The efficiency and spectral photon dose dependence of photochemically induced changes to the bioavailability of dissolved organic carbon

To quantify the effects of photochemistry on the biological lability of dissolved organic carbon in a terrestrially influenced system, a quarterly sampling effort was undertaken at three estuarine locations along the coast of Georgia, producing a total of 15 apparent quantum yield (AQY) spectra for biologically labile products (BLPs). Prefiltered samples were irradiated and then inoculated with natural microbial communities isolated from the same sample. Oxygen consumption was used as a proxy for community carbon utilization over 10-12 d dark incubations. Seasonal microbial response to irradiated samples ranged from -221% to +155% relative to dark controls. AQY spectral shapes were not consistent over the data set, nor were there any apparent patterns in season or location. Progressive photon dose experiments showed that increasing irradiation length strongly decreased the total biolabile product as assayed with microbial community oxygen consumption, as well as altering the spectral shape of the AQY spectra. A conceptual model is presented to explain this dependence on photon dose, by illustrating the competition between the photochemical production and photochemical destruction of BLPs. The varying dependence on photon dose is fundamentally different from published results for other photochemical products and further complicates attempts to quantify the total effect of photochemistry on organic carbon cycles in natural environments.