Photochemical transformations of riverine dissolved organic matter: effects on estuarine bacterial metabolism and nutrient demand

By experimentally simulating the exposure of terrigenous dissolved organic matter (DOM) to sunlight during the mixing of river water and seawater in an estuary (St. Helena Sound, South Carolina), this study examined how photochemical alterations may affect both the magnitude and fate of DOM utilization by estuarine bacteria. In a series of experiments performed with water samples collected from 3 blackwater rivers draining into an estuary of the South Atlantic Bight (USA), exposure to light resulted in significant decreases in dissolved organic carbon (DOC) concentration and DOM absorption at 350 nm relative to dark controls. When these water samples were subsequently inoculated with an estuarine bacterial community, prior exposure to light significantly increased bacterial growth, respiration and total DOC consumption relative to that observed in control treatments. Increases in respiration (46 to 78%) were, however, always greater than increases in growth (19 to 35%). This resulted in decreases in bacterial growth efficiency that ranged from 16 to 33% and appeared predictable from initial differences in DOM optical characteristics. In addition, although light exposure consistently resulted in significant NH4 production, bacterial metabolism in light treatments was accompanied by a significant increase in bacterial demand for inorganic nutrients (117 to 243% over dark controls). These results indicate that irradiation of terrigenous DOM significantly alters its ultimate fate (CO2 VS. bacterial biomass) and that bacterial carbon metabolism of photoaltered DOM is coupled to an enhanced demand for inorganic nutrients, which may considerably influence ecosystem-scale carbon and nutrient interactions in the coastal zone.