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COMPARING RESPIRATORY (ETS) ACTIVITY OF BACTERIA ATTACHED TO PHYTOPLANKTON AND FREELY SUSPENDED IN SURROUNDING SEAWATER

We conducted investigated respiratory (ETS) activity of bacteria attached to phytoplankton and living free in surrounding seawater collected from coastal waters at Charleston, Oregon, USA. We hypothesized that the surface of a phytoplankton cell provides a habitat that permits enhanced metabolic activity in attached bacteria. In order to test this hypothesis, the number of bacteria that reduce 5‐Cyano‐2,3‐di‐tolyl tetrazolium chloride (CTC) were compared to the total number of bacteria, assessed by the nucleic acid stain, 4′,6‐diamidino‐2‐phenylindole (DAPI). Two‐hour incubations with the CTC dye and subsequent staining allowed for direct viewing of the position and the number of ETS active versus inactive bacteria in each sample. We found that the percentage of active bacteria attached to the phytoplankton cells' surface (42%) was 10 times that of bacteria freely suspended in the surrounding seawater (4%). Previous investigations of ETS activity of freely suspended bacteria found that 1% to 8% are actively respiring while non‐living particle associated bacteria showed that 20 to 35% of bacteria are actively respiring. Our results suggest that bacteria in coastal waters are substrate limited, and the extracellular products of photosynthesis may be more important than is generally realized. The surfaces of phytoplankton cell surfaces may provide habitat and substrates for a disproportionate share of bacterial production in coastal waters.