Production of dissolved organic carbon in phytoplankton cultures as measured by high-temperature catalytic oxidation and ultraviolet photo- oxidation methods

Abstract

The production of dissolved organic carbon (DOC) in cultures of the diatoms Chaetoceros gracilis find Phaeodactylum tricornutum, the flagellate Isochrysis galbana, the dinoflagellale Alexandrium tamarense and a natural algal assemblage from the Northwest Arm, Halifax, Nova Scotia, Canada, was followed using a high-temperature catalytic oxidation (HTCO) and a UV photo oxidation method. Molecular weight fractionation of the DOC was performed for two cultures: C. gracilis and I. galbana. While the DOC in the culture medium increased significantly during log-phase growth for all 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 took place during stationary and senescent phases, usually after cell numbers had started to decrease. In both C. gracilis and I. galbana, a major portion (>65%) of the organic matter released to the medium during log-phase growth had mol. wts of <10 000 Da. The increase in DOC in the I. galbana culture in stationary and senescent phases was due to the release of high-molecular-weight materials. The differences in extra- cellular release of DOC between species and between different growth stages 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 order to determine whether the differences in DOC values found by the HTCO and UV oxidation methods are caused by the resistance to UV oxidation of some compounds produced by phytoplankton, rather than by less than optimum efficiency of the UV unit used, standards must be based on naturally occurring compounds, rather than the pure compounds normally used.