Dissolved organic carbon in the sea makes up some 700 gigatons according to conservative estimates. Even more important is the labile part, which constitutes more than 50% of newly produced DOC. The source of DOC production in the sea is photosynthetic algae. The major constituent in rapidly growing phytoplankton cells is protein, with a content up to about 50% of organic dry matter; the content of carbohydrate and lipid being somewhat lower. When growth is slow and in stationary phases, the cellular composition will change very markedly and often carbohydrate will be the most prominent constituent.