1 Département des sciences biologiques, Université de Montréal, Pavillon MIL C. P. 6128, succ. Centre-ville, Montréal, QC H3C 3J7, Canada.
2 Groupe de recherche interuniversitaire en limnologie et environnement aquatique (GRIL), Université de Montréal, C. P. 6128, succ. Centre-ville, Montréal, QC H3C 3J7, Canada.
3 Department of Fisheries and Ocean Canada, Bedford Institute of Oceanography, 1 Challenger Dr., Dartmouth, NS B2Y 4A2, Canada.
4 Department of Earth and Environmental Sciences, Brooklyn College-The City University of New York, 2900 Bedford Avenue, Brooklyn, NY 11210, USA.
5 Geotop and Department of Chemistry and Biochemistry, Concordia University, 7141 Sherbrooke W., Montréal, QC H4B 1R6, Canada.
6 Département des sciences de l'environnement, Université du Québec à Trois-Rivières, 3351 Boulevard des Forges, Trois-Rivières, QC G8Z 4M3, Canada.
7 Pontchartrain Instit

The microbial carbon pump (MCP) hypothesis suggests that successive transformation of labile dissolved organic carbon (DOC) by prokaryotes produces refractory DOC (RDOC) and contributes to the long-term stability of the deep ocean DOC reservoir. We tested the MCP by exposing surface water from a deep convective region of the ocean to epipelagic, mesopelagic, and bathypelagic prokaryotic communities and tracked changes in dissolved organic matter concentration, composition, and prokaryotic taxa over time. Prokaryotic taxa from the deep ocean were more efficient at consuming DOC and producing RDOC as evidenced by greater abundance of highly oxygenated molecules and fluorescent components associated with recalcitrant molecules. This first empirical evidence of the MCP in natural waters shows that carbon sequestration is more efficient in deeper waters and suggests that the higher diversity of prokaryotes from the rare biosphere holds a greater metabolic potential in creating these stable dissolved organic compounds.