biomass in Chlorococcum humicola by indole-3-acetic acid.

Overview of microalgae and cyanobacteria-based biostimulants produced from wastewater and CO2 streams towards sustainable agriculture: A review

For a long time, marine macroalgae (seaweeds) have been used to produce commercial biostimulants in order to ensure both productivity and quality of agricultural crops under abiotic stress. With similar biological properties, microalgae have slowly attracted the scientific community and the biostimulant industry, in particular because of their ability to be cultivated on non-arable lands with high biomass productivity all year long. Moreover, the recent strategies of culturing these photosynthetic microorganisms using wastewater and CO2 opens the possibility to produce large quantity of biomass at moderate costs while integrating local and circular economy approaches. This paper aims to provide a state of the art review on the development of microalgae and cyanobacteria based biostimulants, focusing on the different cultivation, extraction and application techniques available in the literature. Emphasis will be placed on microalgae and cyanobacteria cultivation using liquid and gaseous effluents as well as emerging green-extraction approaches, taking in consideration the actual European regulatory framework.

Effects of phytohormone on Chlorella vulgaris grown in wastewater-flue gas: C/N/S fixation, wastewater treatment and metabolome analysis

Chlorella vulgaris (C. vulgaris) can provide the means to fix CO2 from complicated flue gas, treat wastewater and reach a sustainable production of petrochemical substitutes simultaneously. However, a prerequisite to achieving this goal is to promote C. vulgaris growth and improve the CO2-to-fatty acids conversion efficiency under different conditions of flue gas and wastewater. Thus, the addition of indole-3-acetic acid (IAA) in C. vulgaris cultivation was proposed. Results showed that C. vulgaris were more easily inhibited by 100 ppm NO and 200 ppm SO2 under low nitrogen (N) condition. NO and SO2 decreased the carbon (C) fixation; but increased N and sulfur (S) fixation. IAA adjusted the content of superoxide dismutase (SOD) and malondialdehyde (MDA), improved the expression of psbA, rbcL, and accD, attenuated the toxicity of NO and SO2 on C. vulgaris, and ultimately improved cell growth (2014.64-2458.16 mgdw·L-1) and restored CO2 fixation rate (170.98-220.92 mg CO2·L-1·d-1). Moreover, wastewater was found to have a high treatment efficiency because C. vulgaris grew well in all treatments, and the maximal removal rates of both N and phosphorus (P) reached 100%. Metabonomic analysis showed that IAA, "NO and SO2" were involved in the down-regulated and up-regulated expression of multiple metabolites, such as fatty acids, amino acids, and carbohydrates. IAA was beneficial for improving lipid accumulation with 24584.21-27634.23 μg g-1, especially monounsaturated fatty acids (MUFAs) dominated by 16-18 C fatty acids, in C. vulgaris cells. It was concluded that IAA enhanced the CO2 fixation, fatty acids production of C. vulgaris and its nutrients removal rate.

Salicylic acid treatment and overexpression of a novel polyamine transporter gene for astaxanthin production in Phaffia rhodozyma

Phaffia rhodozyma represents an excellent microbial resource for astaxanthin production. However, the yeast's low astaxanthin productivity poses challenges in scaling up industrial production. Although P. rhodozyma originates from plant material, and phytohormones have demonstrated their effectiveness in stimulating microbial production, there has been limited research on the effects and mechanisms of phytohormones on astaxanthin biosynthesis in P. rhodozyma. In this study, the addition of exogenous salicylic acid (SA) at a concentration as low as 0.5 mg/L significantly enhanced biomass, astaxanthin content, and yield by 20.8%, 95.8% and 135.3% in P. rhodozyma, respectively. Moreover, transcriptomic analysis showed that SA had discernible impact on the gene expression profile of P. rhodozyma cells. Differentially expressed genes (DEGs) in P. rhodozyma cells between the SA-treated and SA-free groups were identified. These genes played crucial roles in various aspects of astaxanthin and its competitive metabolites synthesis, material supply, biomolecule metabolite and transportation, anti-stress response, and global signal transductions. This study proposes a regulatory mechanism for astaxanthin synthesis induced by SA, encompassing the perception and transduction of SA signal, transcription factor-mediated gene expression regulation, and cellular stress responses to SA. Notably, the polyamine transporter gene (PT), identified as an upregulated DEG, was overexpressed in P. rhodozyma to obtain the transformant Prh-PT-006. The biomass, astaxanthin content and yield in this engineered strain could reach 6.6 g/L, 0.35 mg/g DCW and 2.3 mg/L, 24.5%, 143.1% and 199.0% higher than the wild strain at the SA-free condition, respectively. These findings provide valuable insights into potential targets for genetic engineering aimed at achieving high astaxanthin yields, and such advancements hold promise for expediting the industrialization of microbial astaxanthin production.

Metabolic Responses of the Microalga Neochloris oleoabundans to Extracellular Self- and Nonself-DNA

Stressed organisms identify intracellular molecules released from damaged cells due to trauma or pathogen infection as components of the innate immune response. These molecules called DAMPs (Damage-Associated Molecular Patterns) are extracellular ATP, sugars, and extracellular DNA, among others. Animals and plants can recognize their own DNA applied externally (self-exDNA) as a DAMP with a high degree of specificity. However, little is known about the microalgae responses to damage when exposed to DAMPs and specifically to self-exDNAs. Here we compared the response of the oilseed microalgae Neochloris oleoabundans to self-exDNA, with the stress responses elicited by nonself-exDNA, methyl jasmonate (MeJA) and sodium bicarbonate (NaHCO3). We analyzed the peroxidase enzyme activity related to the production of reactive oxygen species (ROS), as well as the production of polyphenols, lipids, triacylglycerols, and phytohormones. After 5 min of addition, self-exDNA induced peroxidase enzyme activity higher than the other elicitors. Polyphenols and lipids were increased by self-exDNA at 48 and 24 h, respectively. Triacylglycerols were increased with all elicitors from addition and up to 48 h, except with nonself-exDNA. Regarding phytohormones, self-exDNA and MeJA increased gibberellic acid, isopentenyladenine, and benzylaminopurine at 24 h. Results show that Neochloris oleoabundans have self-exDNA specific responses.

Phytohormones: novel strategy for removing emerging contaminants and recovering resources

Microalgae have the ability to remove emerging contaminants (ECs) from wastewater and to recover resources, but this is limited by oxidative damage caused by the contaminants. Recently, phytohormones have been found to improve the tolerance of microalgae under oxidative stress, to promote the removal of ECs, and to enhance the synthesis of metabolites.

High-Level Expression of Recombinant VHSV Glycoprotein Using Transformed C. vulgaris and Verification of Vaccine Efficacy

The demand for aquaculture is increasing, but production is declining due to high feed costs and disease outbreaks. Viral hemorrhagic septicemia (VHS) is a viral disease that seriously affects seawater and freshwater fish in aquaculture, including the olive flounder (Paralichthys olivaceus), a major aquaculture fish in Korea. However, very few vaccines are currently available for viral hemorrhagic septicemia virus (VHSV). The nutrient-rich microalga Chlorella vulgaris has been used as a feed additive in aquaculture and as a host for the industrial production of recombinant VHSV glycoprotein as an oral vaccine. In this study, VHSV glycoprotein was cloned with a salt-inducible promoter, and high levels of expression up to 41.1 mg/g wet C. vulgaris, representing 27.4% of total extracted soluble protein, were achieved by growing the transformed C. vulgaris for 5 days in the presence of 250 mM NaCl. The production of a neutralizing antibody was detected in the serum of fish given feed containing 9% VHSV glycoprotein-expressing C. vulgaris. Furthermore, relative survival rates of 100% and 81.9% were achieved following challenges of these fish with VHSV at 10⁶ and 10⁷ pfu/fish, respectively, indicating that C. vulgaris could be used as a platform for the production of recombinant proteins for use as oral vaccines in the control of viral diseases in aquaculture.

A review on microalgal-bacterial co-culture: The multifaceted role of beneficial bacteria towards enhancement of microalgal metabolite production

Mass microalgal-bacterial co-cultures have come to the fore of applied physiological research, in particularly for the optimization of high-value metabolite from microalgae. These co-cultures rely on the existence of a phycosphere which harbors unique cross-kingdom associations that are a prerequisite for the cooperative interactions. However, detailed mechanisms underpinning the beneficial bacterial effects onto microalgal growth and metabolic production are rather limited at the moment. Hence, the main purpose of this review is to shed light on how bacteria fuels microalgal metabolism or vice versa during mutualistic interactions, building upon the phycosphere which is a hotspot for chemical exchange. Nutrients exchange and signal transduction between two not only increase the algal productivity, but also facilitate in the degradation of bio-products and elevate the host defense ability. Main chemical mediators such as photosynthetic oxygen, N-acyl-homoserine lactone, siderophore and vitamin B12 were identified to elucidate beneficial cascading effects from the bacteria towards microalgal metabolites. In terms of applications, the enhancement of soluble microalgal metabolites is often associated with bacteria-mediated cell autolysis while bacterial bio-flocculants can aid in microalgal biomass harvesting. In addition, this review goes in depth into the discussion on enzyme-based communication via metabolic engineering such as gene modification, cellular metabolic pathway fine-tuning, over expression of target enzymes, and diversion of flux toward key metabolites. Furthermore, possible challenges and recommendations aimed at stimulating microalgal metabolite production are outlined. As more evidence emerges regarding the multifaceted role of beneficial bacteria, it will be crucial to incorporate these findings into the development of algal biotechnology.

Unraveling the intracellular and extracellular self-defense of Chlorella sorokiniana toward highly toxic pyridine stress

A bottleneck of microalgae-based techniques for wastewater bioremediation is activity inhibition of microalgae by toxic pollutants. The defense strategies of Chlorella sorokinana against toxic pyridine were studied. Results indicated that pyridine caused photoinhibition and reactive oxygen species overproduction in a concentration-dependent manner. The 50% inhibitory concentration of pyridine (147 mg L^-1) destroyed C/N balance, disrupted multiple metabolic pathways of C. sorokinana. In response to pyridine stress, ascorbate peroxidase and catalase activities increased to scavenge reactive oxygen species under pyridine concentrations lower than 23 mg L^-1. At higher pyridine concentrations, the activation of calcium signaling pathways and phytohormones represented the predominant defense response. Extracellular polymeric substances increased 3.6-fold in 147 mg L^-1 group than control, which interacted with pyridine through hydrophobic and aromatic stacking to resist pyridine entering algal cells. Unraveling the intracellular and extracellular self-defense mechanisms of microalgae against pyridine stress facilitates the development of microalgal-based technology in wastewater bioremediation.

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.

Hormone released by the microalgae Neochlorisaquatica and alkalinization influence growth of terrestrial and floating aquatic plants

The microalgae Neochloris aquatica were previously evaluated as a potential biological control agent and source of bioactive compounds against immature stages of Culex quinquefasciatus. Larvae reared on microalgae suspension showed mortality or drastic effects with morphological alterations and damage in the midgut. N. aquatica have nutritional and toxic effects, resulting in delayed life cycle and incomplete adult development. Given the possibility of its use as a biological control agent, in this work we evaluate the effect of microalgae on other organisms of the environment, such as plants. Arabidopsis thaliana, a terrestrial plant, and Lemna sp., a floating aquatic plant, were selected as examples. Interaction assays and compound evaluations showed that the microalgae release auxins causing root inhibition, smaller epidermal cells and hairy root development. In Lemna sp., a slight decrease in growth rate was observed, with no deleterious effects on the fronds. On the other hand, we detected a detrimental effect on plants when interactions were performed in a closed environment, in a medium containing soluble carbonate, in which microalgae culture rapidly modifies the pH. The experiments showed that alkalinization of the medium inhibits plant growth, causing bleaching of leaves or fronds. This negative effect in plants was not observed when plants and microalgae were cultured in carbonate-free media. In conclusion, the results showed that N. aquatica can modify plant growth without being harmful, but the rapid alkalinization produced by carbon metabolism of microalgae under CO₂-limiting conditions, could regulate the number of plants.

Phytohormones enhance heavy metal responses in Euglena gracilis: Evidence from uptake of Ni, Pb and Cd and linkages to hormonomic and metabolomic dynamics

Over the last decade, significant effort has been made to understand phytohormonal functions (e.g., cytokinins (CKs) and abscisic acid (ABA)) in metal stress responses of higher plants and algae. Despite the potential for these phytohormones to improve industrial remediation by Euglena gracilis (Euglenophyceae), no such roles have been elucidated for this highly adaptive species and its response to heavy metals. This study demonstrates that toxic metals (nickel, lead, cadmium) modify hormonal activity profiles (i.e., CK forms and their concentrations) in E. gracilis. Furthermore, exogenous ABA or CK (tZ) enabled higher metal uptake efficiency (i.e., 9.35% in lead and 9.2% in cadmium uptake with CK) and alleviated metal toxicity through the regulation of endogenous CKs (i.e., total CK, isoprenoid CK) and gibberellin (GAs, GA₁ and GA₃) levels. These responses suggest that E. gracilis regulates multiple phytohormone signals during metal stress acclimation. A deeper approach, using untargeted metabolomic analyses, gave more detailed insight into phytohormone-controlled pathways and associated modified metabolites, which were frequently related to metal accumulation and the physiological acclimation to metal presence. Significant changes in the levels of cellular metabolites, especially those involved in acclimation to metal stress, were under the influence of phytohormones in algal cells. When grown under metal stress conditions, the presence of exogenous ABA or CKs, caused changes in cellular metabolites which included those from: lipid pathways, riboflavin metabolism, the biosynthesis of cofactors/vitamins, and carbohydrate metabolism. Also, bioactive secondary metabolites (e.g., terpenoids, alkaloids, flavonoids, carotenoids) were modified in algal cells treated with phytohormones. Thus, the study gives a detailed view on the regulatory functions of ABA and CKs in algal metal bioremediation strategies, which are attributed to enhanced metal uptake and in the fine-tuning of plant hormone levels during metal stress response. The results can guide efforts to develop efficient, low-cost and environmentally friendly methods for bioremediation.

Plant Growth Promotion, Phytohormone Production and Genomics of the Rhizosphere-Associated Microalga, Micractinium rhizosphaerae sp. nov

Microalgae are important members of the soil and plant microbiomes, playing key roles in the maintenance of soil and plant health as well as in the promotion of plant growth. However, not much is understood regarding the potential of different microalgae strains in augmenting plant growth, or the mechanisms involved in such activities. In this work, the functional and genomic characterization of strain NFX-FRZ, a eukaryotic microalga belonging to the Micractinium genus that was isolated from the rhizosphere of a plant growing in a natural environment in Portugal, is presented and analyzed. The results obtained demonstrate that strain NFX-FRZ (i) belongs to a novel species, termed Micractinium rhizosphaerae sp. nov.; (ii) can effectively bind to tomato plant tissues and promote its growth; (iii) can synthesize a wide range of plant growth-promoting compounds, including phytohormones such as indole-3-acetic acid, salicylic acid, jasmonic acid and abscisic acid; and (iv) contains multiple genes involved in phytohormone biosynthesis and signaling. This study provides new insights regarding the relevance of eukaryotic microalgae as plant growth-promoting agents and helps to build a foundation for future studies regarding the origin and evolution of phytohormone biosynthesis and signaling, as well as other plant colonization and plant growth-promoting mechanisms in soil/plant-associated Micractinium.

On the human health benefits of microalgal phytohormones: An explorative in silico analysis

Phytohormones represent a group of secondary metabolites with different chemical structures, in which belong auxins, cytokinins, gibberellins, or brassinosteroids. In higher plants, they cover active roles in growth or defense function, while their potential benefits for human health protection were noted for some phytohormones and little explored for many others. In this study, we developed a target fishing strategy on fifty-three selected naturally occurring phytohormones covering different families towards proteins involved in key cellular functions related to human metabolism and health protection/disease. This in silico analysis strategy aims to screen the potential human health-driven bioactivity of more than fifty phytohormones through the analysis of their interactions with specific targets. From this analysis, twenty-eight human targets were recovered. Some targets e.g., the proteins mitochondrial glutamate dehydrogenase (GLUD1) or nerve growth factor (NGF) bound many phytohormones, highlighting their involvement in amino acid metabolism and/or in the maintenance or survival of neurons. Conversely, some phytohormones specifically interacted with some proteins, e.g., SPRY domain-containing SOCS box protein 2 (SPSB2) or Inosine-5'-monophosphate dehydrogenase 1 (IMPDH1), both involved in human immune response. They were then investigated with a molecular docking analysis approach. Our bioprospecting study indicated that many phytohormones may endow human health benefits, with potential functional role in multiple cellular processes including immune response and cell cycle progression.

Efficient microalgal lipid production driven by salt stress and phytohormones synergistically

In this study, a novel method of coupling phytohormones with saline wastewater was proposed to drive efficient microalgal lipid production. All the six phytohormones effectively promoted microalgae growth in saline wastewater, and further increased the microalgal lipid content based on salt stress, so as to achieve a large increase in microalgal lipid productivity. Among the phytohormones used, abscisic acid had the most significant promoting effect. Under the synergistic effect of 20 g/L salt and 20 mg/L abscisic acid, the microalgal lipid productivity reached 3.7 times that of the control. Transcriptome analysis showed that differentially expressed genes (DEGs) of microalgae in saline wastewater were mainly up-regulated under the effects of phytohormones except brassinolide. Common DEGs analysis showed that phytohormones all regulated the expression of genes related to DNA repair and substance synthesis. In conclusion, synergistic effect of salt stress and phytohormones can greatly improve the microalgal lipid production efficiency.

Resource recovery from hydroponic wastewaters using microalgae-based biorefineries: A circular bioeconomy perspective

As the world's population grows, it is necessary to rethink how countries throughout the world produce food in order to replace the conventional and unsustainable agricultural techniques. Microalgae cultivation using a nutrient-rich solution from hydroponic systems not only presents a novel approach to solving problems pertaining to the impact of the discharges on the natural environment but also provides a plethora of other biotechnological applications particularly in the productions of high value-added products and plants growth stimulants, which can be potentially assimilated into the circular bioeconomy (CBE) in the hydroponic sector. In this review, the potential and practicability of microalgae to be merged into hydroponics CBE are reviewed. Overall, the integration of microalgal biorefineries in hydroponics systems can be realized after considering their Technology Readiness Level and System Readiness Level beforehand. Several suggestions on strains and hydroponics system improvement using existing biotechnological tools, Artificial Intelligence (AI) and nanobiotechnology in support of the CBE will be covered.

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

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

Photosynthesis Monitoring in Microalgae Cultures Grown on Municipal Wastewater as a Nutrient Source in Large-Scale Outdoor Bioreactors

Simple Summary

The remediation of waste nutrients, mainly nitrogen, phosphorous, and carbon, results in low-cost biomass production. In this work, raceway ponds with an area of 1 and 0.5 ha were built in Mérida, Spain next to a municipal wastewater (WW) treatment plant. These DEMO bioreactors are operated continuously all year round. At the start of the trials, the bioreactors were filled with urban WW as a sole source of nutrients and left to be populated by the sewage-born phytoplankton. The fitness and growth of the phytoplankton population (dominated by the green microalga Micractinium) were monitored by photosynthesis measuring techniques. The cultures showed suitable photosynthetic activity. Productivity values ranged between 15 and 25 g DW m⁻² day⁻¹ and processed biomass revealed biostimulating activity. In these trials, photosynthesis measuring techniques, i.e., oxygen evolution and chlorophyll (Chl) fluorescence techniques, were validated to monitor large-scale bioreactors using municipal WW remediation for biomass production, which can be used for agricultural purposes as biofertilizer/biostimulant.

Abstract

Microalgae cultures were used for a WW treatment to remediate nutrients while producing biomass and recycling water. In these trials, raceway ponds (RWPs; 1 and 0.5 ha) were located next to a municipal (WW) treatment plant in Mérida, Spain. The ponds were used for continuous, all-year-round microalgae production using WW as a source of nutrients. Neither CO₂ nor air was supplied to cultures. The objective was to validate photosynthesis monitoring techniques in large-scale bioreactors. Various in-situ/ex-situ methods based on chlorophyll fluorescence and oxygen evolution measurements were used to follow culture performance. Photosynthesis variables gathered with these techniques were compared to the physiological behavior and growth of cultures. Good photosynthetic activity was indicated by the build-up of dissolved oxygen concentration up to 380% saturation, high photochemical yield (Fv/Fm = 0.62–0.71), and relative electron transport rate rETR between 200 and 450 μmol e⁻ m⁻² s⁻¹ at midday, which resulted in biomass productivity of about 15–25 g DW m⁻² day⁻¹. The variables represent reliable markers reflecting the physiological status of microalgae cultures. Using waste nutrients, the biomass production cost can be significantly decreased for abundant biomass production in large-scale bioreactors, which can be exploited for agricultural purposes.

Influence of exogenous phytohormone supplementation on the pigment and fatty acid content of three marine diatoms

Diatoms are ubiquitous photosynthetic microorganisms with great potential for biotechnological applications. However, their commercialisation is hampered by production costs, requiring hence optimisation of cultivation methods. Phytohormones are plant growth regulators which may be used to influence physiological processes in microalgae, including diatoms. In this study, the model species Phaeodactylum tricornutum (Phaeodactylaceae) and two Irish isolates of Stauroneis sp. (Stauroneidaceae) and Nitzschia sp. (Bacillariaceae) were grown with varying amounts of the phytohormones indoleacetic acid (IAA), gibberellic acid (GA3), methyl jasmonate (MJ), abscisic acid (ABA) or salicylic acid (SA), and their influence on pigment and fatty acid profiles was monitored. The application of GA3 (200 mg/l) stimulated the growth of P. tricornutum which accumulated 52% more dry biomass compared to the control and concomitantly returned the highest eicosapentaenoic acid (EPA) yield (0.6 mg/l). The highest fucoxanthin yield (0.18 mg/l) was obtained for P. tricornutum cultivated with GA3 (2 mg/l) supplementation. In Stauroneis sp., SA (1 mg/l) had the most positive effect on EPA, the content of which was enhanced up to 45.7 μg/mg (4.6% of total dry weight). The SA (1 mg/l) treatment also boosted carotenogenesis in Nitzschia sp., leading to 1.7- and 14-fold increases in fucoxanthin and β-carotene compared to the control, respectively. Of note, MJ (0.5 mg/l) increased the EPA content of all diatom species compared to their controls. These results indicate that phytohormone-based treatments can be used to alter the pigment and lipid content of microalgae, which tend to respond in dose- and species-specific manners to individual compounds.Key points• Response to phytohormones was investigated in diatoms from distinct families.• MJ (0.5 mg/l) caused an increase in EPA cellular content in all three diatoms.• Phytohormones mostly caused dose-dependent and species-specific responses.

Physiological and proteomic dissection of the rice roots in response to iron deficiency and excess

Iron (Fe) disorder is a pivotal factor that limits rice yields in many parts of the world. Extensive research has been devoted to studying how rice molecularly copes with the stresses of Fe deficiency or excess. However, a comprehensive dissection of the whole Fe-responsive atlas at the protein level is still lacking. Here, different concentrations of Fe (0, 40, 350, and 500 μM) were supplied to rice to demonstrate its response differences to Fe deficiency and excess via physiological and proteomic analysis. Results showed that compared with the normal condition, the seedling growth and contents of Fe and manganese were significantly disturbed under either Fe stress. Proteomic analysis revealed that differentially accumulated proteins under Fe deficiency and Fe excess were commonly enriched in localization, carbon metabolism, biosynthesis of amino acids, and antioxidant system. Notably, proteins with abundance retuned by Fe starvation were individually associated with phenylpropanoid biosynthesis, cysteine and methionine metabolism, while ribosome- and endocytosis-related proteins were specifically enriched in treatment of Fe overdose of 500 μM. Moreover, several novel proteins which may play potential roles in rice Fe homeostasis were predicted. These findings expand the understanding of rice Fe nutrition mechanisms, and provide efficient guidance for genetic breeding work. SIGNIFICANCE: Both iron (Fe) deficiency and excess significantly inhibited the growth of rice seedlings. Fe deficiency and excess disturbed processes of localization and cellular oxidant detoxification, metabolisms of carbohydrates and amino acids in different ways. The Fe-deficiency and Fe-excess-responsive proteins identified by the proteome were somewhat different from the reported transcriptional profiles, providing complementary information to the transcriptomic data.

Trace phenolic acids simultaneously enhance degradation of chlorophenol and biofuel production by Chlorella regularis

Coupling the cultivation of microalgae with wastewater treatment is a promising technology to recover bioresources from wastewater. However, toxic pollutants in wastewater seriously inhibit the growth of microalgae and the removal of pollutants. Phenolic acids are similar to phytohormones, could potentially relieve the toxicity to microalgae and simultaneously promote pollutant degradation and lipid accumulation. Chlorella and 4-chlorophenol (4-CP) were utilized to simulate the toxic wastewater treatment, and the roles of two typical phenolic acids, such as p-hydroxybenzoic acid (p-HBA) and caffeic acid (CA), were explored. The 0.2 μM concentration of p-HBA or CA improved the specific growth rate by 7.6% by enhancing photosynthesis and DNA replication. The oxidative damage caused by 4-CP was reduced by 30.3-49.7% via the synthesis of more antioxidant enzymes and the direct scavenging of free radicals by phenolic acids. Furthermore, the 4-CP removal rate increased by 27.0%, and toxic 4-CP was degraded into non-toxic compounds. The phenolic acids did not change the 4-CP degradation pathway but accelerated its removal and detoxification by enhancing the expression of 4-CP degradation enzymes. Simultaneously, lipid production increased by 20.5-23.1% due to the upregulation of enzymes related to fatty acid and triacylglycerol synthesis. Trace phenolic acids stimulated the mitogen-activated protein kinase signaling cascade and the calcium signaling pathway to regulate the physiology of the microalgae and protect cells from toxic stress. This study provides a promising new strategy for toxic wastewater treatment and bioresource recovery.

Microalgae Polysaccharides: An Alternative Source for Food Production and Sustainable Agriculture

Carbohydrates or polysaccharides are the main products derived from photosynthesis and carbon fixation in the Calvin cycle. Compared to other sources, polysaccharides derived from microalgae are safe, biocompatible, biodegradable, stable, and versatile. These polymeric macromolecules present complex biochemical structures according to each microalgal species. In addition, they exhibit emulsifying properties and biological characteristics that include antioxidant, anti-inflammatory, antitumor, and antimicrobial activities. Some microalgal species have a naturally high concentration of carbohydrates. Other species can adapt their metabolism to produce more sugars from changes in temperature and light, carbon source, macro and micronutrient limitations (mainly nitrogen), and saline stress. In addition to growing in adverse conditions, microalgae can use industrial effluents as an alternative source of nutrients. Microalgal polysaccharides are predominantly composed of pentose and hexose monosaccharide subunits with many glycosidic bonds. Microalgae polysaccharides can be structural constituents of the cell wall, energy stores, or protective polysaccharides and cell interaction. The industrial use of microalgae polysaccharides is on the rise. These microorganisms present rheological and biological properties, making them a promising candidate for application in the food industry and agriculture. Thus, microalgae polysaccharides are promising sustainable alternatives for potential applications in several sectors, and the choice of producing microalgal species depends on the required functional activity. In this context, this review article aims to provide an overview of microalgae technology for polysaccharide production, emphasizing its potential in the food, animal feed, and agriculture sector.

Humic Substances as Microalgal Biostimulants—Implications for Microalgal Biotechnology

Humic substances (HS) act as biostimulants for terrestrial photosynthetic organisms. Their effects on plants are related to specific HS features: pH and redox buffering activities, (pseudo)emulsifying and surfactant characteristics, capacity to bind metallic ions and to encapsulate labile hydrophobic molecules, ability to adsorb to the wall structures of cells. The specific properties of HS result from the complexity of their supramolecular structure. This structure is more dynamic in aqueous solutions/suspensions than in soil, which enhances the specific characteristics of HS. Therefore, HS effects on microalgae are more pronounced than on terrestrial plants. The reported HS effects on microalgae include increased ionic nutrient availability, improved protection against abiotic stress, including against various chemical pollutants and ionic species of potentially toxic elements, higher accumulation of value-added ingredients, and enhanced bio-flocculation. These HS effects are similar to those on terrestrial plants and could be considered microalgal biostimulant effects. Such biostimulant effects are underutilized in current microalgal biotechnology. This review presents knowledge related to interactions between microalgae and humic substances and analyzes the potential of HS to enhance the productivity and profitability of microalgal biotechnology.

Promoting effect of plant hormone gibberellin on co-metabolism of sulfamethoxazole by microalgae Chlorella pyrenoidosa

In this study, sodium acetate (NaAC) as a co-substrate effectively promoted the metabolism of sulfamethoxazole (SMX) by microalgae Chlorella pyrenoidosa. In the cultivation supplied with 5.0 and 10.0 g L^-1 NaAC, 51.1% and 61.2% SMX was removed, respectively. On this basis, the improvement effect of plant hormone gibberellin (GA₃) on SMX removal by 5 g L^-1 NaAC supplied as co-substrate was further investigated. The results showed that biodegradation played decisive role in the removal of SMX. As a plant hormone, GA₃ effectively improved the co-metabolic removal efficiency of SMX by C. pyrenoidosa. Especially when GA₃ dosage reached 10.0 and 50.0 mg L^-1, C. pyrenoidosa showed a very high SMX removal rate of 83.5% and 95.3%, respectively. Transcriptome analysis showed that GA₃ promoted the removal of SMX by C. pyrenoidosa was the result of the combined action of exogenous and endogenous plant hormones.

Supplementation with rac-GR24 Facilitates the Accumulation of Biomass and Astaxanthin in Two Successive Stages of Haematococcus pluvialis Cultivation

The unicellular freshwater green alga Haematococcus pluvialis has attracted much research attention due to its biosynthetic ability for large amounts of astaxanthin, a blood-red ketocarotenoid that is used in cosmetics, nutraceuticals, and pharmaceuticals. Recently, numerous studies have investigated the functions of natural astaxanthin; however, the high cost of the production of astaxanthin from H. pluvialis cultures restricts its commercial viability. There is an urgent need to fulfill commercial demands by increasing astaxanthin accumulation from H. pluvialis cultures. In this study, we discovered that treatment of H. pluvialis cultures at the beginning of the macrozooid stage (day 0) with 1 μM rac-GR24, a synthetic analogue of strigolactones (a class of phytohormones), led to significant increases in biomass [up to a maximum dry cell weight (DCW) of 0.53 g/L] during the macrozooid stage and astaxanthin (from 0.63 to 5.32% of DCW) during the hematocyst stage. We elucidated that this enhancement of biomass accumulation during the macrozooid stage by rac-GR24 is due to its increasing CO₂ utilization efficiency in photosynthesis and carbohydrate biosynthesis. We also found that rac-GR24 stimulated the overproduction of nicotinamide adenine dinucleotide phosphate (NADPH) and antioxidant enzymes in H. pluvialis cultures, which alleviated the oxidative damage caused by reactive oxygen species generated during the hematocyst stage due to the exhaustion of nitrogen supplies. Moreover, rac-GR24 treatment of H. pluvialis synergistically altered the activity of the pathways of fatty acid biosynthesis and astaxanthin esterification, which resulted in larger amounts of astaxanthin being generated by rac-GR24-treated cultures than by controls. In summary, we have developed a feasible and economic rac-GR24-assisted strategy that increases the amounts of biomass and astaxanthin generated by H. pluvialis cultures, and have provided novel insights into the mechanistic roles of rac-GR24 to achieve these effects.

The Growth, Lipid Accumulation and Fatty Acid Profile Analysis by Abscisic Acid and Indol-3-Acetic Acid Induced in Chlorella sp. FACHB-8

Microalgae are considered a promising source for biodiesel. The addition of plant hormone can exert a significant impact on the production of microalgae biomass and lipid accumulation. Nevertheless, the response of microalgae cells to hormones is species- or strain-dependent. It remains controversial which genes involved in strong increase of fatty acids production in response to abscisic acid (ABA) in Chlorella sp. FACHB-8 strain. We investigated cell growth, lipid accumulation, and fatty acid composition when ABA and indol-3-acetic acid (IAA) were used in the growth medium of Chlorella sp. FACHB-8. The four treatments, including 5 mg/L IAA (E1), 10 mg/L IAA (E2), 10 mg/L ABA (E3), the combination of 5 mg/L IAA and 5 mg/L ABA (E4), were found to increase cell growth, but only 10 mg/L ABA treatment could enhance the lipid accumulation. The fatty acid profile was changed by the addition of ABA, making fatty acids afflux from polyunsaturated fatty acids to monounsaturated and saturated fatty acids, which were suitable for diesel application. Furthermore, a transcriptome analysis was conducted, unraveling the differentially expressed genes enriched in fatty acid biosynthesis, fatty acid metabolism, and biosynthesis of the unsaturated fatty acid pathway in response to ABA. Our results clarified the correlation of fatty acid synthesis-related genes and fatty acid profiles, helping understand the potential response mechanism of Chlorella sp. FACHB-8 strain respond to ABA treatment.

Biochemical and Morphological Changes Triggered by Nitrogen Stress in the Oleaginous Microalga Chlorella vulgaris

Oleaginous microalgae have been considered promising sources of biodiesel due to their high lipid content. Nitrogen limitation/starvation is one of the most prominent strategies to induce lipid accumulation in microalgae. Nonetheless, despite numerous studies, the mechanism underlying this approach is not well understood. The aim of this study was to investigate the effect of nitrogen limitation and starvation on biochemical and morphological changes in the microalga Chlorella vulgaris FACHB-1068, thereby obtaining the optimal nitrogen stress strategy for maximizing the lipid productivity of microalgal biomass. The results showed that nitrogen limitation (nitrate concentration < 21.66 mg/L) and starvation enhanced the lipid content but generally decreased the biomass productivity, pigment concentration, and protein content in algal cells. Comparatively, 3-day nitrogen starvation was found to be a more suitable strategy to produce lipid-rich biomass. It resulted in an increased biomass production and satisfactory lipid content of 266 mg/L and 31.33%, respectively. Besides, nitrogen starvation caused significant changes in cell morphology, with an increase in numbers and total size of lipid droplets and starch granules. Under nitrogen starvation, saturated fatty acids (C-16:0, C-20:0, and C-18:0) accounted for the majority of the total fatty acids (~80%), making C. vulgaris FACHB-1068 a potential feedstock for biodiesel production. Our work may contribute to a better understanding of the biochemical and morphological changes in microalgae under nitrogen stress. Besides, our work may provide valuable information on increasing the lipid productivity of oleaginous microalgae by regulating nitrogen supply.