Biostimulant potential of Limnospira platensis biomass fermented with lactic acid bacteria in common bean (Phaseolus vulgaris): strain characterization, biochemical shifts, and growth promotion

The excessive use of chemical fertilizers and pesticides in conventional agriculture poses significant risks to soil health, environmental quality, and food security. Plant biostimulants derived from microorganisms, microalgae and cyanobacteria offer a sustainable alternative. Lactic acid bacteria (LAB) and cyanobacteria such as Limnospira platensis are recognized for their plant growth-promoting characteristics. However, the combined effects of LAB-fermented L. platensis biomass on common bean ( Phaseolus vulgaris L.) remain underexplored. This investigation assessed the phytostimulant potential of L. platensis extracts, live LAB, and LAB-fermented L. platensis extract on growth parameters, yield, soil microbiota, and chlorophyll content of P. vulgaris under field conditions. Five treatments were evaluated: T 1 (water, negative control), T 2 (commercial biostimulant containing humic/fulvic acids, amino acids, N-P-K, and micronutrients), T 3 (0.5% L. platensis biomass extract), T 4 (live LAB culture, 1.4 × 10 8 cells mL -1 ), and T 5 (0.5% extract of L. platensis biomass fermented with LAB). Foliar applications were applied weekly from week 2 to week 9. Phenological parameters, chlorophyll concentration (SPAD-calibrated), root nodulation, soil CFU, and final yield (kg ha -1 ) were measured. Results indicated that T 5 consistently yielded the highest values across all parameters. Compared to the commercial biostimulant (CUMPLIDOR ® ), T 5 increased flower buds by 30.13%, pods per plant by 26,67%, average pod weight by 27.13%, grain weight per pod by 14.86%, chlorophyll content by 11.11%, and final yield by 24.73% (3482.36 kg ha -1 vs. 2790.31 kg ha -1 ). Notably, root nodulation doubled with T 5 (26.9 nodules/plant) but was suppressed by T 4 (5.98) and T 2 (2.40). Soil CFU increased 70- to 148-fold in T 4 and T 5 , respectively, which indicates robust microbial growth. Fermentation of L. platensis biomass with LAB significantly enhances its phytostimulant efficacy, surpassing both the commercial biostimulant and the non-fermented extract. T 5 promotes plant growth, chlorophyll content, and yield, and supports beneficial soil microbiota, whereas live LAB alone suppresses nodulation, but the fermented extract enhances it. This innovative fermented biostimulant presents a promising, eco-friendly strategy for sustainable common bean cultivation.