DOC Molecule Transporter and Transformation in Marine Microbes

The ocean acts as a “sink” of atmospheric CO₂, thus mitigating global warming. The recognized biological mechanism for this sink is the “biological pump”, which is based on the photosynthetic fixation of CO₂ and subsequent carbon export driven mainly by sinking of particulate organic carbon (POC). The ocean possesses a huge dissolved organic carbon (DOC) pool which accounts for about 95% of the total remaining organic carbon. The majority of this DOC pool is recalcitrant to biological degradation and can persist in the water column for thousands of years, constituting carbon sequestration in the ocean. Recently a new concept has been proposed to address this matter, the “microbial carbon pump (MCP)”, which refers to the microbial processes that transform labile DOC (LDOC) to recalcitrant DOC (RDOC). The transformation of DOC is carried out by marine microbes, and the pathways and rates of DOC transformation determine the fate and the amount of carbon converted ultimately to CO₂ or RDOC. The DOC pool consists of thousands of organic carbon compounds with different biological turnover rates, biological availabilities, and biogeochemical features. While microbial processes modify the composition of the DOC pool, the availability of DOC compounds to microbes shapes microbial diversity and community structure. For instance, the Roseobacter clade and SAR11 clade are dominant bacterial groups in relatively eutrophic and oligotrophic waters respectively. Each clade has different strategies for carbon utilization, and thus different responses to and impacts on the DOC pool in the ocean.