Nitrogen source-dependent biofilm formation in A.Brasilense as an approcach to effective biostimulant formulas

Abstract

In a global growing population, the need to raise productivity and resources in an efficient manner with minimal environmental impact has become a main challenge to achieve in the agricultural sector. Diazotrophic A.brasilense is known for their ability to fix-nitrogen and promote plant growth, being a promising tool for agriculture as plant biostimulant (PB). Although the market of these products is flourishing, many biostimulants still result in variable outcomes and therefore low reliability. For that reason, it is of interest to know which environment favors its growth and survival, and is more efficient as plant growth promoter bacteria (PGPB). As growing bacteria is commonly carried out in rich medium to ensure rapid replication, the present study wanted to take into account environments that bacteria would find after the application of biostimulant formulas. Thinking that plant biostimulants will most likely be compensated with inorganic fertilizers and applied in many areas where soils are poor in nutrients, the following nitrogen sources were used: nitrate (NO3 - ), ammonium (NH4 + ), atmospheric nitrogen (N2) and rich medium (LB). In addition, many of these biostimulants use model strains as they are widely studied and available on the market. However, it is known that wild strains isolated from ecosystems where environmental conditions are not ideal, are better adapted for survival and therefore are of great interest for a successful viability of biostimulant formulas. For that reason, A.brasilense model strains such as SP7 and SP245 and isolated strains such as 29145 and ARG2 were used in this study. Bacterial growth (OD600), biofilm biomass (OD550), biofilm patterns and early stages of biofilm formation as well as their survival strategies and their carbon and nitrogen usage for each strain in each nitrogen scenario was analyzed. Overall results suggest that isolated strains perform better than model strains. In particular, ARG2 strain shows great fitness. Moreover, this strain tends to form cysts in all studied medium compared to other strains. Encysting is a survival strategy that A.brasilense can adopt to face environmental stress. Taking into account that such structures are capable of fixing nitrogen under aerobic conditions, it would be of great interest the research of their potential for future biostimulant formulas.