PRODUCTION OF DISSOLVED ORGANIC-CARBON IN PHYTOPLANKTON CULTURES AS MEASURED BY HIGH-TEMPERATURE CATALYTIC-OXIDATION AND ULTRAVIOLET PHOTOOXIDATION METHODS

The production of dissolved organic carbon (DOC) in cultures of the di atoms Chaetoceros gracilis and Phaeodactylum tricornutum, the flagella te Isochrysis galbana, the dinoflagellate Alexandrium tamarense and a natural algal assemblage from the Northwest Arm, Halifax, Nova Scotia, Canada, was followed using a high-temperature catalytic oxidation (HT CO) and a UV photo-oxidation method. Molecular weight fractionation of the DOC was performed for two cultures: C.gracilis and L.galbana. Whi le the DOC in the culture medium increased significantly during log-ph ase growth for ail organisms except the dinoflagellate, this increase was proportional to the increase in cell numbers; the increase in DOC per cell was either small or zero. In all cultures, maximum release to ok place during stationary and senescent phases, usually after cell nu mbers had started to decrease. In both C.gracilis and L.galbana, a maj or portion (>65%) of the organic matter released to the medium during Log-phase growth had mel. wts of <10 000 Da. The increase in DOC in th e L.galbana culture in stationary and senescent phases was due to the release of high-molecular weight materials. The differences in extrace llular release of DOC between species and between different growth sta ges in the same species suggest that both the species composition and physiological state of phytoplankton populations must be known before interpretations and predictions based on field data can be made. In or der to determine whether the differences in DOC Values found by the HT CO and UV oxidation methods are caused by the resistance to UV oxidati on of some compounds produced by phytoplankton, rather than by less th an optimum efficiency of the UV unit used, standards must be based on naturally occurring compounds, rather than the pure compounds normally used.