Evaluation of biostimulant and biocidal activities of four Nostoc strains and perspective of exploitation in green agriculture

Since the first green revolution, the diffusion and massive use of agrochemicals have caused multiple ecological damages. However, over the last century, the main challenge has remained the same: achieving food security. This is becoming increasingly difficult as agricultural production is under pressure from non-climate stressors and climate change causing, among others, increased exposure to abiotic stresses and the spread of pests. In this context, the search for sustainable sources of growth promoters and plant protection products is crucial, and microalgae can represent a huge and mostly unexplored source of bioactivity to be screened. In fact, cyanobacteria are known producers of antifungal and, to a lesser extent, antibacterial compounds which have been so far investigated mainly for pharmaceutical applications and only in few cases for the agricultural sector. Moreover, besides their more common use as biofertilizer and soil amendments, cyanobacteria are known to produce several bioactive compounds that may promote plant growth and increase tolerance to abiotic stresses suggesting their use as biostimulants. In this work, the bioactivity spectrum of four Nostoc strains (FACHB261, PTA251, F&M-C98 and F&M-C177) was determined on different target organisms to assess their possible exploitation in agriculture. Strains were cultivated for 13 days in BG11 medium in bubble tubes photobioreactors (surface/volume ratio 37 m-1) bubbled with air:CO2 (98:2, v:v) and illuminated with continuous light (150 µmol hotons m-2 s-1 ). Then hydrophilic (HE) and lipophilic (LE) extracts were prepared form the lyophilised biomass and tested at different concentrations in bioassays on Arabidopsis thaliana and against a panel of phytopathogenic organisms including 4 bacteria, 1 fungus and one insect. Cytotoxicity against Artemia salina was also determined for a first assessment of extracts toxicity.