Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 112 (2015): 453-457, doi:10.1073/pnas.1413137112 . ; About half the carbon fixed by phytoplankton in the ocean is taken up and metabolized by marine bacteria, a transfer that is mediated through the seawater dissolved organic carbon (DOC) pool. The chemical complexity of marine DOC, along with a poor understanding of which compounds form the basis of trophic interactions between bacteria and phytoplankton, have impeded efforts to identify key currencies of this carbon cycle link. Here, we used transcriptional patterns in a bacterial-diatom model system based on vitamin B12 auxotrophy as a sensitive assay for metabolite exchange between marine plankton. The most highly upregulated genes (up to 374-fold) by a marine Roseobacter clade bacterium when co-cultured with the diatom Thalassiosira pseudonana were those encoding the transport and catabolism of 2,3- dihydroxypropane-1-sulfonate (DHPS). This compound has no currently recognized role in the marine microbial food web. As the genes for DHPS catabolism have limited distribution among bacterial taxa, T. pseudonana may use this novel sulfonate for targeted feeding of beneficial associates. Indeed, DHPS was both a major component of the T. pseudonana cytosol and an abundant microbial metabolite in a diatom bloom in the eastern North Pacific Ocean. Moreover, transcript analysis of the North Pacific samples provided evidence of DHPS catabolism by Roseobacter populations. Other such biogeochemically important metabolites may be common in the ocean but difficult to discriminate against the complex chemical background of seawater. Bacterial transformation of this diatom-derived sulfonate represents a new and likely sizeable link in both the marine carbon and sulfur cycles. ; This research was partially funded by NSF grants OCE-1356010 to M.A.M., OCE-1205233 to E.V.A., OCE-0928424 to E.B.K., and OCE-1233964 to S.R.C., and by the Gordon and Betty Moore Foundation grants 538.01 to M.A.M. and 537.01 to E.V.A. ; 2015-06-29