Community-level interactions control proliferation of Azospirillum brasilense Cd in microcosms

We tested the hypothesis that community-level interactions, rather than the genetically-derived capability to fix N 2 control proliferation, as monitored by phospholipid-linked fatty acid methyl ester (PL-FAME) analysis, of A. brasilense Cd in physicochemically-defined microcosms. In N 2-fixation microcosms, microflora from compost or soil did not proliferate, either alone or in mixed cultures with A. brasilense Cd. In P-mobilization microcosms, the amount of 2-OH19:0, a biomarker for A. brasilense Cd, was generally higher in mixed cultures with compost microflora than in those with soil microflora. P mobilized from hydroxylapatite by A. brasilense was significantly less than that by compost microflora, but not different from that by soil microflora. Profiles of phospholipid-linked fatty acid methyl esters (PL-FAME) from cultures of compost microflora alone generally contained more 18:2(9 c,12 c), indicating a larger fungal component in the community, than did profiles from cultures of soil microflora alone. Cluster analysis of PL-FAME profiles confirmed that A. brasilense comprised a larger proportion of the community in mixed culture with compost microflora than with soil microflora. Our results in combination with the literature support our hypothesis; further research is warranted to refine PL-FAME analyses for monitoring soil microbial communities.