Screening of microalgae liquid extracts for their bio stimulant properties on plant growth, nutrient uptake and metabolite profile of Solanum lycopersicum L

Growth enhancement, metabolic profile improvement, and DXR and TPS2 gene expression changes in Thymus vulgaris L. by cyanobacterial inoculation

BACKGROUND

In recent decades cyanobacterial species have attracted research attention as potential sources of new biostimulants. In this study, the biostimulant effects of five cyanobacterial suspensions on the growth and essential oil composition of Thymus vulgaris L. were evaluated. The expression of key genes involved in the biosynthesis of thymol and carvacrol, such as DXR and TPS2, were investigated.

RESULTS

A pot culture experiment revealed that cyanobacterial application significantly improved T. vulgaris L. growth indices, including plant height, dry and fresh weight, leaf and flower number, leaf area, and photosynthetic pigment content. Total phenol and flavonoid content in inoculated plants also showed a significant increase compared with the control. Anabaena torulosa ISB213 inoculation significantly increased root and shoot biomass by about 65.38% and 92.98% compared with the control, respectively. Nostoc calcicola ISB215 inoculation resulted in the highest amount of essential oil accumulation (18.08 ± 0.62) in T. vulgaris leaves, by about 72.19% compared with the control (10.5 ± 0.50%). Interestingly, the amount of limonene in the Nostoc ellipsosporum ISB217 treatment (1.67%) increased significantly compared with the control and other treatments. The highest expression rates of DXR and TPS2 genes were observed in the treatment of N. ellipsosporum ISB217, with 5.92-fold and 5.22-fold increases over the control, respectively.

CONCLUSION

This research revealed the potential of the cyanobacteria that were studied as promising biostimulants to increase the production of biomass and secondary metabolites of T. vulgaris L., which could be a suitable alternative to chemical fertilizers. © 2024 Society of Chemical Industry.

Applications of the Microalgae Chlamydomonas and Its Bacterial Consortia in Detoxification and Bioproduction

The wide metabolic diversity of microalgae, their fast growth rates, and low-cost production make these organisms highly promising resources for a variety of biotechnological applications, addressing critical needs in industry, agriculture, and medicine. The use of microalgae in consortia with bacteria is proving valuable in several areas of biotechnology, including the treatment of various types of wastewater, the production of biofertilizers, and the extraction of various products from their biomass. The monoculture of the microalga Chlamydomonas has been a prominent research model for many years and has been extensively used in the study of photosynthesis, sulphur and phosphorus metabolism, nitrogen metabolism, respiration, and flagellar synthesis, among others. Recent research has increasingly recognised the potential of Chlamydomonas-bacteria consortia as a biotechnological tool for various applications. The detoxification of wastewater using Chlamydomonas and its bacterial consortia offers significant potential for sustainable reduction of contaminants, while facilitating resource recovery and the valorisation of microalgal biomass. The use of Chlamydomonas and its bacterial consortia as biofertilizers can offer several benefits, such as increasing crop yields, protecting crops, maintaining soil fertility and stability, contributing to CO2 mitigation, and contributing to sustainable agricultural practises. Chlamydomonas-bacterial consortia play an important role in the production of high-value products, particularly in the production of biofuels and the enhancement of H2 production. This review aims to provide a comprehensive understanding of the potential of Chlamydomonas monoculture and its bacterial consortia to identify current applications and to propose new research and development directions to maximise their potential.

The Microalgae Chlamydomonas for Bioremediation and Bioproduct Production

The extensive metabolic diversity of microalgae, coupled with their rapid growth rates and cost-effective production, position these organisms as highly promising resources for a wide range of biotechnological applications. These characteristics allow microalgae to address crucial needs in the agricultural, medical, and industrial sectors. Microalgae are proving to be valuable in various fields, including the remediation of diverse wastewater types, the production of biofuels and biofertilizers, and the extraction of various products from their biomass. For decades, the microalga Chlamydomonas has been widely used as a fundamental research model organism in various areas such as photosynthesis, respiration, sulfur and phosphorus metabolism, nitrogen metabolism, and flagella synthesis, among others. However, in recent years, the potential of Chlamydomonas as a biotechnological tool for bioremediation, biofertilization, biomass, and bioproducts production has been increasingly recognized. Bioremediation of wastewater using Chlamydomonas presents significant potential for sustainable reduction in contaminants and facilitates resource recovery and valorization of microalgal biomass, offering important economic benefits. Chlamydomonas has also established itself as a platform for the production of a wide variety of biotechnologically interesting products, such as different types of biofuels, and high-value-added products. The aim of this review is to achieve a comprehensive understanding of the potential of Chlamydomonas in these aspects, and to explore their interrelationship, which would offer significant environmental and biotechnological advantages.

Advances in the Development of Biofertilizers and Biostimulants from Microalgae

Microalgae have commercial potential in different sectors of the industry. Specifically in modern agriculture, they can be used because they have the ability to supply nutrients to the soil and produce plant growth hormones, polysaccharides, antimicrobial compounds, and other metabolites that improve agricultural productivity. Therefore, products formulated from microalgae as biofertilizers and biostimulants turn out to be beneficial for agriculture and are positioned as a novel and environmentally friendly strategy. However, these bioproducts present challenges in preparation that affect their shelf life due to the rapid degradation of bioformulated products. Therefore, this work aimed to provide a comprehensive review of biofertilizers and biostimulants from microalgae, for which a bibliometric analysis was carried out to establish trends using scientometric indicators, technological advances were identified in terms of formulation methods, and the global market for these bioproducts was analyzed.

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.

A Comprehensive Review of Microalgae and Cyanobacteria-Based Biostimulants for Agriculture Uses

Significant progress has been achieved in the use of biostimulants in sustainable agricultural practices. These new products can improve plant growth, nutrient uptake, crop yield and quality, stress adaptation and soil fertility, while reducing agriculture's environmental footprint. Although it is an emerging market, the biostimulant sector is very promising, hence the increasing attention of the scientific community and agro-industry stakeholders in finding new sources of plant biostimulants. Recently, pro- and eucaryotic microalgae have gained prominence and can be exploited as biostimulants due to their ability to produce high-value-added metabolites. Several works revealed the potential of microalgae- and cyanobacteria-based biostimulants (MCBs) as plant growth promoters and stress alleviators, as well as encouraging results pointing out that their use can address current and future agricultural challenges. In contrast to macroalgae biostimulants, the targeted applications of MBs in agriculture are still in their earlier stages and their commercial implementation is constrained by the lack of research and cost of production. The purpose of this paper is to provide a comprehensive overview on the use of this promising new category of plant biostimulants in agriculture and to highlight the current knowledge on their application prospects. Based on the prevailing state of the art, we aimed to roadmap MCB formulations from microalgae and cyanobacteria strain selection, algal biomass production, extraction techniques and application type to product commercialization and farmer and consumer acceptance. Moreover, we provide examples of successful trials demonstrating the beneficial applications of microalgal biostimulants as well as point out bottlenecks and constraints regarding their successful commercialization and input in sustainable agricultural practices.

Effect of a Fortified Biostimulant Extract on Tomato Plant Productivity, Physiology, and Growing Media Properties

The pursuit of sustainable and productive agriculture demands the exploration of innovative approaches to improve plant productivity and soil health. The utilization of natural agricultural biostimulants, such as extracts from seaweed, fish, and humus, has gained prominence as an ecological strategy to achieve this goal. In this study we investigated the effectiveness of a fortified biostimulant extract (FBE), composed of extracts from seaweed, fish, and humus, on tomato plant physiology, productivity, and growing media properties, and estimated carbon emissions associated with tomato production. The FBE was applied to the growing media of tomato plants produced in a greenhouse, in experiments over two growing seasons. The productivity assessments demonstrated that the application of FBE significantly increased tomato fruit yield by 20% and relative marketable fruit yield by 27%, and reduced estimated greenhouse gas (GHG) emissions associated with production by 29%. FBE treatment improved plant shoot and root biomass, accelerated flower and fruit set initiation, and increased chlorophyll content in leaves, resulting in enhanced plant physiology and advanced development. FBE treatment positively influenced the availability of crucial nutrients such as nitrogen, phosphorus, and iron in the growing media. FBE promoted the growth of total active microbes in the growing media, particularly the fungal population, which plays an important role in nutrient cycling and health. These findings highlight the beneficial effects of the FBE due to enhanced plant productivity and growth, improved fertility, the promotion of beneficial plant and growing media interactions, and the reduction in estimated GHG emissions.

Mitigation of Huanglongbing: Implications of a Biologically Enhanced Nutritional Program on Yield, Pathogen Localization, and Host Gene Expression Profiles

Huanglongbing (HLB, citrus greening disease), the most destructive disease affecting citrus production, is primarily linked to the gram-negative, insect-vectored, phloem-inhabiting α-proteobacterium 'Candidatus Liberibacter asiaticus' (CLas). With no effective treatment available, management strategies have largely focused on the use of insecticides in addition to the destruction of infected trees, which are environmentally hazardous and cost-prohibitive for growers, respectively. A major limitation to combating HLB is the inability to isolate CLas in axenic culture, which hinders in vitro studies and creates a need for robust in situ CLas detection and visualization methods. The aim of this study was to investigate the efficacy of a nutritional program-based approach for HLB treatment, and to explore the effectiveness of an enhanced immunodetection method to detect CLas-infected tissues. To achieve this, four different biologically enhanced nutritional programs (bENPs; P1, P2, P3, and P4) were tested on CLas-infected citrus trees. Structured illumination microscopy preceded by a modified immunolabeling process and transmission electron microscopy were used to show treatment-dependent reduction of CLas cells in phloem tissues. No sieve pore plugging was seen in the leaves of P2 trees. This was accompanied by an 80% annual increase in fruit number per tree and 1,503 (611 upregulated and 892 downregulated) differentially expressed genes. These included an MLRQ subunit gene, UDP-glucose transferase, and genes associated with the alpha-amino linolenic acid metabolism pathway in P2 trees. Taken together, the results highlight a major role for bENPs as a viable, sustainable, and cost effective option for HLB management.

Nano-silicone and Ascophyllum nodosum-based biostimulant down-regulates the negative effect of in vitro induced-salinity in Rosa damascena

BACKGROUND

Rosa damascena is extensively cultivated in various regions of Iran due to its aesthetic attributes, medicinal qualities, and essential oil production. This study investigated the efficacy of Ascophyllum nodosum extract (AnE) at concentrations of 0, 2, and 3 g L- 1 and Nano-silicon (nSiO2) at concentrations of 0, 50, and 100 mg L- 1 in ameliorating the impact of salinity on two genotypes of Damask rose ('Chaharfasl' and 'Kashan') under in vitro culture conditions. Additionally, various physio-chemical characteristics of R. damascena explants were assessed.

RESULTS

The findings revealed that exposure to 100 mM NaCl resulted in a substantial reduction in the Relative Water Content (RWC), Membrane Stability Index (MSI), leaf pigments (Chlorophyll b, Chlorophyll a, total Chlorophyll, and carotenoids), chlorophyll fluorescence parameters, and protein content in both genotypes when compared to control conditions. Salinity induced a significant increase in the parameter F0 and a decrease in the parameter Fv/Fm compared to the control conditions in both genotypes. Nonetheless, the genotype Kashan treated with 3 g L- 1 AnE + 100 mg L- 1 nSiO2 exhibited the maximum Fm value under control conditions, with a significant difference compared to other treatments. Furthermore, salinity caused a considerable reduction in Fm in both 'Kashan' and 'Chaharfasl' by 22% and 17%, respectively, when compared to the control condition. 'Kashan' displayed the maximum Fv/Fm compared to the other genotype. The maximum levels of Malondialdehyde (MAD) and hydrogen peroxide (H2O2) were also observed in explants affected by salinity. The combination of 3 g L- 1 AnE + 100 mg L- 1 nSiO2, followed by 2 g L- 1 AnE + 100 mg L- 1 nSiO2, exhibited substantial positive effects. Salinity also led to an increase in proline content and the activity of peroxidase (POD), superoxide dismutase (SOD), guaiacol peroxidase (GPX), and catalase (CAT) in both genotypes. The activity of these enzymes was further enhanced when AnE was applied at concentrations of 2 and 3 g L- 1 in combination with 100 mg L- 1 nSiO2.

CONCLUSIONS

The 'Kashan' genotype displayed greater tolerance to salinity by enhancing water balance, maintaining membrane integrity, and augmenting the activity of antioxidant enzymes compared to 'Chaharfasl'. The utilization of nSiO2 and AnE biostimulants demonstrated potential benefits for R. damascena, both under salinity and control conditions. These findings hold substantial importance for researchers, policymakers, and farmers, offering valuable insights into the development of salinity-tolerant crop varieties.

Algal Bio-Stimulants Enhance Salt Tolerance in Common Bean: Dissecting Morphological, Physiological, and Genetic Mechanisms for Stress Adaptation

Salinity adversely affects the plant's morphological characteristics, but the utilization of aqueous algal extracts (AE) ameliorates this negative impact. In this study, the application of AE derived from Chlorella vulgaris and Dunaliella salina strains effectively reversed the decline in biomass allocation and water relations, both in normal and salt-stressed conditions. The simultaneous application of both extracts in salt-affected soil notably enhanced key parameters, such as chlorophyll content (15%), carotene content (1%), photosynthesis (25%), stomatal conductance (7%), and transpiration rate (23%), surpassing those observed in the application of both AE in salt-affected as compared to salinity stress control. Moreover, the AE treatments effectively mitigated lipid peroxidation and electrolyte leakage induced by salinity stress. The application of AE led to an increase in GB (6%) and the total concentration of free amino acids (47%) by comparing with salt-affected control. Additionally, salinity stress resulted in an elevation of antioxidant enzyme activities, including superoxide dismutase, ascorbate peroxidase, catalase, and glutathione reductase. Notably, the AE treatments significantly boosted the activity of these antioxidant enzymes under salinity conditions. Furthermore, salinity reduced mineral contents, but the application of AE effectively counteracted this decline, leading to increased mineral levels. In conclusion, the application of aqueous algal extracts, specifically those obtained from Chlorella vulgaris and Dunaliella salina strains, demonstrated significant efficacy in alleviating salinity-induced stress in Phaseolus vulgaris plants.

Effects of Poultry Manure on the Growth, Physiology, Yield, and Yield-Related Traits of Maize Varieties

Industries play a significant role in the improvement of lifestyle and in the development of a country. However, the byproducts from these industries are a source of environmental pollution. The proper use of the byproducts of these industries can help to cope with environmental pollution. Some byproducts have high nutritional content and are good for crop plants. The purpose of this research was to investigate the effect of different rates of poultry manure on the soil chemical properties, growth, and yield of maize. A pot experiment was conducted in the botanical garden of the Department of Botany, University of Sargodha, Pakistan to investigate the effect of various treatments of poultry manure (0, 25, 50, 75, and 100 g/pot) on the morphological, physiological, and yield attributes of two maize varieties, Pearl and MMRI. Treatment T₁ was a mixture of soil and 75 g/pot poultry manure, T₂ was a mixture of soil and 50 g/pot poultry manure, T₃ was a mixture of soil and 25 g/pot poultry manure, and T₄ was 100 g/pot poultry manure. Soil without any industrial byproduct (100% soil only) was used as the control (T₀). The results revealed that the use of poultry manure enhanced the physical properties of the soil. Available P and soil organic matter were improved in soil amended with poultry manure. It is evident from the results that the vegetative growth of both maize varieties was significantly enhanced by growing in soil amended with poultry manure as compared to their respective control. Similar responses were also recorded for the physiological attributes of leaf area, photosynthetic rate, transpiration rate, stomatal conductance, and water use efficiency of both varieties. Yield and yield-contributing traits of both maize varieties were significantly improved by growing plants in soil amended with 50 and 75 g/pot of poultry manure. It is also inferred that the use of 50 g/pot poultry manure in soil amendment is an eco-friendly and economically effective option for maize growers of arid and semiarid regions to enhance the kernel yield and profit per annum. Poultry manure could be useful to ameliorate the adverse effects of salinity stress on all parameters, particularly the grain yield. Furthermore, this would be a useful and economical method for the safe disposal of byproducts.

Biostimulant Properties of Protein Hydrolysates: Recent Advances and Future Challenges

Over the past decade, plant biostimulants have been increasingly used in agriculture as environment-friendly tools that improve the sustainability and resilience of crop production systems under environmental stresses. Protein hydrolysates (PHs) are a main category of biostimulants produced by chemical or enzymatic hydrolysis of proteins from animal or plant sources. Mostly composed of amino acids and peptides, PHs have a beneficial effect on multiple physiological processes, including photosynthetic activity, nutrient assimilation and translocation, and also quality parameters. They also seem to have hormone-like activities. Moreover, PHs enhance tolerance to abiotic stresses, notably through the stimulation of protective processes such as cell antioxidant activity and osmotic adjustment. Knowledge on their mode of action, however, is still piecemeal. The aims of this review are as follows: (i) Giving a comprehensive overview of current findings about the hypothetical mechanisms of action of PHs; (ii) Emphasizing the knowledge gaps that deserve to be urgently addressed with a view to efficiently improve the benefits of biostimulants for different plant crops in the context of climate change.

Cyanobacteria as a Valuable Natural Resource for Improved Agriculture, Environment, and Plant Protection

Taking into consideration, the challenges faced by the environment and agro-ecosystem make increased for suggestions more reliable methods to help increase food security and deal with difficult environmental problems. Environmental factors play a critical role in the growth, development, and productivity of crop plants. Unfavorable changes in these factors, such as abiotic stresses, can result in plant growth deficiencies, yield reductions, long-lasting damage, and even death of the plants. In reflection of this, cyanobacteria are now considered important microorganisms that can improve the fertility of soils and the productivity of crop plants due to their different features like photosynthesis, great biomass yield, ability to fix the atmospheric N2, capability to grow on non-arable lands, and varied water sources. Furthermore, numerous cyanobacteria consist of biologically active substances like pigments, amino acids, polysaccharides, phytohormones, and vitamins that support plant growth enhancement. Many studies have exposed the probable role of these compounds in the alleviation of abiotic stress in crop plants and have concluded with evidence of physiological, biochemical, and molecular mechanisms that confirm that cyanobacteria can decrease the stress and induce plant growth. This review discussed the promising effects of cyanobacteria and their possible mode of action to control the growth and development of crop plants as an effective method to overcome different stresses.

Graphical Abstract

Aphanothece sp. as promising biostimulant to alleviate heavy metals stress in Solanum lycopersicum L. by enhancing physiological, biochemical, and metabolic responses

Heavy metals (H.M) are a major environmental concern around the world. They have harmful impact on plant productivity and pose a serious risk to humans and animals health. In the present study, we investigated the effect of Aphanothece crude extract (ACE) on physiological, biochemical, and metabolic responses of tomato plant exposed to 2 mM Pb and Cd. The results showed a significant reduction of tomato plant weights and perturbation in nutrients absorption under 2 mM Pb and Cd conditions. Moreover, ACE treatment showed a significant enhancement of plant biomass compared to plants under Pb and Cd. On the other hand, ACE application favoured H.M accumulation in root and inhibited their translocation to shoot. In addition, ACE treatment significantly enhanced several stress responses in plant under Pb and Cd stress such as scavenging enzymes and molecules: POD, CAT, SOD, proline, and polyphenols etc. Furthermore, ACE treatment showed remodulation of metabolic pathways related to plant tolerance such as wax construction mechanism, particularly SFA, UFA, VLFA, alkanes, alkenes, and sterols biosynthesis to enhance tolerance and resistance to H.M stress. In the present study, we emphasized that ACE alleviates H.M stress by minimizing metal translocation to above-part of plant and enhancing plant growth, nutrients absorption, and biochemical responses.

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.

Microalgae from Nordic collections demonstrate biostimulant effect by enhancing plant growth and photosynthetic performance

We investigated the biostimulant potential of six microalgal species from Nordic collections extracted with two different procedures: thermal hydrolysis with a weak solution of sulfuric acid accompanied by ultrasonication and bead-milling with aqueous extraction followed by centrifugation. To this aim, we designed a phenotyping pipeline consisting of a root growth assay in the model plant Arabidopsis thaliana, complemented with greenhouse experiments to evaluate lettuce yield (Lactuca sativa L. cv. Finstar) and photosynthetic performance. The best-performing hydrolyzed extracts stimulated Arabidopsis root elongation by 8%-13% and lettuce yield by 12%-15%. The in situ measured photosynthetic performance of lettuce was upregulated in the efficient extracts: PSII quantum yield increased by 26%-34%, and thylakoid proton flux increase was in the range of 34%-60%. In contrast, aqueous extracts acquired by bead-milling showed high dependence on biomass concentration in the extract and an overall plant growth enhancement was not attained in any of the applied dosages. Our results indicate that hydrolysis of the biomass can be a decisive factor for rendering effective plant biostimulants from microalgae.

The bioherbicidal potential of isolated fungi cultivated in microalgal biomass

This study evaluated the bioherbicidal potential of wild fungi grown on microalgal biomass from the digestate treatment of biogas production. Four fungal isolates were used and the extracts were evaluated for the activity of different enzymes and characterized by gas chromatography coupled with mass spectrometry. The bioherbicidal activity was assessed by application on Cucumis sativus, and the leaf damage was visually estimated. The microorganisms showed potential as agents producing an enzyme pool. The obtained fungal extracts presented different organic compounds, most acids, and when applied to Cucumis sativus, showed high levels of leaf damage (80-100 ± 3.00%, deviation relative to the observed average damage). Therefore, the microbial strains are potential biological control agents of weeds, which, together with the microalgae biomass, offer the appropriate conditions to obtain an enzyme pool of biotechnological relevance and with favorable characteristics to be explored as bioherbicides, addressing aspects within the environmental sustainability.

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.

Fermentation as a Promising Tool to Valorize Rice-Milling Waste into Bio-Products Active against Root-Rot-Associated Pathogens for Improved Horticultural Plant Growth

In this study, water extracts from fermented (F), ultrasonicated (US), and enzyme-hydrolyzed (E) rice bran (RB) were evaluated against sixteen fungal plant stem and root-rot-associated pathogens. The effects of pre-treated RB additives on plant growth substrate (PGS) on bean and tomato seed germination, stem height and root length of seedlings, and chlorophyll concentration in plants were analyzed. The results showed that US-assisted pre-treatments did not affect protein content in RB, while 36 h semi-solid fermentation (SSF) reduced protein content by 10.3–14.8%. US initiated a 2.9- and 2-fold increase in total sugar and total phenolics (TPC) contents compared to the untreated RB (3.89 g/100 g dw and 0.61 mg GAE/g dw, respectively). Lactic acid (19.66–23.42 g/100 g dw), acetic acid (10.54–14.24 g/100g dw), propionic acid (0.40–1.72 g/100 g dw), phenolic compounds (0.82–1.04 mg GAE/g dw), among which phenolic acids, such as p-coumaric, cinnamic, sinapic, vanillic, and ferulic, were detected in the fermented RB. The RBF extracts showed the greatest growth-inhibition effect against soil-born plant pathogens, such as Fusarium, Pythium, Sclerotinia, Aspergillus, Pseudomonas, and Verticillium. Beans and tomatoes grown in RBUS+E- and RBF-supplemented PGS increased the germination rate (14–75%), root length (21–44%), and stem height (25–47%) compared to seedlings grown in PGS. The RB additives increased up to 44.6–48.8% of the chlorophyll content in both plants grown under greenhouse conditions. The results indicate that the biological potential of rice-milling waste as a plant-growth-promoting substrate component can be enhanced using solid-state fermentation with antimicrobial LABs and US processing.

Application of microalgae Chlamydomonas applanata M9V and Chlorella vulgaris S3 for wheat growth promotion and as urea alternatives

Excessive use of chemical fertilizers to meet the global food demand has caused extensive environmental pollution. Microalgae can be used to enhance agricultural crop production as a potentially sustainable and eco-friendly alternative. In this study, Chlamydomonas applanata M9V and Chlorella vulgaris S3 were isolated from the soil and mass-cultured for use as microalgal fertilizers. The influence of microalgae M9V and S3 on the growth of wheat (Triticum aestivum L.) and soil properties was evaluated and compared with that of chemical urea fertilizer. A pot experiment was conducted with six treatments, i.e., living M9V (M9VL), dead M9V (M9VD), living S3 (S3L), dead S3 (S3D), urea fertilizer (urea), and control without fertilizer (control). M9VL was found to have the best effect on wheat growth promotion, followed by M9VD and S3D. In addition, M9VL resulted in the highest enhancement of shoot fresh weight (166.67 and 125.68%), root dry weight (188.89 and 77.35%), leaf length (26.88 and 14.56%), root length (46.04 and 43.93%), chlorophyll a (257.81 and 82.23%), and chlorophyll b contents (269.00 and 247.27%) comparing to the control and urea treatments, respectively. Moreover, all microalgal fertilizer treatments increased soil organic matter (SOM) by 1.77–23.10%, total carbon (TC) by 7.14–14.46%, and C:N ratio by 2.99–11.73% compared to the control and urea treatments. Overall, this study provided two microalgae strains, M9V and S3, that could promote wheat growth and improve soil properties, thus highlighting the use of microalgae as biofertilizers to reduce the use of chemical fertilizers and promoting sustainable agricultural production.

Fungal Contamination in Microalgal Cultivation: Biological and Biotechnological Aspects of Fungi-Microalgae Interaction

In the last few decades, the increasing interest in microalgae as sources of new biomolecules and environmental remediators stimulated scientists’ investigations and industrial applications. Nowadays, microalgae are exploited in different fields such as cosmeceuticals, nutraceuticals and as human and animal food supplements. Microalgae can be grown using various cultivation systems depending on their final application. One of the main problems in microalgae cultivations is the possible presence of biological contaminants. Fungi, among the main contaminants in microalgal cultures, are able to influence the production and quality of biomass significantly. Here, we describe fungal contamination considering both shortcomings and benefits of fungi-microalgae interactions, highlighting the biological aspects of this interaction and the possible biotechnological applications.

Presence and future of plant phenotyping approaches in biostimulant research and development

Commercial interest in biostimulants as a tool for sustainable green economics and agriculture concepts is on a steep rise, being followed by increasing demand to employ efficient scientific methods to develop new products and understand their mechanisms of action. Biostimulants represent a highly diverse group of agents derived from various natural sources. Regardless of their nutrition content and composition, they are classified by their ability to improve crop performance through enhanced nutrient use efficiency, abiotic stress tolerance, and quality of crops. Numerous reports have described modern, non-invasive sensor-based phenotyping methods in plant research. This review focuses on applying phenotyping approaches in biostimulant research and development, and maps the evolution of interaction of these two intensively growing domains. How phenotyping served to identify new biostimulants, the description of their biological activity, and the mechanism/mode of action are summarized. Special attention is dedicated to the indoor high-throughput methods using model plants suitable for biostimulant screening and developmental pipelines, and high-precision approaches used to determine biostimulant activity. The need for a complex method of testing biostimulants as multicomponent products through integrating other -omic approaches followed by advanced statistical/mathematical tools is emphasized.

Biostimulants for Resilient Agriculture: A Preliminary Assessment in Italy

In agriculture, plant biostimulants have become necessary to meet the United Nations sustainable development goals (UN-SDGs) and advance the European Green Deal. In particular, seaweed-based biostimulants have received a greater acceptance for their several benefits in crop growth and yield. In this study, we evaluated the effects of foliar applications of a vegetable- and brown-algae-based extract (Ascophyllum nodosum (L.) Le Jol. on grapes (Vitis vinifera L. cv. Montepulciano) and olives (Olea europaea L. cv. Coratina) and its agronomic performance in two field experiments in the Apulia region, which is known for its modern agricultural sector. The results highlight that the crop responses differ in grape and olive orchards. The biostimulant application determined significant increases in bunch development (+9.5%) and bunch weight (+10%) compared to the untreated control. In the olive orchard, the yield was not significantly influenced by biostimulant application, whereas we observed quality improvement in the olive oil of the treated plants compared to the control. To better understand the mechanisms behind this difference, the research concludes by suggesting that further research pursues in-depth studies and high scientific and technical proficiency to determine and optimise the rates and timing of applications.

Impact of the microalga Dunaliella salina (Dunal) Teodoresco culture and its β-carotene extract on the development of salt-stressed squash (Cucurbita pepo L. cv. Mabrouka)

Salinity is a major threat to crop production and global food security. Algae and their extracts containing bioactive compounds can enhance the salt tolerance of plants, including the salt-sensitive plants. The current study evaluated the efficacy of Dunaliella salina (Dunal) Teodoresco culture and/or its β-carotene extract in improving the salt tolerance of squash (Cucurbita pepo L. cv. Mabrouka). Amendment of C. pepo with D. salina culture and/or its β-carotene extract was more effective in alleviating the impact of moderate salinity imposed by seawater dilution of 2.5 dS m^-1 than either low (0.55 dS m^-1) or high (3.5 dS m^-1) salinity, with a comparable effect to that of salicylic acid (SA). Plants that received a combination of D. salina culture and its β-carotene extract showed significantly higher growth (total biomass, fruit productivity) and physiological attributes (photosynthetic pigments, nitrogen (N), phosphorus (P), and potassium (K⁺) contents) than those receiving either amendment alone, reaching up to 80-90% of the SA-treated plants at moderate salinity (2.5 dS m^-1). The combination could enhance the antioxidant activity of moderately salt-stressed C. pepo via increasing carotenoids and phenolics contents, suggesting that this combination could enhance the adaptation of C. pepo to the moderate salinity. The present study recommends using the blooms of D. salina and its β-carotene that is naturally secreted in situ in natural or synthetic open systems in improving the salt tolerance of C. pepo instead of using the expensive synthetic hormones.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01176-6.

Microalgae based wastewater treatment coupled to the production of high value agricultural products: Current needs and challenges

One of the main social and economic challenges of the 21st century will be to overcome the worlds' water deficit expected by the end of this decade. Microalgae based wastewater treatment has been suggested as a strategy to recover nutrients from wastewater while simultaneously producing clean water. Consortia of microalgae and bacteria are responsible for recovering nutrients from wastewater. A better understanding of how environmental and operational conditions affect the composition of the microalgae-bacteria consortia would allow to maximise nutrient recoveries and biomass productivities. Most of the studies reported to date showed promising results, although up-scaling of these processes to reactors larger than 100 m² is needed to better predict their industrial relevance. The main advantage of microalgae based wastewater treatment is that valuable biomass with unlimited applications is produced as a co-product. The aim of the current paper was to review microalgae based wastewater treatment processes focusing on strategies that allow increasing both biomass productivities and nutrient recoveries. Moreover, the benefits of microalgae based agricultural products were also discussed.

Screening of Leafy Vegetable Varieties with Low Lead and Cadmium Accumulation Based on Foliar Uptake

Leafy vegetables cultivated in kitchen gardens and suburban areas often accumulate excessive amounts of heavy metals and pose a threat to human health. For this reason, plenty of studies have focused on low accumulation variety screening. However, identifying specific leafy vegetable varieties according to the foliar uptake of air pollution remains to be explored (despite foliar uptake being an important pathway for heavy-metal accumulation). Therefore, in this study, the lead (Pb) and cadmium (Cd) contents, leaf morphology, and particle matter contents were analyzed in a micro-area experiment using 20 common vegetables. The results show that the Pb content in leaves ranged from 0.70 to 3.86 mg kg⁻¹, and the Cd content ranged from 0.21 to 0.99 mg kg⁻¹. Atmospheric particles were clearly scattered on the leaf surface, and the particles were smaller than the stomata. Considering the Pb and Cd contents in the leaves and roots, stomata width-to-length ratio, leaf area size, enrichment factor, and translocation factor, Yidianhongxiancai, Qingxiancai, Baiyuanyexiancai, Nanjingjiangengbai and Sijixiaobaicai were recommended for planting in kitchen gardens and suburban areas as they have low accumulation characteristics. Identifying the influencing factors in the accumulation of heavy metals in vegetables through foliar uptake is important to help plant physiologists/environmentalists/policy makers to select suitable varieties for planting in air-polluted areas and thus reduce their threat to human health.

Integration of Phenomics and Metabolomics Datasets Reveals Different Mode of Action of Biostimulants Based on Protein Hydrolysates in Lactuca sativa L. and Solanum lycopersicum L. Under Salinity

Plant phenomics is becoming a common tool employed to characterize the mode of action of biostimulants. A combination of this technique with other omics such as metabolomics can offer a deeper understanding of a biostimulant effect in planta. However, the most challenging part then is the data analysis and the interpretation of the omics datasets. In this work, we present an example of how different tools, based on multivariate statistical analysis, can help to simplify the omics data and extract the relevant information. We demonstrate this by studying the effect of protein hydrolysate (PH)-based biostimulants derived from different natural sources in lettuce and tomato plants grown in controlled conditions and under salinity. The biostimulants induced different phenotypic and metabolomic responses in both crops. In general, they improved growth and photosynthesis performance under control and salt stress conditions, with better performance in lettuce. To identify the most significant traits for each treatment, a random forest classifier was used. Using this approach, we found out that, in lettuce, biomass-related parameters were the most relevant traits to evaluate the biostimulant mode of action, with a better response mainly connected to plant hormone regulation. However, in tomatoes, the relevant traits were related to chlorophyll fluorescence parameters in combination with certain antistress metabolites that benefit the electron transport chain, such as 4-hydroxycoumarin and vitamin K1 (phylloquinone). Altogether, we show that to go further in the understanding of the use of biostimulants as plant growth promotors and/or stress alleviators, it is highly beneficial to integrate more advanced statistical tools to deal with the huge datasets obtained from the -omics to extract the relevant information.

Physiology of microalgae and their application to sustainable agriculture: A mini-review

Concern that depletion of fertilizer feedstocks, which are a finite mineral resource, threatens agricultural sustainability has driven the exploration of sustainable methods of soil fertilization. Given that microalgae, which are unicellular photosynthetic organisms, can take up nutrients efficiently from water systems, their application in a biological wastewater purification system followed by the use of their biomass as a fertilizer alternative has attracted attention. Such applications of microalgae would contribute to the accelerated recycling of nutrients from wastewater to farmland. Many previous reports have provided information on the physiological characteristics of microalgae that support their utility. In this review, we focus on recent achievements of studies on microalgal physiology and relevant applications and outline the prospects for the contribution of microalgae to the establishment of sustainable agricultural practices.

The potential use of Chlamydomonas reinhardtii and Chlorella sorokiniana as biostimulants on maize plants

Nitrogen deficiency and drought stress are among the major stresses faced by plants with negative consequence on crop production. The use of plant biostimulants is a very promising application in agriculture to improve crop yield, but especially to prevent the effect of abiotic stresses. Algae-derived biostimulants represent an efficient tool to stimulate the root development: while macroalgae have already been widely adopted as a source of biostimulants to improve plants growth and resilience, far less information is available for microalgae. The objective of this work is to investigate the stimulant ability on maize roots of two green algae species, Chlamydomonas reinhardtii and Chlorella sorokiniana, being respectively the model organism for Chlorophyta and one of the most promising species for microalgae cultivation at industrial scale. The results obtained demonstrate that both C. reinhardtii and C. sorokiniana cells promoted the development of maize root system compared to the untreated negative control. C. sorokiniana specifically increased the number of secondary roots, while improved micro-nutrients accumulation on roots and shoots was measured in the case of C. reinhardtii treated plants. When these microalgae-derived biostimulants were applied on plants grown in stress conditions as nitrogen deficiency, improved development of the root system was measured in the case of plants treated with C. sorokiniana biomass. Microalgae cultivation for biostimulant production can thus be considered as a bio-based process providing solutions for improving plant resilience toward stress conditions.

Plant-cyanobacteria interactions: Beneficial and harmful effects of cyanobacterial bioactive compounds on soil-plant systems and subsequent risk to animal and human health

Plant-cyanobacteria interactions occur in different ways and at many different levels, both beneficial and harmful. Plant-cyanobacteria interactions, as a beneficial symbiosis, have long been demonstrated in rice-growing areas (Poaceae) where the most efficient nitrogen-fixing cyanobacteria are present in paddies. Moreover, cyanobacteria may in turn produce and/or secrete numerous bioactive compounds that have plant growth-promoting abilities or that may make the plant more resistant to abiotic or biotic stress. In recent years, there has been a growing worldwide interest in the use of cyanobacterial biomass as biofertilizers to replace chemical fertilizers, in part to overcome increasing organic-farming demands. However, the potential presence of harmful cyanotoxins has delayed the use of such cyanobacterial biomass, which can be found in large quantities in freshwater ecosystems around the world. In this review, we describe the existing evidence for the positive benefit of plant-cyanobacteria interactions and discuss the use of cyanobacterial biomass as biofertilizers and its growing worldwide interest. Although mass cyanobacterial blooms and scums are a current and emerging threat to the degradation of ecosystems and to animal and human health, they may serve as a source of numerous bioactive compounds with multiple positive effects that could be of use as an alternative to chemical fertilizers in the context of sustainable development.

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.

Alginate and fucoidan changes the bacterial community in different directions and the alginate or fucoidan degrading bacteria isolated from paddy soil promotes the plant growth

Seaweed and its extracts have been developed as fertilizers because they possess plant-growth-promoting and antibacterial compounds. For use as fertilizers, the major carbohydrates in seaweed, including fucoidan and alginate, need to be efficiently digested in the soil. We isolated fucoidan/alginate degrading bacteria from paddy soil and verified its use as a biofertilizer. Results show that Stenotrophomonas pavanii has a high alginate degrading activity, and also stimulating melon, pepper, and tomato growth. The growth stimulation effect of the bacteria was enhanced by alginate treatment. Bacillus sp. was isolated as a fucoidan degrading bacterium and this bacterium was also able to stimulate melon growth. Using 16S ribosomal DNA analysis, fucoidan/alginate resistant or susceptible bacteria were successively selected. Bacteria with increased population due to fucoidan and alginate had specificity to each carbohydrate, whereas those with decreased population showed susceptibility to both carbohydrates. This report demonstrates some bacteria for their use as biofertilizers with seaweed and demonstrated that a high throughput method is efficient in identifying bacteria with specific properties.

Consumer Attitudes towards Microalgae Production and Microalgae-Based Agricultural Products: The Cases of Almería (Spain) and Livorno (Italy)

The production of microalgal biomass and products derived thereof for a wide variety of applications is a hot research topic, with the number of facilities being built and products and biologically active molecules launched into the market increasing every year. The aim of the current study was to identify the attitudes of citizens in Almería (Spain) and Livorno (Italy) towards the construction of a microalgae production plant and a biorefinery in their cities and also their opinions about the microalgae-based products that could be produced. Overall, in Almería (Spain), a NIMBY (not in my back yard) attitude towards the construction of a microalgal production facility and especially towards a microalgal biorefinery was observed, despite the strong microalgal industry in the region and the higher knowledge of citizens about microalgae. In both locations, but especially in Livorno (Italy), microalgae-based biostimulants, biofertilisers, and aquafeeds were well accepted. Proximity was the main factor affecting the acceptance of a microalgae producing facility. Consumer knowledge about microalgal biotechnology and the health and environmental benefits of this valuable raw material are scarce, and opinions are based on drivers other than knowledge. After gaining more knowledge about microalgal biorefineries, most of the responses in Almería (47%) and Livorno (61%) were more positive.

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.

Plant Biostimulants from Cyanobacteria: An Emerging Strategy to Improve Yields and Sustainability in Agriculture

Cyanobacteria can be considered a promising source for the development of new biostimulants as they are known to produce a variety of biologically active molecules that can positively affect plant growth, nutrient use efficiency, qualitative traits of the final product, and increase plant tolerance to abiotic stresses. Moreover, the cultivation of cyanobacteria in controlled and confined systems, along with their metabolic plasticity, provides the possibility to improve and standardize composition and effects on plants of derived biostimulant extracts or hydrolysates, which is one of the most critical aspects in the production of commercial biostimulants. Faced with these opportunities, research on biostimulant properties of cyanobacteria has undergone a significant growth in recent years. However, research in this field is still scarce, especially as regards the number of investigated cyanobacterial species. Future research should focus on reducing the costs of cyanobacterial biomass production and plant treatment and on identifying the molecules that mediate the biostimulant effects in order to optimize their content and stability in the final product. Furthermore, the extension of agronomic trials to a wider number of plant species, different application doses, and environmental conditions would allow the development of tailored microbial biostimulants, thus facilitating the diffusion of these products among farmers.

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.

Evaluation of microalgae polysaccharides as biostimulants of tomato plant defense using metabolomics and biochemical approaches

Microalgal polysaccharides (PSs) may be an effective elicitor agent that can efficiently protect plants against biotic stresses. In this study, wee investigates, the effect of PS obtained from microalgae and cyanobacteria (D. salina MS002, P. tricorontum MS023, Porphyridium sp. MS081, Desmodesmus sp., D. salina MS067 and A. platensis MS001) on the biochemical and metabolomics markers linked to defense pathways in tomato plants. The phenylalanine ammonia lyase (PAL), chitinase, 1,3-beta-glucanase and peroxidase (POX) activities have been improved in tomato plants leaves treated by polysaccharides extracted from P. triocnutum (238.26%); Desmodesmus sp. (19.95%); P. triocnutum (137.50%) and Porphyridium sp. (47.28%) respectively. For proteins, polyphenols and H2O2, the maximum effect was induced by D. salina 067 (55.01%), Porphyridium sp. (3.97%) and A. platensis (35.08%) respectively. On the other hand, Gas Chromatography-mass spectrometry (GC-MS) metabolomics analysis showed that PSs induced the modification of metabolite profile involved in the wax construction of tomato leaves, such as fatty acids, alkanes, alkenes and phytosterol. PS treatments improved the accumulation of fatty acids C16:3, C18:2 and C18:3 released from the membrane lipids as precursors of oxylipin biosynthesis which are signaling molecules of plant defense. In addition, PS treatment induced the accumulation of C18:0 and Azelaic acid which is a regulator of salicylic acid-dependent systemic acquired resistance. However, molecular and metabolic studies can determine more precisely the mode of action of microalgal polysaccharides as biostimulants/elicitors plant defense.

Physiological, Nutritional and Metabolomic Responses of Tomato Plants After the Foliar Application of Amino Acids Aspartic Acid, Glutamic Acid and Alanine

Agriculture is facing a great number of different pressures due to the increase in population and the greater amount of food it demands, the environmental impact due to the excessive use of conventional fertilizers, and climate change, which subjects the crops to extreme environmental conditions. One of the solutions to these problems could be the use of biostimulant products that are rich in amino acids (AAs), which substitute and/or complement conventional fertilizers and help plants adapt to climate change. To formulate these products, it is first necessary to understand the role of the application of AAs (individually or as a mixture) in the physiological and metabolic processes of crops. For this, research was conducted to assess the effects of the application of different amino acids (Aspartic acid (Asp), Glutamic acid (Glu), L-Alanine (Ala) and their mixtures Asp + Glu and Asp + Glu + Ala on tomato seedlings (Solanum lycopersicum L.). To understand the effect of these treatments, morphological, physiological, ionomic and metabolomic studies were performed. The results showed that the application of Asp + Glu increased the growth of the plants, while those plants that received Ala had a decreased dry biomass of the shoots. The greatest increase in the growth of the plants with Asp + Glu was related with the increase in the net CO2 assimilation, the increase of proline, isoleucine and glucose with respect to the rest of the treatments. These data allow us to conclude that there is a synergistic effect between Aspartic acid and Glutamic acid, and the amino acid Alanine produces phytotoxicity when applied at 15 mM. The application of this amino acid altered the synthesis of proline and the pentose-phosphate route, and increased GABA and trigonelline.

Enhancing Sustainability by Improving Plant Salt Tolerance through Macro- and Micro-Algal Biostimulants

Algal biomass, extracts, or derivatives have long been considered a valuable material to bring benefits to humans and cultivated plants. In the last decades, it became evident that algal formulations can induce multiple effects on crops (including an increase in biomass, yield, and quality), and that algal extracts contain a series of bioactive compounds and signaling molecules, in addition to mineral and organic nutrients. The need to reduce the non-renewable chemical input in agriculture has recently prompted an increase in the use of algal extracts as a plant biostimulant, also because of their ability to promote plant growth in suboptimal conditions such as saline environments is beneficial. In this article, we discuss some research areas that are critical for the implementation in agriculture of macro- and microalgae extracts as plant biostimulants. Specifically, we provide an overview of current knowledge and achievements about extraction methods, compositions, and action mechanisms of algal extracts, focusing on salt-stress tolerance. We also outline current limitations and possible research avenues. We conclude that the comparison and the integration of knowledge on the molecular and physiological response of plants to salt and to algal extracts should also guide the extraction procedures and application methods. The effects of algal biostimulants have been mainly investigated from an applied perspective, and the exploitation of different scientific disciplines is still much needed for the development of new sustainable strategies to increase crop tolerance to salt stress.

Characterization of Endogenous Auxins and Gibberellins Produced by Chlorella sorokiniana TH01 under Phototrophic and Mixtrophic Cultivation Modes toward Applications in Microalgal Biorefinery and Crop Research

Microalgae have been reported to produce endogenous phytohormones including auxins, gibberellins, cytokinins, brassinosteroids, and abscisic acid. Methanol residual released from microalgal lipid extraction usually contains a variety of bioactive compounds including the phytohormones; however, they are poorly characterized and used for other applications. This study aimed at investigating auxin, gibberellin, and cytokinin production of C. sorokiniana TH01 under phototrophic and mixtrophic cultivations. Moreover, endogenous auxins, gibberellins, and cytokinins in methanol residual obtained from the algal lipid extraction were characterized using HPLC-ESI-MS/MS toward application for crop and biorefinery research. Data showed that endogenous indole-3-acetic acid (IAA), 3-indolepropionic acid (IPA), gibberellin A4 (GA4), and gibberellin A7 (GA7) were detected in C. sorokiniana TH01 biomass. Under the phototrophic mode, total auxin and GA levels were reduced to 0.98 and 9.65 μg/g DW under salt stress (20 g NaCl/L) from 3.59 to 24.71 μg/g DW, respectively, measured for the control. Similarly, total auxins and GAs were also decreased to 0.56 and 2.86 μg/g DW, respectively, under mixtrophic growth with 6 g glucose/L. Total auxins and GAs determined in the water algal extract were 1062.7 and 2000.1 μg/L, respectively. Treatment with higher 40% (v/v) of the algal extract triggered earlier seed germination of rice and tomato plants in 2 and 1 days, respectively. Our new findings in capability of C. sorokiniana TH01 in endogenous phytohormone production contain fundamental merits for further optimization of the algal production (i.e., cultivation modes, conditions, lipids, biomass productivity, and hormone levels) to be used for biorefinery.