Ihe microbial carbon pump (MCP) contributes to ocean carbon sequestration by converting reactive organic matter into recalcitrant dissolved organic carbon (RDOC) that can remain in seawater for thousands of years.The MCP is a potentially important ecosystem pathway operating in parallel with the well-known biological pump (BP),which tus atmospheric CO2 into particulate organic matter that sinks to deep waters and the ocean bottom,where its carbon is sequestered.Since the MCP was proposed byJiao et al.[1],it has become an important impetus for new research in the ocean carbon cycle (e.g.Legendre et al.[2]).A study of bacterial exometabolites recently showed that these dissolved molecules share many compositional and structural characteristics of recalcitrant DOC present in the deep ocean [3].