Full article: Biological pump
Particulate organic carbon, created through biological production, can be exported from the upper ocean in a flux commonly termed the biological pump, or respired (equation 6) back into inorganic carbon. In the former, dissolved inorganic carbon is biologically converted into organic matter by photosynthesis (equation 5) and other forms of autotrophy that then sinks and is, in part or whole, digested by heterotrophs. Particulate organic carbon can be classified, based on how easily organisms can break them down for food, as labile, semilabile, or refractory. Photosynthesis by phytoplankton is the primary source for labile and semilabile molecules, and is the indirect source for most refractory molecules. Labile molecules are present at low concentrations outside of cells (in the picomolar range) and have half-lives of only minutes when free in the ocean. They are consumed by microbes within hours or days of production and reside in the surface oceans, where they contribute a majority of the labile carbon flux. Semilabile molecules, much more difficult to consume, are able to reach depths of hundreds of meters below the surface before being metabolized. Refractory DOM largely comprises highly conjugated molecules like Polycyclic aromatic hydrocarbons or lignin. Refractory DOM can reach depths greater than 1000 m and circulates through the oceans over thousands of years. Over the course of a year, approximately 20 gigatons of photosynthetically-fixed labile and semilabile carbon is taken up by heterotrophs, whereas fewer than 0.2 gigatons of refractory carbon is consumed. Marine dissolved organic matter (DOM) can store as much carbon as the current atmospheric CO supply, but industrial processes are altering the balance of this cycle.