1. Department of Marine Sciences, University of Georgia, Athens, Georgia 30602, USA
2. Department of Computer Science, San Diego State University, San Diego, California 92182, USA

Correspondence to: Mary Ann Moran¹ Correspondence and requests for materials should be addressed to M.A.M. (Email: mmoran@uga.edu).

The assimilation and mineralization of dissolved organic carbon (DOC) by marine bacterioplankton is a major process in the ocean carbon cycle¹. However, little information exists on the specific metabolic functions of participating bacteria and on whether individual taxa specialize on particular components of the marine DOC pool². Here we use experimental metagenomics to show that coastal communities are populated by taxa capable of metabolizing a wide variety of organic carbon compounds. Genomic DNA captured from bacterial community subsets metabolizing a single model component of the DOC pool (either dimethylsulphoniopropionate or vanillate) showed substantial overlap in gene composition as well as a diversity of carbon-processing capabilities beyond the selected phenotypes. Our direct measure of niche breadth for bacterial functional assemblages indicates that, in accordance with ecological theory, heterogeneity in the composition and supply of organic carbon to coastal oceans may favour generalist bacteria. In the important interplay between microbial community structure and biogeochemical cycling, coastal heterotrophic communities may be controlled less by transient changes in the carbon reservoir that they process and more by factors such as trophic interactions and physical conditions.

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