Soil microalga-derived L-glutamic acid enhances the growth and yield of pepper by recruiting nitrogen-fixing bacteria

Background and aims Reducing chemical fertilizer dependence while sustaining crop productivity is a key challenge for sustainable agriculture. Soil microalgae are promising biofertilizers, yet their growth-promoting mechanisms remain largely unclear. This study aimed to evaluate the effects of soil microalgae on pepper growth and rhizosphere processes, and to clarify the mechanisms underlying yield enhancement. Methods This study isolated two soil microalgae Anabaena azotica and Scenedesmus sp., which were applied in pot and field experiments, with three groups: chemical fertilizer alone, chemical fertilizer plus Anabaena azotica (A), and chemical fertilizer plus a mixed inoculum of A. azotica and Scenedesmus sp. (AS). Transcriptomic profiling of root and third-generation amplicon sequencing of rhizosphere soil were conducted to explore the underlying mechanism. Results The results showed that A and AS application increased pepper yield by 36.42% and 107.87%, respectively. Especially, AS application increased soil available nitrogen content by 11.98%. Moreover, Scenedesmus sp. enriched nitrogen-fixing bacteria, and thereby promoted nitrogen uptake and utilization in the roots, which consequently increased pepper yield. Furthermore, L-glutamic acid was found to be the key metabolite through which Scenedesmus sp. exerted its yield-enhancing effects. Conclusions Microalga-derived L-glutamic acid could recruit nitrogen-fixing bacteria, strengthened rhizosphere nitrogen cycling, increased plant nitrogen acquisition, and enhanced pepper yield. This study provided important scientific evidence for the development and utilization of microalgae-based biofertilizer to improve crop yields in the sustainable agriculture.