Agronomic Biofertilizer Potential of a Cyanobacterium-Enriched Culture.

Microalgae- and cyanobacteria-based biofertilizers are valuable resources for sustainable agricultural practices. Though abundant in soils, their biotechnological potential remains underexplored, with only a few species acknowledged for agricultural applications. In this study, the plant growth-promoting effects of a nonaxenic culture, dominated by a filamentous cyanobacterium, were evaluated. In an initial screening, the conditioned culture medium increased root length and stimulated the formation of lateral roots in Arabidopsis thaliana and Lactuca sativa seedlings, consistent with the cyanobacterium-enriched culture capacity to produce auxins and/or modulate plant auxin metabolism, as observed using auxin-GUS A. thaliana reporter plants. Furthermore, cocultivation of the cyanobacterium-enriched culture with L. sativa plants, both in pots and more notably in a commercial hydroponic system, resulted in a significant plant growth-promoting effect. In the hydroponic system, cocultivation increased root and shoot biomass by approximately 60%, root length by 30%, and the number of leaves by 20%. This effect was concurrent with enhanced plant carbon and nitrogen metabolism, including higher photosynthesis rate, increased carbohydrate and pigment content, and enhanced activity of nitrogen-related enzymes. Shotgun sequencing led to the identification of a novel cyanobacterial species belonging to the Tildeniella genus that harbors genomic traits related to nitrogen fixation, siderophore production and auxin biosynthesis. This study contributes to a better understanding of the molecular mechanisms underlying the nonsymbiotic interaction between cyanobacteria and plants and opens new avenues for the use of cyanobacterial species in sustainable hydroponic agricultural practices.