The main biological processes are shown. The biological pump is a process whereby CO2 in the upper ocean is fixed by primary producers and transported to the deep ocean as sinking biogenic particles (particulate organic matter; POM) or as dissolved organic matter (DOM). The microbial loop is a pathway in the aquatic food web whereby DOM is taken up by bacteria and archaea, which are consumed by protists, which are in turn consumed by metazoans (not shown). The viral shunt reflects virus-mediated lysis of microorganisms, which returns the POM to the DOM pool. The proposed microbial carbon pump is a conceptual framework for understanding the role of microbial processes in the production of recalcitrant DOM (RDOM). RDOM can persist in the ocean for millennia and is therefore a reservoir for carbon storage in the ocean. Three major pathways have been identified in the microbial carbon pump: direct exudation of microbial cells during production and proliferation (path 1); viral lysis of microbial cells to release microbial cell wall and cell surface macromolecules (path 2); and POM degradation (path 3). The grey shading roughly indicates the total flux of carbon metabolism in the water column.