Dissolved Organic Phosphorus Excretion by Marine Phytoplankton¹,²

SUMMARY The amounts of extracellular dissolved organic phosphorus (DOP) produced by 8 species of marine planktonic algae under various conditions of light, salinity, and nutrition were compared. Large amounts, more than 20% of the total P in the system, were excreted by Cyclotella cryptica, Thalassiosira fluviatilis, Dunaliella tertiolecta , and Synechococcus under 1 or more of the experimental conditions. Excretion of DOP was proportional to light intensity in Dunaliella, Rhodomonas, Chlorella sp., and Coccolithus huxleyi. Phosphorus limitation reduced DOP production by Cyclotella and Thalassiosira , nitrogen limitation reduced DOP production by Phaeodactylum, Dunaliella , and Rhodomonas , and lack of iron reduced DOP levels in Cyclotella cultures. Salinity affected growth, but no clear relationship to DOP excretion was evident. The DOP eliminated during growth was reassimilated by the species that produced it and by other species, but lack of alkaline phosphatase reduced the amount of DOP that was available to certain algae.