Rates of microbial degradation of dissolved organic carbon from phytoplankton cultures

Dissolved organic carbon (DOC) decay was measured for samples from cultures of the diatoms Chaetoceros gracilis and Phaeodactylum tricornutum, the flagellate Isochrysis galbana, the dinoflagellate Alexandrium tamarense, and a natural algal assemblage from the Northwest Arm, Nova Scotia, Canada, by a high-temperature catalytic oxidation (HTCO) method. Decay rate constants were determined using first-order reaction kinetics in the multi-G model of Berner (In Early Diagenesis, a Theoretical Approach, Princeton University Press, 1980). Decay rates as high as 0.37 day(-1) were obtained, which demonstrated that DOC released by phytoplankton might be highly labile to bacterial utilization and could be degraded significantly within hours. Decay rates for most species tested followed much the same pattern, with K-01 values around 0.3-0.4, K-02 values around 0.03, and K-03 and K-04 values around 10(-3) day(-1). DOC released by the senescent cells of A.tamarense was found to be essentially bacteria resistant, in contrast to that of the other species tested. The decay of DOC was directly temperature dependent over the 10-20 degrees C range. Six methods for DOC preservation were tested. Acidification with HCl and refrigerated storage was demonstrated to be the most convenient and practical method. This method can be used for both short- and long-term preservation of DOC samples containing highly labile organic compounds.