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.