Spontaneous Cell Lysis by <i>Pelomonas saccharophila</i> MRB3 Provides Plant-Available Macronutrients in Hydroponic Growth Media and Accelerates Biomass Production of Duckweed

Abstract

This study reports an intriguing observation regarding the betaproteobacterium Pelomonas saccharophila MRB3 (NITE P-01647), a candidate plant growth-promoting bacterium that accelerates biomass production of the aquatic plant, duckweed. In a series of experiments in which strain MRB3 cells were inoculated into duckweed cultures, MRB3 improved duckweed growth with decreasing cell density in the culture medium. By monitoring nutrient dynamics of plants and bacteria, we found that MRB3 exhibited spontaneous cell lysis and released NH 4 + and PO 4 3-, which were subsequently utilized for duckweed growth. Surprisingly, the amount of NH 4 + released by MRB3 corresponded to approximately half of the total nitrogen contained in the inoculated cells, notwithstanding the fact that the majority of nitrogen elements in bacterial cells should be present in plant-unavailable forms, such as proteins and nucleic acids. Spontaneous cell lysis, provision of macronutrients, and duckweed growth promotion by MRB3 were robustly observed in different inoculation amounts and growth media, including natural pond water. These observations indicated the possibility of utilizing autolytic bacteria as an alternative nutrient source in hydroponic cultivation of duckweeds and other plants.