सूक्ष्म शैवाल और बैक्टीरिया: पर्यावरण-अनुकूल सब्जी उत्पादन के लिए एक अदभुत जोड़ी

Contemporary agriculture's reliance on chemical fertilizers has led to significant environmental pollution and degradation of soil fertility. As an alternative, microalgae and plant growth-promoting bacteria (PGPB) have emerged as potential biofertilizers due to their ability to enhance soil fertility through the production of bioactive compounds, such as phytohormones, amino acids, and carotenoids, and their capacity to suppress plant pathogens.

Traditionally, agricultural treatments have focused on single-species applications of either microalgae or bacteria. However, recent experimental evidence highlights that a symbiotic relationship between microalgae and bacteria can positively influence each other's physiological and metabolic processes. This synergistic interaction is particularly promising for biotechnological applications, such as wastewater treatment and efficient biomass production, due to its enhanced effectiveness. Despite the promising potential, the interactions between microalgae and bacteria in agricultural contexts remain underexplored. This article explores the impact of microalgae and PGPB as biofertilizers on vegetable production and discusses the potential benefits of co-culturing these organisms for environmentally sustainable and improved vegetable cultivation.

Ecological Impact of Chemical Fertilizers in Agriculture

The widespread use of chemical fertilizers in agriculture has significantly increased crop yields and cultivation efficiency. However, this practice has led to substantial environmental problems, including water, soil, and air pollution. Excessive chemical fertilizer use causes soil acidification and hardening, reducing root vigor and respiration.

This practice also diminishes the population of beneficial microorganisms, resulting in decreased soil fertility and a higher incidence of root diseases. Nitrogen fertilizers, in particular, are absorbed by crops in reactive forms such as nitrate (NO₃), ammonia (NH₃), and nitrogen oxides (NOx).

While these forms can be naturally produced by soil microbes, their excessive presence from chemical fertilizers leads to soil and groundwater contamination and contributes to greenhouse gas emissions like nitrous oxide (N₂O).

Benefits of Biofertilizers in Agriculture

Biofertilizers offer a promising alternative to chemical fertilizers, helping to mitigate associated environmental issues. These preparations contain living or dormant cells that promote plant growth through the production of phytohormones and other beneficial substances, facilitating eco-friendly and sustainable agriculture.

Biofertilizers enhance the decomposition of organic matter, aiding soil mineralization, which increases nutrient availability and improves crop yields. Additionally, biofertilizers boost the quantity and diversity of beneficial bacteria, such as plant growth-promoting rhizobacteria (PGPR), including species like Azotobacter, Bacillus, Burkholderia, Pantoea, Pseudomonas, Serratia, and Streptomyces.

These bacteria, which have co-evolved with plants, often act as facultative intracellular endophytes, providing numerous benefits through direct or indirect pathways. Some PGPR strains enhance plant growth by synthesizing and metabolizing phytohormones or influencing their biosynthesis, while others produce substances that combat soil-borne pathogens.

As consumer interest in safe agricultural products has grown, the use of beneficial microbes as biofertilizers has become increasingly important for sustainable crop production and food safety.

Evaluation of Microalgae as Biofertilizers for Vegetable Production

Impact of Microalgae as Biofertilizers on Crop Cultivation

Recent research indicates that microalgae are increasingly used in agriculture, not just for bioremediation and biofuel production, but also as biofertilizers. Microalgae produce a variety of bioactive compounds, including plant growth-promoting substances such as phytohormones, amino acids, carotenoids, and phycobilins, which enhance crop productivity by promoting plant growth and providing resistance against pathogens with minimal environmental impact.

Microalgal extracts contain phytohormones like auxin, cytokinin, abscisic acid, ethylene, and gibberellin, which are crucial for regulating plant growth and development. Consequently, these extracts serve as renewable plant biostimulants. Auxin, a key regulator of plant developmental processes such as cell division and elongation, is found in microalgae as indole-3-acetic acid (IAA) and indole-3-butanoic acid (IBA), both of which can influence plant growth and metabolism. Cytokinins are involved in root and shoot development, leaf senescence, nutrient mobilization, and seed germination, while gibberellins promote cell division, pigment and protein accumulation, and cell elongation and expansion. Ethylene helps plants tolerate abiotic stresses such as drought, low temperatures, and high salinity, and biotic stresses like pathogen attacks. Thus, phytohormones not only promote plant growth and development but also enhance plant defense mechanisms through complex interaction networks.

Microalgae Species Used in Vegetable Cultivation

The growing body of evidence on the beneficial effects of microalgae on plant growth and pathogen resistance has led to the use of various microalgae species in cultivating key vegetables. Specifically, species such as Chlorella vulgaris, Chlorella fusca, and Spirulina platensis have been utilized to enhance the production and market quality of tomatoes, cucumbers, onions, lettuce, and peppers.

Table 1: Effects of Microalgae on Vegetable Production

Source: Kang et al., (2021)

Evaluation of Bacteria as Biofertilizers for Vegetable Production

Impact of Bacteria as Biofertilizers on Crop Cultivation

Bacteria play a crucial role as decomposers and recyclers in soil ecosystems, contributing to nutrient cycling, energy flow, and bioconversion processes. Agricultural production systems heavily depend on soil bacterial biomass, which allows for rapid adaptation to environmental changes. Effective microorganisms (EM) are microbial inoculums comprising mixed cultures of beneficial microorganisms, such as lactic acid bacteria, photosynthetic bacteria, yeast, Actinomyces, and fermenting fungi, which enhance soil microbial diversity.

Plant growth-promoting bacteria (PGPB) form specific symbiotic relationships with plants, directly promoting growth by facilitating resource acquisition and modulating plant hormone levels. The application of PGPB in vegetable cultivation can reduce the reliance on chemical fertilizers by up to 30%, lowering production costs and minimizing pollution. Additionally, PGPB can enhance plant defenses against soil-borne pathogens by producing antibiotics such as 2,4-diacetylphloroglucinol (2,4-DAPG), pyoluteorin (PLT), pyrrolnitrin, and phenazine-1-carboxylate.

Bacterial Species Used in Vegetable Cultivation

Numerous bacterial species have been demonstrated to promote the growth and development of vegetables while controlling pathogens through various mechanisms, including the production of inhibitory substances that confer disease resistance to crops. Particularly, several Bacillus species, such as Bacillus amyloliquefaciens, B. subtilis, B. cereus, B. licheniformis, and B. pumilus, are commercially utilized to enhance the growth of vegetables like tomatoes, cucumbers, onions, lettuce, and peppers, as well as to manage plant pathogens. For example, B. cereus UW85 produces zwittermicin A and kanosamine, which suppress damping-off disease caused by Phytophthora medicaginis in alfalfa.

Other bacterial species like Serratia liquefaciens and Pseudomonas putida produce N-acyl-L-homoserine lactone (AHL) signaling molecules that enhance systemic resistance in tomato plants against the leaf fungal pathogen Alternaria alternata. Furthermore, Burkholderia anthina has been observed to improve the growth traits of tomato plants by increasing soil phosphorus content under greenhouse conditions. Azotobacter chroococcum and Pseudomonas fluorescens also enhance vegetative growth and yield in onion production by producing indole-3-acetic acid (IAA), siderophores, and solubilizing tricalcium phosphate (TCP).

Table 2: Bacterial Effects on Vegetable Production

Source: Kang et al., (2021)

Relationship between Microalgae and Bacteria

Microalgae–Bacteria Interactions

In natural environments, microalgae and bacteria coexist and engage in various interactions that can be both beneficial and harmful, depending on the species involved and environmental conditions. These interactions influence the growth and development of both groups through the production of growth factors or exotoxins.

Beneficial Interactions

In beneficial relationships, microalgae facilitate bacterial growth by supplying photosynthetic oxygen and dissolved organic matter, such as organic carbon, calcium carbonate, and 2,3-dihydroxypropane-1-sulfonate (DHPS). Photosynthetic oxygen from microalgae or cyanobacteria acts as an electron acceptor in bacterial organic matter degradation. Bacteria, in return, support the photoautotrophic growth of microalgae by providing carbon dioxide and other stimulatory compounds. Additionally, bacteria can supply essential micronutrients like B-vitamins, which serve as co-factors for enzyme activities critical to the central metabolism of microalgae.

Microalgae also benefit from bacterial activity through the acquisition of nutrients such as inorganic carbon, nitrogen, phosphorus, and sulfate, which are made available from organic matter decomposition by extracellular bacterial enzymes.

Harmful Interactions

Conversely, in harmful interactions, microalgae can inhibit bacterial growth by releasing antibacterial metabolites and altering the culture medium's pH, dissolved oxygen concentration, and temperature.

Combined Benefits for Plant Growth

Both microalgae and plant growth-promoting bacteria (PGPB) can enhance plant growth through the production of polysaccharides and phytohormones such as auxin and cytokinin. Additionally, they contribute to plant disease prevention by activating plant defense systems and secreting antifungal enzymes and antibiotics.

These intricate interactions between microalgae and bacteria, whether beneficial or harmful, highlight the complex dynamics within natural ecosystems and their potential applications in sustainable agriculture. By leveraging these natural relationships, we can develop biofertilizers that enhance crop productivity and resilience while minimizing environmental impacts.

Co-Culturing/ Combination of Microalgae–Bacteria

Microalgae–Bacteria Co-culture for Wastewater Treatment and Biomass Production

The co-culturing of bacteria and microalgae has emerged as a valuable approach in wastewater treatment and biomass production. Studies, such as those conducted by Mujtaba et al. (2017), have showcased the effectiveness of symbiotic co-cultures in removing nutrients (e.g., ammonium, phosphate) and reducing chemical oxygen demand (COD) from wastewater, employing microalgae like immobilized Chlorella vulgaris alongside bacteria such as Pseudomonas putida. Symbiotic co-culture systems have demonstrated superior performance compared to monocultures by simultaneously targeting a broader range of nutrients in wastewater.

For instance, Ogbonna et al. (2000) observed that monocultures of Rhodobacter sphaeroides, Chlorella sorokiniana, and Spirulina platensis were incapable of removing all wastewater nutrients such as acetate, propionate, ammonia, nitrate, and phosphorus simultaneously. In contrast, co-culture systems effectively removed these nutrients, highlighting the efficiency of symbiotic interactions in wastewater treatment. Moreover, the co-cultivation of Azospirillum brasilense and Scenedesmus sp. has shown promise in biofuel production, resulting in increased biomass yields and fatty acid content, particularly in nitrogen-deficient media. This underscores the potential of co-culturing for enhancing microalgal colony size and biofuel production.

Table 3: Examples of Co-inoculation of Microalgae–Bacteria in Agriculture

Source: Kang et al., (2021)

Enhanced Efficiency in Detoxification and Nutrient Removal

The combination of cyanobacteria/microalgae with bacteria represents a powerful approach for enhancing the detoxification of organic and inorganic pollutants and the removal of nutrients from wastewater, surpassing the capabilities of either group alone. Heavy metal contamination in soils and aquatic systems poses significant threats to crop production and food safety. Microalgae play a crucial role in detoxifying and volatilizing heavy metals through metabolic processes and the formation of metal-binding peptides like class III metallothionein (MtIII), which help regulate cytoplasmic heavy metal concentrations and mitigate toxicity.

For instance, microalgae have been shown to bioabsorb and biotransform arsenate (As) in rice fields, reducing its availability to plants and enhancing the safety of food grains for consumption. The synergistic interactions between algae and bacteria have demonstrated increased efficiency in removing heavy metals when bacterial inoculum is added, facilitating algal growth through the provision of additional CO₂ and organic compounds by bacteria.

Furthermore, microalgae used in these processes can be recycled as biofertilizing agents, providing an additional benefit to agricultural practices. This dual application highlights the versatility and environmental advantages of employing microalgae–bacteria co-cultures in sustainable agricultural and environmental management practices.

Potential of Microalgae–Bacteria Co-cultures/Combination for Vegetable Cultivation

The simultaneous use of microalgae and bacteria in vegetable cultivation offers two distinct application methods: co-culturing the microbes from the beginning or preparing a mixture containing microalgae/microalgal extract and bacteria obtained from pure cultures.

Synergistic Effects in Vegetable Cultivation

The combined application of specific bacteria, acting as plant growth promoters or biocontrol agents against plant pathogens, can create synergies ideal for vegetable cultivation. For instance, co-culturing Pantoea ananatis and Pseudomonas fluorescens (CPP-2) resulted in higher production of indole-3-acetic acid (IAA) and phosphate solubilization compared to either strain alone. Moreover, this co-culture promoted root and shoot elongation of pea (Pisum sativum) plants more effectively than individual strain cultures. Similarly, combining Pseudomonas putida WCS358 and RE8 enhanced the suppression of Fusarium wilt in radish by approximately 50% compared to the untreated control, surpassing the effects of single-strain treatments. Even when one strain failed to suppress disease individually, the combination treatment still exhibited a suppressive effect, indicating the additive/combinational effect of different disease-suppressive mechanisms.

Conclusion

Microalgae and bacteria have received great interest as biofertilizers in ecofriendly vegetable production. Until now, monoculture systems using certain agricultural microorganisms have been highlighted to improve the yield and quality of agricultural products. However, co-culture/combination systems of microorganisms can be more effective in enhancing microbial diversity in the soil, resistance to plant diseases, and productivity of vegetable crops. Therefore, further investigations to uncover the molecular mechanisms underlying the effect of microalgae–bacteria co-culture/combination on vegetable growth, and/or plant disease suppression, will be necessary for the extension of sustainable agriculture.

Authors:

Shivam Kumar Rai, Akanksha, Abhinav Dayal, and Prashant Kumar Rai

Department of Genetics and Plant Breeding,

Sam Higginbottom University of Agriculture, Technology and Sciences, Prayagraj

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