Diazotrophic bacterium Azotobacter vinelandii as a mutualistic growth promoter of an aquatic plant: Lemna minor

Abstract

Lemnaceae plants, commonly referred to as duckweeds, are small planktonic terrestrial freshwater plants that live in symbiosis with various microbial communities. Azotobacter vinelandii are typical free-living nitrogen fixing soil bacteria that indirectly benefit plants by providing nitrogen compounds. In this study, Lemna minor RDSC 5512 and A. vinelandii ATCC 12837 = NBRC 13581 were co-cultured under gnotobiotic conditions. The growth of L. minor colonized by A. vinelandii was accelerated in both nitrogen-containing and nitrogen-free water conditions. The growth promotion effect is attributed to several plant growth promotion factors produced by the bacterium as well as biological nitrogen fixation in nitrogen-free condition. Moreover, L. minor elevated the nitrogen fixing activity of A. vinelandii and the cell number of A. vinelandii on L. minor increased continuously over 30 d. These observations indicated that L. minor provides a favorable environment for A. vinelandii colonization, allowing them to mutually benefit and flourish through syntrophism.