Release of dissolved organic matter from marine phytoplankton

Release of dissolved organic matter from marine phytoplankton was examined by conducting batch culture and chemostat culture. In a batch culture of the diatom Skeletonema costatum using disphotic zone seawater from a depth of 530m collected in Sagami Bay that contained low dissolved organic carbon (DOC) - 0.7mgC / l**-1, the concentration of DOC in the culture medium increased significantly (to - 1.7mgC / l**-1) at the end of the exponential phase, and remained fairly constant (1.5 - 1.9 mgC / l**-1) in the stationary phase. About 50% of the DOC concentration in the culture medium is considered to be excreted from living phytoplankton cells. Absorbance of filtered culture medium at 250nm [A (250)] and 380nm [A (380)] were measured to know the property of the dissolved organic matter. A (250)/DOC, A (380)/DOC and A (380)/A (250) were higher in culture medium in high concentrations of phytoplankton than those in disphotic zone seawater. They were also high in coastal surface water containing a high concentration of phytoplankton demonstrating that a considerable part of the DOC in the coastal surface water is excreted from living phytoplankton cells. In a chemostat culture of the diatom Chaetoceros ceratosporum, the concentration of dissolved organic nitrogen (DON) was low (3 micro M) in the inflow medium (disphotic zone seawater from a depth of 320m collected off Muroto), and increased in the outflow medium (to - l0 micro M). About 70% of the DON in the outflow medium is considered to be excreted from living phytoplankton cells. The concentration of DON in coastal surface water of Toyama Bay was about 10 micro M, which is close to that in outflow medium of the chemostat culture. The rate of DON released from cultured living phytoplankton cells was calculated. The release rate from a single cell was 14.6 - 19.6 f mole / cell sup(-1) / day sup(-1), and the release rate in a unit volume of seawater was 2.3 - 3.8 micro mole / l sup(-1) / day sup(-1).