Changes in bacterial diversity in response to dissolved organic matter supply in a continuous culture experiment

Heterotrophic bacterial metabolism is tightly linked to the availability of dissolved organic matter (DOM), but the relationship between substrate supply and bacterial diversity remains unresolved. The objective of the present study was to determine how an increase in the supply of bioavailable DOM affects bacterial metabolism and diversity. We addressed this issue by growing a natural bacterial community over 5 generations in continuous cultures either on seawater (dissolved organic carbon concentration equal to 74 mu M) or on seawater amended with 13 mu M of diatom-derived DOM. Bacteria markedly responded to the additional DOM supply, as revealed by 1.6-fold higher bacterial abundances and 10-fold higher cell-specific alpha- and beta-glucosidase activities in the +DOM treatment than in the control throughout the experimental period. Analysis of diversity as determined by tag pyrosequencing of 16S rRNA genes after 3 and 5 bacterial generations showed that the bacterial communities in the +DOM treatment were more similar to each other than to communities in the control. Diversity indices such as richness and phylogenetic distance were higher in the DOM-amended cultures than in the control. Gammaproteobacteria dominated the communities selected in our cultures, representing between 87 and 99% of the sequences. The addition of DOM led to a higher relative abundance of several bacterial groups, in particular Alphaproteobacteria, Bacteroidetes, and Verrucomicrobia, which accounted for up to 14% of the sequences, compared to the control in which they represented <5% of the sequences. Our results demonstrate that in this experimental context, an increased supply of bioavailable DOM sustained a higher bacterial diversity.