Nitrogen-fixation by Azospirillum brasilense Cd is promoted when co-cultured with a mangrove rhizosphere bacterium (Staphylococcus sp.)

Azospirillum brasilense Cd fixed more N2 when grown in a mixed culture with Staphylococcus sp., a non-N2-fixing bacterium isolated from mangrove roots. This was not the result of an increase in the bacterial population nor of decreased O2 concentration in the mixed culture. In mixed culture, the Staphylococcus population declined sharply, but not because A. brasilense Cd was more effective in competing for the available N. The addition of a cell-free dialyzate of Staphylococcus sp. culture medium to the A. brasilense culture significantly promoted the N2-fixing capacity of the latter. When this dialyzate was produced by culturing Staphylococcus in N-free medium without yeast extract, the increased dialyzate activity depended on the concentration. When the dialyzate was diluted by volume to 50 and 25% of its original concentration, N2-fixation by A. brasilense Cd increased significantly; when undiluted, the dialyzate failed to enhance N2-fixation. Chemical analyses of the dialyzate by thin layer chromatography identified aspartic acid; gas chromatography revealed succinic acid to be the major organic acid component. When artificially added to the A. brasilense Cd culture, only aspartic acid significantly promoted N2-fixation by A. brasilense Cd. The N2-fixing ability of A. brasilense Cd increased significantly when grown in mixed culture with the non-N2 fixing bacterium Staphylococcus epidermidis, but not with Micrococcus lylae, both isolated from mangrove roots.