Production of chromophoric dissolved organic matter by Sargasso Sea microbes

Time series of chromophoric dissolved organic matter (CDOM) light absorption coefficients indicate a local origin for a large fraction of the CDOM in the upper water column of the Sargasso Sea. In the present study, we demonstrate that CDOM is produced in bacterial culture experiments using Sargasso Sea water and naturally occurring microbial assemblages. Seawater cultures were prepared and grown at in situ temperatures in the dark for periods of weeks. Selected cultures were treated with amendments including inorganic nutrients, glucose, phytoplankton exudates, and zooplankton excretia. In all experiments, when bacterial biomass increased, CDOM increased during the first week of the experiment, followed by a decrease over a longer period of time. Cultures amended with both glucose and inorganic nitrogen and phosphorus produced more CDOM than controls or cultures amended with glucose or inorganic nutrients alone. However, when complex DOM substrates (derived from phytoplankton or zooplankton cultures) were added to seawater cultures, there was a net accumulation of CDOM over the course of the experiments. These data suggest that, in addition to microbial growth, the quality of the substrate plays an important role in net CDOM production. ‘New’ CDOM produced in culture was spectroscopically similar to CDOM appearing below the surface during summer stratification. The results of the present study support a new paradigm for CDOM in the open ocean, which allows for local origin and significant dynamics. Appreciation of CDOM dynamics will, in turn, add to our understanding of microbial productivity, photochemical rate processes, and ultraviolet radiation availability in the global ocean.