Auxin and cytokinin relationships in 24 microalgal strains(1)

Review of biological algal fertilizer technology: Alleviating salinization, sequestering carbon, and improving crop productivity

Periphyton-based biofertilizer have a high potential for soil remediation, particularly for controlling soil salinization. This global environmental problem leads to low soil utilization and insufficient crop yields. Efficient and sustainable methods of managing saline soils are needed to reduce salinization and improve soil fertility and crop quality. Traditional methods such as physical mulching and chemical amendments, while improving soil conditions, exhibit limited effectiveness and may damage soil structure. This study aims to evaluate the feasibility of algae-based fertilizers in remediating saline-alkali soils and improving crop performance. The review delves into the and application prospects of algae-based fertilizers, highlighting their potential from both sustainable development and economic perspectives. It further advocates integrating other emerging technologies with the production and application of algae-based fertilizers to address the increasingly severe challenges posed by degraded soil resources and environmental instability. The review found that algal fertilizers are more environmentally friendly than traditional chemical fertilizers but are not inferior in function. This approach offers more efficient and sustainable solutions for managing saline-alkaline soils and effectively achieves sustainable agricultural production. Furthermore, it is necessary to conduct experimental research and monitoring evaluations of algal fertilizers to formulate scientific and rational fertilization plans to meet the increasingly serious challenges facing soil resources and unstable environments. The findings of this study will provide theoretical and technical support for using algae biofertilizers for soil remediation, improving crop quality and sequestering carbon.

Plant Biostimulants to Enhance Abiotic Stress Resilience in Crops

The escalating impact of abiotic stress on crop productivity requires innovative strategies to ensure sustainable agriculture. This review examines the promising role of biostimulants in mitigating the adverse effects of abiotic stress on crops. Biostimulants, ranging from simple organic compounds to complex living microorganisms, have demonstrated significant potential in enhancing plant resilience, stress tolerance, and overall performance. The mechanisms underlying biostimulant action-such as enhancing antioxidant defenses, regulating hormonal pathways, and inducing metabolic adjustments-are reviewed. Furthermore, we incorporate the latest research findings, methodologies, and advancements in biostimulant applications for addressing abiotic stressors, including drought, salinity, high temperatures, and nutrient deficiencies. This review also highlights current challenges and future opportunities for optimizing biostimulant use in sustainable crop production. This revision aims to guide researchers and agronomists in applying biostimulants to improve crop resilience in the context of climate change.

Hydroponics with Microalgae and Cyanobacteria: Emerging Trends and Opportunities in Modern Agriculture

The global population is expected to reach 9.5 billion, which means that crop productivity needs to double to meet the growing population's food demand. Soil degradation and environmental factors, such as climate events, significantly threaten crop production and global food security. Furthermore, rapid urbanization has led to 55% of the world's population migrating to cities, and this proportion is expected to increase to 75% by 2050, which presents significant challenges in producing staple foods through conventional hinterland farming. Numerous studies have proposed various sustainable farming techniques to combat the shortage of farmable land and increase food security in urban areas. Soilless farming techniques such as hydroponics have gained worldwide popularity due to their resource efficiency and production of superior-quality fresh products. However, using chemical nutrients in a conventional hydroponic system can have significant environmental impacts, including eutrophication and resource depletion. Incorporating microalgae into hydroponic systems as biostimulants offers a sustainable and ecofriendly approach toward circular bioeconomy strategies. The present review summarizes the plant growth-promoting activity of microalgae as biostimulants and their mechanisms of action. We discuss their effects on plant growth parameters under different applications, emphasizing the significance of integrating microalgae into a closed-loop circular economy model to sustainably meet global food demands.

The role of cis-zeatin in enhancing high-temperature resistance and fucoxanthin biosynthesis in Phaeodactylum tricornutum

Phaeodactylum tricornutum a prominent source of industrial fucoxanthin production, faces challenges in its application due to its tolerance to high-temperature environments. This study investigates the physiological responses of P. tricornutum to high-temperature stress and its impact on fucoxanthin content, with a specific focus on the role of cis-zeatin. The results reveal that high-temperature stress inhibits P. tricornutum's growth and photosynthetic activity, leading to a decrease in fucoxanthin content. Transcriptome analysis shows that high temperature suppresses the expression of genes related to photosynthesis (e.g., psbO, psbQ, and OEC) and fucoxanthin biosynthesis (e.g., PYS, PDS1, and PSD2), underscoring the negative effects of high temperature on P. tricornutum. Interestingly, genes associated with cis-zeatin biosynthesis and cytokinesis signaling pathways exhibited increased expression under high-temperature conditions, indicating a potential role of cis-zeatin signaling in response to elevated temperatures. Content measurements confirm that high temperature enhances cis-zeatin content. Furthermore, the exogenous addition of cytokinesis mimetics or inhibitors significantly affected P. tricornutum's high-temperature resistance. Overexpression of the cis-zeatin biosynthetic enzyme gene tRNA DMATase enhanced P. tricornutum's resistance to high-temperature stress, while genetic knockout of tRNA DMATase reduced its resistance to high temperatures. Therefore, this research not only uncovers a novel mechanism for high-temperature resistance in P. tricornutum but also offers a possible alga species that can withstand high temperatures for the industrial production of fucoxanthin, offering valuable insights for practical utilization.IMPORTANCEThis study delves into Phaeodactylum tricornutum's response to high-temperature stress, specifically focusing on cis-zeatin. We uncover inhibited growth, reduced fucoxanthin, and significant cis-zeatin-related gene expression under high temperatures, highlighting potential signaling mechanisms. Crucially, genetic engineering and exogenous addition experiments confirm that the change in cis-zeatin levels could influence P. tricornutum's resistance to high-temperature stress. This breakthrough deepens our understanding of microalgae adaptation to high temperatures and offers an innovative angle for industrial fucoxanthin production. This research is a pivotal step toward developing heat-resistant microalgae for industrial use.

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.

Cytokinin-deficient Chlamydomonas reinhardtii CRISPR-Cas9 mutants show reduced ability to prime resistance of tobacco against bacterial infection

Although microalgae have only recently been recognized as part of the plant and soil microbiome, their application as biofertilizers has a tradition in sustainable crop production. Under consideration of their ability to produce the plant growth-stimulating hormone cytokinin (CK), known to also induce pathogen resistance, we have assessed the biocontrol ability of CK-producing microalgae. All pro- and eukaryotic CK-producing microalgae tested were able to enhance the tolerance of tobacco against Pseudomonas syringae pv. tabaci (PsT) infection. Since Chlamydomonas reinhardtii (Cre) proved to be the most efficient, we functionally characterized its biocontrol ability. We employed the CRISPR-Cas9 system to generate the first knockouts of CK biosynthetic genes in microalgae. Specifically, we targeted Cre Lonely Guy (LOG) and isopentenyltransferase (IPT) genes, the key genes of CK biosynthesis. While Cre wild-type exhibits a strong protection, the CK-deficient mutants have a reduced ability to induce plant defence. The degree of protection correlates with the CK levels, with the IPT mutants showing less protection than the LOG mutants. Gene expression analyses showed that Cre strongly stimulates tobacco resistance through defence gene priming. This study functionally verifies that Cre primes defence responses with CK, which contributes to the robustness of the effect. This work contributes to elucidate microalgae-mediated plant defence priming and identifies the role of CKs. In addition, these results underscore the potential of CK-producing microalgae as biologicals in agriculture by combining biofertilizer and biocontrol ability for sustainable and environment-friendly crop management.

A Holistic Approach to Circular Bioeconomy Through the Sustainable Utilization of Microalgal Biomass for Biofuel and Other Value-Added Products

Emissions from transportation and industry primarily cause global warming, leading to floods, glacier melt, and rising seas. Widespread greenhouse gas emissions and resulting global warming pose significant risks to the environment, economy, and society. The need for alternative fuels drives the development of third-generation feedstocks: microalgae, seaweed, and cyanobacteria. These microalgae offer traits like rapid growth, high lipid content, non-competition with human food, and growth on non-arable land using brackish or waste water, making them promising for biofuel. These unique phototrophic organisms use sunlight, water, and carbon dioxide (CO2) to produce biofuels, biochemicals, and more. This review delves into the realm of microalgal biofuels, exploring contemporary methodologies employed for lipid extraction, significant value-added products, and the challenges inherent in their commercial-scale production. While the cost of microalgae bioproducts remains high, utilizing wastewater nutrients for cultivation could substantially cut production costs. Furthermore, this review summarizes the significance of biocircular economy approaches, which encompass the utilization of microalgal biomass as a feed supplement and biofertilizer, and biosorption of heavy metals and dyes. Besides, the discussion extends to the in-depth analysis and future prospects on the commercial potential of biofuel within the context of sustainable development. An economically efficient microalgae biorefinery should prioritize affordable nutrient inputs, efficient harvesting techniques, and the generation of valuable by-products.

Mitigating Effect of Trans-Zeatin on Cadmium Toxicity in Desmodesmus armatus

Phytohormones, particularly cytokinin trans-zeatin (tZ), were studied for their impact on the green alga Desmodesmus armatus under cadmium (Cd) stress, focusing on growth, metal accumulation, and stress response mechanisms. Using atomic absorption spectroscopy for the Cd level and high-performance liquid chromatography for photosynthetic pigments and phytochelatins, along with spectrophotometry for antioxidants and liquid chromatography-mass spectrometry for phytohormones, we found that tZ enhances Cd uptake in D. armatus, potentially improving phycoremediation of aquatic environments. Cytokinin mitigates Cd toxicity by regulating internal phytohormone levels and activating metal tolerance pathways, increasing phytochelatin synthase activity and phytochelatin accumulation essential for Cd sequestration. Treatment with tZ and Cd also resulted in increased cell proliferation, photosynthetic pigment and antioxidant levels, and antioxidant enzyme activities, reducing oxidative stress. This suggests that cytokinin-mediated mechanisms in D. armatus enhance its capacity for Cd uptake and tolerance, offering promising avenues for more effective aquatic phycoremediation techniques.

Features of the microalga Raphidocelis subcapitata: physiology and applications

The microalga Raphidocelis subcapitata was isolated from the Nitelva River (Norway) and subsequently deposited in the collection of the Norwegian Institute of Water Research as "Selenastrum capricornutum Printz". This freshwater microalga, also known as Pseudokirchneriella subcapitata, acquired much of its notoriety due to its high sensitivity to different chemical species, which makes it recommended by different international organizations for the assessment of ecotoxicity. However, outside this scope, R. subcapitata continues to be little explored. This review aims to shed light on a microalga that, despite its popularity, continues to be an "illustrious" unknown in many ways. Therefore, R. subcapitata taxonomy, phylogeny, shape, size/biovolume, cell ultra-structure, and reproduction are reviewed. The nutritional and cultural conditions, chronological aging, and maintenance and preservation of the alga are summarized and critically discussed. Applications of R. subcapitata, such as its use in aquatic toxicology (ecotoxicity assessment and elucidation of adverse toxic outcome pathways) are presented. Furthermore, the latest advances in the use of this alga in biotechnology, namely in the bioremediation of effluents and the production of value-added biomolecules and biofuels, are highlighted. To end, a perspective regarding the future exploitation of R. subcapitata potentialities, in a modern concept of biorefinery, is outlined. KEY POINTS: • An overview of alga phylogeny and physiology is critically reviewed. • Advances in alga nutrition, cultural conditions, and chronological aging are presented. • Its use in aquatic toxicology and biotechnology is highlighted.

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.

Genetic evidence for algal auxin production in Chlamydomonas and its role in algal-bacterial mutualism

Interactions between algae and bacteria are ubiquitous and play fundamental roles in nutrient cycling and biomass production. Recent studies have shown that the plant auxin indole acetic acid (IAA) can mediate chemical crosstalk between algae and bacteria, resembling its role in plant-bacterial associations. Here, we report a mechanism for algal extracellular IAA production from L-tryptophan mediated by the enzyme L-amino acid oxidase (LAO1) in the model Chlamydomonas reinhardtii. High levels of IAA inhibit algal cell multiplication and chlorophyll degradation, and these inhibitory effects can be relieved by the presence of the plant-growth-promoting bacterium (PGPB) Methylobacterium aquaticum, whose growth is mutualistically enhanced by the presence of the alga. These findings reveal a complex interplay of microbial auxin production and degradation by algal-bacterial consortia and draws attention to potential ecophysiological roles of terrestrial microalgae and PGPB in association with land plants.

Impact of Chlorella vulgaris, Nannochloropsis salina, and Arthrospira platensis as bio-stimulants on common bean plant growth, yield and antioxidant capacity

The use of bio-stimulants in agriculture has emerged as a promising strategy to improve crop growth and yield. One type of bio-stimulant that has gained attention is microalgae extracts, which are known for their high metabolic activity, bioactive compounds, and ability to enhance plant growth and development. To investigate their effectiveness, a pot experiment was conducted at the Experimental Farm of Helwan University in Egypt during the 2022 season. The experiment aimed to evaluate the efficacy of Chlorella vulgaris, Nannochloropsis salina, and Arthrospira platensis (Spirulina platensis) extracts as bio-stimulants, applied through foliar spray at concentrations ranging from 0.25 to 2.0%, on common bean plants. Analysis of algal extract showed that . N. salina had the highest content of promotive growth hormones gibberellins (GA3) (74.85 ± 2.7mg100 g-1 d.wt). and auxins (IAA) (34.57 ± 2.7µg 100 g-1 d.wt.) compared to Chlorella and Arthrospira..The results revealed that the application of C. vulgaris, N. salina, and A. platensis extracts at concentrations up to 1.0% significantly improved various growth parameters, such as root, and shoot length, number of leaves and flowers per plant, leaf area, and total fresh and dry weight per plant. These extracts also positively affected yield attributes, including the number and fresh weight of pods per plant, seed index, seed yield per plant, and per feddan [a unit of land area]. Furthermore, the application of these extracts increased the chlorophyll content index with the maximum values of CCI (17.95. and 17.81%) was obtained at 0.50% N. salina, followed by 0.50% C.vulgaris. In addition to increase in the capacity of both non-enzymatic antioxidants [such as total antioxidant capacity, phenolics, and flavonoids] and enzymatic antioxidants [including catalase and ascorbic oxidase]. The most promising results were observed with the application of N. salina, and C. vulgaris extracts at a concentration of 0.5%. Additionally, the extracts significantly reduced the content of oxidative stress markers, such as malondialdehyde, percentage of electrolyte leakage, and hydrogen peroxide, in common bean plants compared to the control group. Contrarily, the measured parameters were reduced, while the levels of oxidative stress markers and some antioxidants including peroxidase, ascorbic peroxidase, superoxide dismutase, glutathione peroxidase, and glutathione transferase were increased by three algal extracts at a concentration of 2.0%, compared to control plants. Additionally, the application of these microalgae extracts improved the quality parameters, proximate composition, seed energy, and mineral contents of the harvested seeds, with the most significant positive impact was observed at 0.5% concentration of algal extract. These findings demonstrate the successful and safe utilization of extracts from C. vulgaris, N. salina, and A. platensis at concentrations up to 1.0% as bio-stimulants to enhance common bean yields and improve the nutritional quality of dried beans for consumers.

Enhancing sustainability through microalgae cultivation in urban wastewater for biostimulant production and nutrient recovery

Microalgae can produce biostimulants in form of phytohormones, which are compounds that, even if applied in low concentrations, can have stimulant effects on plants growth and can enhance their quality and their resistance to stress. Considering that microalgal biomass can grow recovering nutrients from wastewater, this circular approach allows to use residues for the production of high added value compounds (such as phytohormones) at low cost. The interest on biostimulants production from microalgae have recently raised. Scientists are focused on the direct application of these cellular extracts on plants, while the number of studies on the identification of bioactive molecules, such as phytohormones, is very scarce. Two cyanobacteria strains (Synechocystis sp. (SY) and Phormidium sp. (PH)) and a chlorophyte (Scenedesmus sp. (SC)) were cultured in laboratory-scale PBRs with a working volume of 2.5 L in secondary urban wastewater varying N:P ratio in the cultures to obtain the highest productivity. The variation of N:P ratio affects microalgae growth, and SY and PH presented higher productivities (73 and 48 mg L-1 d, respectively) under higher N:P ratio (> 22:1). Microalgal biomass was freeze-dried and phytohormones content was measured with ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The three microalgae showed similar phytohormones profiles, being the auxin (indole-3-acetic acid, IAA) the most abundant (72 ng g-1DW in SY). Proteins were major macronutrient for all strains, reaching 48 %DW in PH culture. To optimize the biostimulants production, a balance between the production of such compounds, biomass productivity and nutrients removal should be taken into consideration. In this sense, SC was the most promising strain, showing the highest N and P removal rates (73 % and 59 %, respectively) while producing phytohormones.

Photosynthesis and biochemical characterization of the green alga Chlamydopodium fusiforme (Chlorophyta) grown in a thin-layer cascade

Photosynthesis, growth and biochemical composition of the biomass of the freshwater microalga Chlamydopodium fusiforme cultures outdoors in a thin-layer cascade were investigated. Gross oxygen production measured off-line in samples taken from the outdoor cultures was correlated with the electron transport rate estimated from chlorophyll a fluorescence measurements. According to photosynthesis measurements, a mean of 38.9  ±  10.3 mol of photons were required to release one mole of O2, which is 4.86 times higher than the theoretical value (8 photons per 1 O2). In contrast, according to the fluorescence measurements, a mean of 11.7  ±  0.74 mol of photons were required to release 1 mol of O2. These findings indicate that fluorescence-based photosynthesis rates may not be fully replace oxygen measurements to evaluate the performance of an outdoor culture. Daily gross biomass productivity was 0.3 g DW L-1 day-1 consistently for 4 days. Biomass productivity was strongly affected by the suboptimal concentration at which the culture was operated and by the respiration rate, as the substantial volume of culture was kept in the dark (about 45% of the total volume). As the cells were exposed to excessive light, the photosynthetic activity was mainly directed to the synthesis of carbohydrates in the biomass. In the morning, carbohydrate content decreased because of the dark respiration. Per contra, protein content in the biomass was lower at the end of the day and higher in the morning due to carbohydrate consumption by respiration. The data gathered in these trials are important for the future exploitation of Chlamydopodium fusiforme as a potential novel species in the field of microalgae for the production of bio-based compounds.

Biomass valorization of agriculture wastewater grown freshwater diatom Nitzschia sp. for metabolites, antibacterial activity, and biofertilizer

The sustainable utilization of agricultural wastewater is a major global challenge. This study evaluated the impact of agricultural fertilizer on the biomass potential of Nitzschia sp. for metabolite production, antibacterial activity, and slow release biofertilizer. Cultivation of Nitzschia sp. in agriculture wastewater (0.5 mg ml^-1) exhibited maximum cell density (12×10⁵ cells ml^-1), protein content (10.0 mg g^-1), and lipid content (14.96%). Carbohydrate and phenol content increases in a dose-dependent manner with 8.27 mg g^-1 and 2.05 mg g^-1 at a concentration of 2 mg ml^-1 respectively. There was a 2.1-fold increment in chrysolaminarin content. Both gram-negative and gram-positive bacteria were susceptible to the antibacterial activity of the biomass. The effects of using diatom biomass as a biofertilizer were evaluated on the growth of periwinkle plants, which showed significant improvements in leaf development, branching at an early stage, flowering, and a marked increase in shoot length. Diatom biorefinery holds immense potential in addressing agriculture wastewater recycling and sustainable generation of high-value compounds.

Application of Chlorella vulgaris Beijerinck as a Biostimulant for Growing Cucumber Seedlings in Hydroponics

Hydroponics is a promising method for growing agricultural plants and is especially relevant in the context of global climate change. Microscopic algae, including Chlorella vulgaris, has great potential for use in hydroponic systems as natural growth stimulators. The effect of the suspension of an authentic strain of Chlorella vulgaris Beijerinck on the length of cucumber shoots and roots, as well as its dry biomass, was studied. During cultivation in a Knop medium with the addition of Chlorella suspension, the length of the shoots was shortened from 11.30 to 8.15 cm, while the length of the roots also decreased from 16.41 to 10.59 cm. At the same time, the biomass of the roots increased from 0.04 to 0.05 g. The data obtained indicate the positive effect of the suspension of the Chlorella vulgaris authentic strain on the dry biomass of cucumber plants in hydroponic conditions and make it possible to recommend this strain for use when growing plants in hydroponic systems.

Microalgae from Cold Environments and Their Possible Biotechnological Applications

Cold environments include deep ocean, alpine, and polar areas. Even if the cold conditions are harsh and extreme for certain habitats, various species have been adapted to survive in them. Microalgae are among the most abundant microbial communities which have adapted to live in low light, low temperature, and ice coverage conditions typical of cold environments by activating different stress-responsive strategies. These species have been shown to have bioactivities with possible exploitation capabilities for human applications. Even if they are less explored compared to species living in more accessible sites, various activities have been highlighted, such as antioxidant and anticancer activities. This review is focused on summarizing these bioactivities and discussing the possible exploitation of cold-adapted microalgae. Thanks to the possibility of mass cultivating algae in controlled photobioreactors, eco-sustainable exploitation is in fact possible by sampling a few microalgal cells without impacting the environment.

Indole-3-acetic acid increases the survival of brine shrimp challenged with vibrios belonging to the Harveyi clade

Vibrios belonging to the Harveyi clade (including closely related species such as Vibrio campbellii, Vibrio harveyi and Vibrio parahaemolyticus) are important pathogens of aquatic organisms. In this study, we investigated the use of indole-3-acetic acid to control disease caused by Harveyi clade vibrios. Indole-3-acetic acid, which can be produced by various seaweeds and microalgae, was added to the rearing water of brine shrimp larvae challenged with 12 different Harveyi clade Vibrio strains. Indole-3-acetic acid significantly decreased the virulence of 10 of the strains without any effect on their growth. The latter is important as it will minimize the selective pressure for resistance development. The survival rate of brine shrimp larvae increased from 1.2-fold to 4.8-fold upon treatment with 400 μM indole-3-acetic acid. Additionally, indole-3-acetic acid significantly decreased the swimming motility in 10 of the strains and biofilm formation in eight of the strains. The mRNA levels of the pirA and pirB toxin genes were decreased to 46% and 42% by indole-3-acetic acid in the AHPND-causing strain V. parahaemolyticus M0904. Hence, our data demonstrate that indole-3-acetic acid has the potential to be an effective virulence inhibitor to control infections in aquaculture.

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.

Microalgae as next generation plant growth additives: Functions, applications, challenges and circular bioeconomy based solutions

Sustainable agriculture practices involve the application of environment-friendly plant growth promoters and additives that do not negatively impact the health of the ecosystem. Stringent regulatory frameworks restricting the use of synthetic agrochemicals and the increase in demand for organically grown crops have paved the way for the development of novel bio-based plant growth promoters. In this context, microalgae biomass and derived agrochemicals offer novel sources of plant growth promotors that enhance crop productivity and impart disease resistance. These beneficial effects could be attributed to the presence of wide range of biomolecules such as soluble amino acid (AA), micronutrients, polysaccharides, phytohormones and other signaling molecules in microalgae biomass. In addition, their phototrophic nature, high photosynthetic efficiency, and wide environmental adaptability make them an attractive source of biostimulants, biofertilizers and biopesticides. The present review aims to describe the various plant growth promoting metabolites produced by microalgae and their effects on plant growth and productivity. Further, the effects elicited by microalgae biostimulants with respect to different modes of applications such as seed treatments, foliar spray and soil/root drenching is reviewed in detail. In addition, the ability of microalgae metabolites to impart tolerance against various abiotic and biotic stressors along with the mechanism of action is discussed in this paper. Although the use of microalgae based biofertilizers and biostimulants is gaining popularity, the high nutrient and water requirements and energy intensive downstream processes makes microalgae based technology commercially unsustainable. Addressing this challenge, we propose a circular economy model of microalgae mediated bioremediation coupled with biorefinery approaches of generating high value metabolites along with biofertilizer applications. We discuss and review new trends in enhancing the sustainability of microalgae biomass production by co-cultivation of algae with hydroponics and utilization of agriculture effluents.

Effect of Abscisic Acid on Growth, Fatty Acid Profile, and Pigment Composition of the Chlorophyte Chlorella (Chromochloris) zofingiensis and Its Co-Culture Microbiome

Microalga Chlorella (Chromochloris) zofingiensis has been gaining increasing attention of investigators as a potential competitor to Haematococcus pluvialis for astaxanthin and other xanthophylls production. Phytohormones, including abscisic acid (ABA), at concentrations relevant to that in hydroponic wastewater, have proven themselves as strong inductors of microalgae biomass productivity and biosynthesis of valuable molecules. The main goal of this research was to evaluate the influence of phytohormone ABA on the physiology of C. zofingiensis in a non-aseptic batch experiment. Exogenous ABA stimulated C. zofingiensis cell division, biomass production, as well as chlorophyll, carotenoid, and lipid biosynthesis. The relationship between exogenous ABA concentration and the magnitude of the observed effects was non-linear, with the exception of cell growth and biomass production. Fatty acid accumulation and composition depended on the concentration of ABA tested. Exogenous ABA induced spectacular changes in the major components of the culture microbiome of C. zofingiensis. Thus, the abundance of the representatives of the genus Rhodococcus increased drastically with an increase in ABA concentration, whereas the abundance of the representatives of Reyranella and Bradyrhizobium genera declined. The possibilities of exogenous ABA applications for the enhancing of the biomass, carotenoid, and fatty acid productivity of the C. zofingiensis cultures are discussed.

Diversity and Distribution of Carotenogenic Algae in Europe: A Review

Microalgae are the richest source of natural carotenoids, which are valuable pigments with a high share of benefits. Often, carotenoid-producing algae inhabit specific biotopes with unfavorable or even extremal conditions. Such biotopes, including alpine snow fields and hypersaline ponds, are widely distributed in Europe. They can serve as a source of new strains for biotechnology. The number of algal species used for obtaining these compounds on an industrial scale is limited. The data on them are poor. Moreover, some of them have been reported in non-English local scientific articles and theses. This review aims to summarize existing data on microalgal species, which are known as potential carotenoid producers in biotechnology. These include Haematococcus and Dunaliella, both well-known to the scientific community, as well as less-elucidated representatives. Their distribution will be covered throughout Europe: from the Greek Mediterranean coast in the south to the snow valleys in Norway in the north, and from the ponds in Amieiro (Portugal) in the west to the saline lakes and mountains in Crimea (Ukraine) in the east. A wide spectrum of algal secondary carotenoids is reviewed: β-carotene, astaxanthin, canthaxanthin, echinenone, adonixanthin, and adonirubin. For convenience, the main concepts of biology of carotenoid-producing algae are briefly explained.

Transitional traits determine the acclimation characteristics of the coccolithophore Chrysotila dentata to ocean warming and acidification

Ocean warming and acidification interactively affect the coccolithophore physiology and drives major biogeochemical changes. While numerous studies investigated coccolithophore under short-term conditions, knowledge on how different transitional periods over long-exposure could influence the element, macromolecular and metabolic changes for its acclimation are largely unknown. We cultured the coccolithophore Chrysotila dentata, (culture generations of 1st, 10th, and 20th) under present (low-temperature low-carbon-dioxide [LTLC]) and projected (high-temperature high-carbon-dioxide [HTHC]) ocean conditions. We examined elemental and macromolecular component changes and sequenced a transcriptome. We found that with long-exposure, most physiological responses in HTHC cells decreased when compared with those in LTLC, however, HTHC cell physiology showed constant elevation between each generation. Specifically, compared to 1st generation, the 20th generation HTHC cells showed increases in quota carbon (Qc:29%), nitrogen (Q_N :101%), and subsequent changes in C:N-ratio (68%). We observed higher lipid accumulation than carbohydrates within HTHC cells under long-exposure, suggesting that lipids were used as an alternative energy source for cellular acclimation. Protein biosynthesis pathways increased their efficiency during long-term HTHC condition, indicating that cells produced more proteins than required to initiate acclimation. Our findings suggest that the coccolithophore resilience increased between the 1st-10th generation to initiate the acclimation process under ocean warming and acidifying conditions.

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.

Cost Reduction in the Production of Spirulina Biomass and Biomolecules from Indole-3-Acetic Acid Supplementation in Different Growth Phases

Despite the great potential for the industrial application of microalgae, production costs are still too high to make them a competitive raw material for commodities. Therefore, studying more efficient cultivation strategies in biomass production and economic viability is necessary. In this sense, this work aimed to reduce the production costs of biomass and biomolecules using phytohormone indole-3-acetic acid in different phases of Spirulina sp. LEB 18 cultivation. The experiments were conducted on bench scale indoor for 30 days. In each couple of experiments, the phytohormone was added on different days. The supplementation of indole-3-acetic acid on half of the growth deceleration phase of the microalga showed a cost reduction of 27%, 34%, and 75% for biomass, proteins, and carbohydrates, respectively. In addition, the strategy increased the final biomass concentration and carbohydrate content at 31.2 and 33.8%, respectively, compared to the condition without phytohormone. This study is the starting point for implementing phytohormone supplementation in industrial microalgal cultures.

High-throughput interspecies profiling of acidic plant hormones using miniaturised sample processing

BACKGROUND

Acidic phytohormones are small molecules controlling many physiological functions in plants. A comprehensive picture of their profiles including the active forms, precursors and metabolites provides an important insight into ongoing physiological processes and is essential for many biological studies performed on plants.

RESULTS

A high-throughput sample preparation method for liquid chromatography-tandem mass spectrometry determination of 25 acidic phytohormones classed as auxins, jasmonates, abscisates and salicylic acid was optimised. The method uses a small amount of plant tissue (less than 10 mg fresh weight) and acidic extraction in 1 mol/L formic acid in 10% aqueous methanol followed by miniaturised purification on reverse phase sorbent accommodated in pipette tips organised in a 3D printed 96-place interface, capable of processing 192 samples in one run. The method was evaluated in terms of process efficiency, recovery and matrix effects as well as establishing validation parameters such as accuracy and precision. The applicability of the method in relation to the amounts of sample collected from distantly related plant species was evaluated and the results for phytohormone profiles are discussed in the context of literature reports.

CONCLUSION

The method developed enables high-throughput profiling of acidic phytohormones with minute amounts of plant material, and it is suitable for large scale interspecies studies.

Effect of the Application of Hydrolysate of Chlorella vulgaris Extracted by Different Techniques on the Growth of Pelargonium × hortorum

The extraction method used to obtain biologically active compounds from microalgal biomass may affect the biostimulant capacity of the microalgae. The objective of this assay was to determine the most efficient extraction method to release the active components of the biomass of Chlorella vulgaris (C. vulgaris). Plantlets of Pelargonium × hortorum were grown in a greenhouse and five treatments were applied: C-application with water; M-application with untreated C. vulgaris microalgae; M-US-application with C. vulgaris microalgae treated with ultrasound; M-USHY-application with C. vulgaris microalgae treated with ultrasound and enzymatic hydrolysis; and M-USHYAU-application with C. vulgaris microalgae treated with ultrasound, enzymatic hydrolysis, and autoclaving. All microalgae treatments increased shoot number and stem and plant diameter. The US-treated biomass increased the inflorescence of the plant significantly compared to the control. To extract bioactive compounds from eukaryotic microalgae for plant biostimulating purposes, the US-treatment (or any other method damaging the plasma membrane) of microalgae cell is, or seems to be, suitable.. Macronutrient content in leaves was not affected by the microalgae treatment, except for K.

Enhancement of plant growth regulators production from microalgae cultivated in treated sewage wastewater (TSW)

The aim of this work is to develop an efficient method for detection and evaluation of the plant growth regulators produced from cyanobacteria species (Anabaena oryzae and Nostoc muscorum) cultivated on BG11₀, and Chlorophyta alga (Chlorella vulgaris) cultivated on BG11 in addition to the cultivation of these strains on treated sewage wastewater (TSW) combined with control media (BG11 and BG11₀) at different concentrations (100, 75 and 50%). Bioassays were performed on Wheat coleoptile length and Cucumber cotyledons fresh weight for indole acetic acid (IAA) and Benzyl adenine (BA) detection. In addition, application experiments of IAA and BA presence in algal extract were applied on tomato plantlets and soybean callus. The obtained results of A. oryzae and N. muscorum extracts (grown on BG11₀ and 100% sewage media) with optimum conc. of IAA and BA showed moderate shoot length and leaves number as well as high root initiation of tomato explant compared to control. While dimethyl sulfoxide (DMSO), IAA conc. as well as IAA + BA conc. showed no effect on branching and leaf expansion. The results of C. vulgaris (grown on BG11) also revealed that the shoot had high leaves number and greatest root initiation, without branching and leaf expansion. On the other hand, 100% TSW had a moderate shoot, leaves number and high root initiation. Extracts of A. oryzae and N. muscorum (grown on BG11₀) induced 1.5-fold increase in soybean callus fresh weight, while the growth on 100% TSW was shown to be less effective. Moreover, extract of C. vulgaris (grown on BG11) induced a moderate effect, while its growth on 100% TSW was shown to be less effective in soybean callus fresh weight increment.

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.

Phloroglucinol promotes fucoxanthin synthesis by activating the cis-zeatin and brassinolide pathways in Thalassiosira pseudonana

Phloroglucinol improves shoot formation and somatic embryogenesis in several horticultural and grain crops, but its function in microalgae remains unclear. Here, we found that sufficiently high concentrations of phloroglucinol significantly increased fucoxanthin synthesis, growth, and photosynthetic efficiency in the microalga Thalassiosira pseudonana. These results suggested that the role of phloroglucinol is conserved across higher plants and microalgae. Further analysis showed that, after phloroglucinol treatment, the contents of cis-zeatin and brassinolide in T. pseudonana increased significantly, while the contents of trans-zeatin, iP, auxin, or gibberellin were unaffected. Indeed, functional studies showed that the effects of cis-zeatin and brassinolide in T. pseudonana were similar to those of phloroglucinol. Knockout of key enzyme genes in the cis-zeatin synthesis pathway of T. pseudonana or treatment of T. pseudonana with a brassinolide synthesis inhibitor (brassinazole) significantly reduced growth and fucoxanthin content in T. pseudonana, and phloroglucinol treatment partially alleviated these inhibitory effects. However, phloroglucinol treatment was ineffective when the cis-zeatin and brassinolide pathways were simultaneously inhibited. These results suggested that the cis-zeatin and brassinolide signaling pathways are independent regulators of fucoxanthin synthesis in T. pseudonana, and that phloroglucinol affects both pathways. Thus, this study not only characterizes the mechanism by which phloroglucinol promotes fucoxanthin synthesis, but also demonstrates the roles of cis-zeatin and brassinolide in T. pseudonana. IMPORTANCE Here, we demonstrate that phloroglucinol, a growth promoter in higher plants, also increases growth and fucoxanthin synthesis in the microalga Thalassiosira pseudonana, and therefore may have substantial practical application for industrial fucoxanthin production. Phloroglucinol treatment also induced the synthesis of cis-zeatin and brassinolide in T. pseudonana, and the cis-zeatin and brassinolide signaling pathways were implicated in the phloroglucinol-driven increases in T. pseudonana growth and fucoxanthin synthesis. Thus, our work clarified the molecular mechanism of phloroglucinol promoting the growth and fucoxanthin synthesis of Thalassiosira pseudonana, and suggested that cis-zeatin and brassinolide, in addition to phloroglucinol, had potential utility as inducers of increased microalgal fucoxanthin production.

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

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

Microalgae as biostimulants: a new approach in agriculture

This review aims to elucidate the state of the art of microalgae-based biostimulants as a tool in agriculture by summarizing the biologically active compounds factors that influence the use of microalgae biostimulants and their application methods in the field. Additionally, we examined the factors that support the use of microalgal biostimulants to face abiotic and biotic stress in crop plants. The use of microalgae in crop production and the benefits of seed preparation, foliar application, soil drenching, and hydroponic treatments were discussed. Furthermore, the use of these biostimulants in crop plants and their multiple benefits such as, better rooting, higher crop, fruit yields, drought and salinity tolerance, photosynthetic activity and pathogen resistance was thoroughly presented. The present situation of microalgal biostimulants and their difficulties in the market was analyzed, as well as the perspectives of their use. However, data shows that microalgal derived biostimulants can be used as an alternative for the protection of crops and plant growth regulators and play a significant key role in increasing the levels of production, yield and health of crops. Special interest needs to focus on investigating more microalgae species and their biological active compound factors, due to the largely untapped field. Perspectives regarding future research lines and development priorities were included.

Integrated microalgal biorefinery for the production and application of biostimulants in circular bioeconomy

Adverse detrimental impacts of environmental pollution over the health regimen of people has driven a shift in lifestyle towards cleaner and natural resources, especially in the aspects of food production and consumption. Microalgae are considered a rich source of high value metabolites to be utilized as plant growth biostimulants. These organisms however, are underrated compared to other microbial counterparts, due to inappropriate knowledge on the technical, enviro-economical constrains leading to low market credibility. Thus, to avert these issues, the present review comprehensively discusses the biostimulatory potential of microalgae interactively combined with circular bio-economy perspectives. The biochemical content and intracellular action mechanism of microalgal biostimulants were described. Furthermore, detailed country-wise market trends along with the description of the existing regulatory policies are included. Enviro-techno-economic challenges are discussed, and the consensus need for shift to biorefinery and circular bio-economy concept are emphasized to achieve sustainable impacts during the commercialization of microalgal biostimulants.

Microalgal and bacterial auxin biosynthesis: implications for algal biotechnology

Optimization of microalgal growth and high-value metabolite production are key steps in microalgal mass culture for the algae industry. An emerging technology is the use of phytohormones, like indole-3-acetic acid (IAA), to promote microalgal growth. This requires an understanding of the biosynthesis of IAA in microalgae-bacteria associations and its function in regulating algal physiology and metabolite production. We review the current advances in understanding of microalgal and bacterial auxin biosynthesis and their implications for algal biotechnology.

Organomineral fertilizers pastilles from microalgae grown in wastewater: Ammonia volatilization and plant growth

With the increasing demand for food, it is increasingly important to maintain soil fertility with the application of fertilizers to supply the nutritional needs of plants. However, the nutrients applied to the soil can suffer significant losses, impacting the environment, and increasing production costs. Using alternative sources, such as microalgae biomass (MB) generated in the treatment of wastewater, in the production of organomineral fertilizers is a way to recover nutrients from the sewage, in addition to contributing to the improvement in soil fertility and favoring crop growth, which can guarantee agricultural sustainability. In the present study, MB was grown in the effluent 00from the food industry and, subsequently, a pelleted organomineral fertilizer (POF) was produced consisting of the combination of MB and synthetic fertilizer (urea), in different proportions. The performance of the proposed fertilizer was analyzed for losses due to ammonia volatilization (N-NH₃) over time, for nitrogen assimilation capacity (N) by corn plants (Zea mays L.), and its structure was evaluated by scanning electron microscopy. The study concluded that the highest accumulated volatilization of N-NH₃ was in the proportion of 40% of MB and the maximum content of N is reached in the proportion of 24.55% of MB. From the proportion of 25% of MB, there is no increase in N absorbed by plants, at the same time that the volatilization of N-NH₃ grows with the increase in MB. The most important factors for obtaining these results were the interaction between MB and urea in the produced organomineral fertilizer tablet, where an increasingly thicker physical barrier was formed with the increase in the proportion of MB; in addition to the POF pH, in which the increase in MB proportions directly favored the pH increase.

On the Evolutionary Origins of Land Plant Auxin Biology

Indole-3-acetic acid, that is, auxin, is a molecule found in a broad phylogenetic distribution of organisms, from bacteria to eukaryotes. In the ancestral land plant auxin was co-opted to be the paramount phytohormone mediating tropic responses and acting as a facilitator of developmental decisions throughout the life cycle. The evolutionary origins of land plant auxin biology genes can now be traced with reasonable clarity. Genes encoding the two enzymes of the land plant auxin biosynthetic pathway arose in the ancestral land plant by a combination of horizontal gene transfer from bacteria and possible neofunctionalization following gene duplication. Components of the auxin transcriptional signaling network have their origins in ancestral alga genes, with gene duplication and neofunctionalization of key domains allowing integration of a portion of the preexisting transcriptional network with auxin. Knowledge of the roles of orthologous genes in extant charophycean algae is lacking, but could illuminate the ancestral functions of both auxin and the co-opted transcriptional network.

Strain-Specific Biostimulant Effects of Chlorella and Chlamydomonas Green Microalgae on Medicago truncatula

Microalgae have been identified to produce a plethora of bioactive compounds exerting growth stimulating effects on plants. The objective of this study was to investigate the plant-growth-promoting effects of three selected strains of eukaryotic green microalgae. The biostimulatory effects of two Chlorella species (MACC-360 and MACC-38) and a Chlamydomonas reinhardtii strain (cc124) were investigated in a Medicago truncatula model plant grown under controlled greenhouse conditions. The physiological responses of the M. truncatula A17 ecotype to algal biomass addition were characterized thoroughly. The plants were cultivated in pots containing a mixture of vermiculite and soil (1:3) layered with clay at the bottom. The application of live algae cells using the soil drench method significantly increased the plants’ shoot length, leaf size, fresh weight, number of flowers and pigment content. For most of the parameters analyzed, the effects of treatment proved to be specific for the applied algae strains. Overall, Chlorella application led to more robust plants with increased fresh biomass, bigger leaves and more flowers/pods compared to the control and Chlamydomonas-treated samples receiving identical total nutrients.

Marine microalgae co-cultured with floc-forming bacterium: Insight into growth and lipid productivity

This study investigated the effect of co-culturing microalgae with a floc-forming bacterium. Of the six microalgae isolated from a biofloc sample, only Thalassiosira weissflogii, Chlamydomonas sp. and Chlorella vulgaris were propagated successfully in Conway medium. Hence, these species were selected for the experiment comparing microalgae axenic culture and co-culture with the floc-forming bacterium, Bacillus infantis. Results obtained showed that the co-culture had higher microalgae biomass compared to the axenic culture. A similar trend was also observed concerning the lipid content of the microalgae-bacterium co-cultures. The cell number of B. infantis co-cultured with T. weissflogii increased during the exponential stage until the sixth day, but the other microalgae species experienced a significant early reduction in cell density of the bacteria at the exponential stage. This study represents the first attempt at co-culturing microalgae with B. infantis, a floc-forming bacterium, and observed increased biomass growth and lipid accumulation compared to the axenic culture.

Algae biostimulants: A critical look at microalgal biostimulants for sustainable agricultural practices

For the growing human population to be sustained during present climatic changes, enhanced quality and quantity of crops are essential to enable food security worldwide. The current consensus is that we need to make a transition from a petroleum-based to a bio-based economy via the development of a sustainable circular economy and biorefinery approaches. Both macroalgae (seaweeds) and microalgae have been long considered a rich source of plant biostimulants with an attractive business opportunity in agronomy and agro-industries. To date, macroalgae biostimulants have been well explored. In contrast, microalgal biostimulants whilst known to have positive effects on development, growth and yields of crops, their commercial implementation is constrained by lack of research and cost of production. The present review highlights the current knowledge on potential biostimulatory compounds, key sources and their quantitative information from algae. Specifically, we provide an overview on the prospects of microalgal biostimulants to advance crop production and quality. Key aspects such as specific biostimulant effects caused by extracts of microalgae, feasibility and potential of co-cultures and later co-application with other biostimulants/biofertilizers are highlighted. An overview of the current knowledge, recent advances and achievements on extraction techniques, application type, application timing, current market and regulatory aspects are also discussed. Moreover, aspects involved in circular economy and biorefinery approaches are also covered, such as: integration of waste resources and implementation of high-throughput phenotyping and -omics tools in isolating novel strains, exploring synergistic interactions and illustrating the underlying mode of microalgal biostimulant action. Overall, this review highlights the current and future potential of microalgal biostimulants, algal biochemical components behind these traits and finally bottlenecks and prospects involved in the successful commercialisation of microalgal biostimulants for sustainable agricultural practices.

Algae as New Kids in the Beneficial Plant Microbiome

Previously, algae were recognized as small prokaryotic and eukaryotic organisms found only in aquatic habitats. However, according to a recent paradigm shift, algae are considered ubiquitous organisms, occurring in plant tissues as well as in soil. Accumulating evidence suggests that algae represent a member of the plant microbiome. New results indicate that plants respond to algae and activate related downstream signaling pathways. Application of algae has beneficial effects on plant health, such as plant growth promotion and disease control. Although accumulating evidence suggests that secreted compounds and cell wall components of algae induce physiological and structural changes in plants that protect against biotic and abiotic stresses, knowledge of the underlying mechanisms and algal determinants is limited. In this review, we discuss recent studies on this topic, and highlight the bioprotectant and biostimulant roles of algae as a new member of the plant beneficial microbiome for crop improvement.

Exploring the Role of a Cytokinin-Activating Enzyme LONELY GUY in Unicellular Microalga Chlorella variabilis

LONELY GUY has been previously characterized in flowering plants to be involved in the direct activation of cytokinins. In this study, the function of the only LONELY GUY gene (CvarLOG1) from unicellular green microalga Chlorella variabilis NC64A has been investigated. CvarLOG1 expressed mainly in the lag and log phases of growth and was confirmed to be a cytokinin-activating enzyme. Overexpression of CvarLOG1 in Chlorella led to extended life in culture by almost 10-20 days, creating a "stay-green" phenotype. In the transformed alga, the cell cycle was lengthened due to delayed entry into the G2/M phase contrary to the known role of cytokinins in stimulating G2/M transition possibly due to excessive levels of this hormone. However, due to the sustained growth and delayed senescence, there was an increase in cell number by 11% and in biomass by 46% at the stationary phase, indicating a potential application for the biofuel industry. The total carbohydrate and lipid yield increased by approximately 30 and 20%, respectively. RNA-Seq-based transcriptomic analysis revealed that the genes associated with light and dark reactions of photosynthesis were upregulated, which may be the reason for the increased biomass. These data show that LOG plays an essential role during the cell cycle and in the functioning of the chloroplast and that the pathway leading to direct activation of cytokinins via LOG is functional in algae.

Cytokinin N-glucosides: Occurrence, Metabolism and Biological Activities in Plants

Cytokinins (CKs) are a class of phytohormones affecting many aspects of plant growth and development. In the complex process of CK homeostasis in plants, N-glucosylation represents one of the essential metabolic pathways. Its products, CK N7- and N9-glucosides, have been largely overlooked in the past as irreversible and inactive CK products lacking any relevant physiological impact. In this work, we report a widespread distribution of CK N-glucosides across the plant kingdom proceeding from evolutionary older to younger plants with different proportions between N7- and N9-glucosides in the total CK pool. We show dramatic changes in their profiles as well as in expression levels of the UGT76C1 and UGT76C2 genes during Arabidopsis ontogenesis. We also demonstrate specific physiological effects of CK N-glucosides in CK bioassays including their antisenescent activities, inhibitory effects on root development, and activation of the CK signaling pathway visualized by the CK-responsive YFP reporter line, TCSv2::3XVENUS. Last but not least, we present the considerable impact of CK N7- and N9-glucosides on the expression of CK-related genes in maize and their stimulatory effects on CK oxidase/dehydrogenase activity in oats. Our findings revise the apparent irreversibility and inactivity of CK N7- and N9-glucosides and indicate their involvement in CK evolution while suggesting their unique function(s) in plants.

The Role of Phytohormones in Enhancing Metal Remediation Capacity of Algae

Heavy metal (HM) contamination of the environment is a major issue worldwide, creating an ever-increasing demand for remediation techniques. Remediation with algae offers an ecologically safe, cost-effective, and efficient option for HM removal. Similar to plants, growth and development of algae are controlled by the hormonal system, where phytohormones are involved in HM tolerance and thus can regulate remediation ability; however, the underlying mechanisms of phytohormone function remain elusive. This review aims to draw a comprehensive model of phytohormone contributions to algal performance under HM stress. We focus on the mechanisms of HM biosorption, uptake and intracellular storage, and on how phytohormones interact with algal defence systems under HM exposure. We provide examples of successful utilization of algae in remediation, and of post-processing of algal materials. Finally, we discuss the advantages and risks of using algae for remediation. An in-depth understanding of these processes can inform effective HM remediation techniques.

Abundance and Extracellular Release of Phytohormones in Aero-terrestrial Microalgae (Trebouxiophyceae, Chlorophyta) As a Potential Chemical Signaling Source1

Phytohormones are pivotal signaling compounds in higher plants, in which they exert their roles intracellularly, but are also released for cell-to-cell communication. In unicellular organisms, extracellularly released phytohormones can be involved in chemical crosstalk with other organisms. However, compared to higher plants, hardly any knowledge is available on the roles of phytohormones in green algae. Here, we studied phytohormone composition and extracellular release in aero-terrestrial Trebouxiophyceae. We investigated (a) which phytohormones are produced and if they are released extracellularly, and if extracellular phytohormone levels are (b) affected by environmental stimuli, and (c) differ between lichen-forming and non-lichen-forming species. Three free-living microalgae (Apatococcus lobatus, Chloroidium ellipsoideum, and Myrmecia bisecta) and three lichen-forming microalgae (Asterochloris glomerata, Trebouxia decolorans, and Trebouxia sp.) were studied. Algae were grown on solid media and the following cellular phytohormones were identified by ultra-high-performance liquid chromatography coupled with tandem mass spectrometry (UHPLC-MS/MS): indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), abscisic acid (ABA), gibberellin A4 (GA4 ), and zeatin (ZT). Furthermore, IAA, IBA, ABA, jasmonic acid (JA), gibberellin A3 (GA3 ), and GA4 were found to be released extracellularly. IAA and ABA were released by all six species, and IAA was the most concentrated. Phytohormone release was affected by light and water availability, especially IAA in A. glomerata, Trebouxia sp., and C. ellipsoideum. No clear patterns were observed between lichen-forming and non-lichen-forming species. The results are envisaged to contribute valuable baseline information for further studies into the roles of phytohormones in microalgae.