Production of chromophoric dissolved organic matter fluorescence in marine and estuarine environments: an investigation into the role of phytoplankton

We tested whether phytoplankton are a direct source of chromophoric dissolved organic matter (CDOM) fluorescence in a series of experiments. In the first experiment, sonication of 11 dense algal cultures from several algal classes revealed no immediate release of CDOM fluorescence. In a second experiment, using nutrient addition bioassays from a range of sites in the mid-Atlantic region, we found no increase in CDOM fluorescence when nutrient limitation was alleviated despite large increases in chlorophyll a over 3–5 days. In a third experiment, the change in CDOM fluorescence over a 28-day period in five non-axenic algal batch cultures was measured. There was little or no increase in CDOM fluorescence until the cultures entered the stationary phase, whereupon an exponential increase in CDOM fluorescence was observed. In a fourth set of experiments, the production of CDOM fluorescence was examined in a series of cultures of Skeletonema costatum and Prorocentrum minimum. In the dark, in the absence of autotrophic growth, we observed slow rates of CDOM fluorescence production (0.02–0.05 NFlU day−1). Rates were much higher in parallel lighted cultures (0.1–0.2 NFlU day−1) but were more related to bacterial counts than to algal biomass. In a third phase of this experiment, when illuminated, stationary phase cultures were filtered through 1-μm pore size filters and incubated in the dark, CDOM fluorescence production continued unchanged. These results are consistent with the hypothesis that phytoplankton are not a direct source of CDOM fluorescence in marine and estuarine environments and that CDOM fluorescence is produced by bacteria using non-fluorescent organic matter derived from phytoplankton.