Exploring microalga and streptomycete co-cultures as plant growthpromoting systems for sustainable agriculture

The extensive use of chemical fertilizers and pesticides has caused significant environmental damage and increased economic burden, prompting the search for sustainable alternatives. Biofertilizers, which consist of living microorganisms possessing plant growth promoting (PGP) capabilities, represent an eco-friendly approach to enhance plant growth through nature-based strategies improving plant performance through targeted modulation of the soil and plant microbiome. Members of the genus Streptomyces are well known for producing a wide range of bioactive compounds, including antibiotics, antifungals, and plant growth-promoting molecules such as indole-3-acetic acid (IAA), highlighting their remarkable metabolic diversity (Fadetta et al., 2023; Salwan et al., 2020). Similarly, photosynthetic microorganisms, such as microalgae, play a crucial role in carbon fixation and in the production of valuable biomolecules, including antitumor and antimicrobial compounds (Licata et al., 2025). In this study, the bacterium Streptomyces violaceoruber and three microalgal strain -Dactylococcopsis salina, Dunaliella sp., and Chlorella sp. were first evaluated individually for their PGP effects. All tested strains exhibited PGP traits, including salt tolerance, indole-3-acetic acid (IAA) production, and the ability to solubilize phosphates, under in vitro conditions using Solanum lycopersicum as the tester plant. Microalgae treatments significantly increased shoot fresh biomass and enhanced key morphological parameters, including primary root length and hypocotyl elongation. Based on these preliminary results, Chlorella sp., which showed the strongest PGP performance among the tested microalgae, was selected for co-culture experiments with S. violaceoruber to assess potential synergistic effects on bioactive compound production and plant growth promotion. Notably, the Chlorella–Streptomyces co-culture treatments further improved plant performance. Metabolomic analyses are currently underway to identify bioactive compounds potentially involved in the observed plant growth stimulation. These findings highlight the potential of microalgae and microalgae–bacteria consortia as sustainable tools for agriculture, providing a promising platform for developing innovative biotechnological applications aligned with circular economy principles.