Interaction of photochemical and microbial processes in the degradation of refractory dissolved organic matter from a coastal marine environment

The interaction between photochemical and biological processes in the degradation of marine dissolved organic matter (DOM) was investigated with seawater from a coastal southeastern U.S. salt marsh. Seawater supplemented with humic substances was exposed to alternating cycles of sunlight (equivalent to 8 h of midday sun) and dark incubations with natural bacterial populations (1–2 weeks in length). Photochemical degradation of the DOM was monitored during sunlight exposure by direct measurements of dissolved inorganic carbon (DIC) and carbon monoxide (CO) formation in 0.2‐µm filtered seawater. Bacterial degradation was monitored during dark incubations by tritiated leucine uptake and changes in bacterial numbers in bacterivore‐free incubations and by direct measurements of DOM loss. The alternating cycles of sunlight and microbial activity resulted in more complete degradation of bulk DOM and marine humic substances than was found for nonirradiated controls (i.e. with microbial activity alone) by a factor of up to 3‐fold. Increased decomposition was due both to direct losses of carbon gas photoproducts (DIC and CO in a 15 : 1 ratio) and to enhanced microbial degradation of photodegraded DOM, with approximately equal contributions from each pathway. Mass balance calculations indicated that low‐molecular‐weight carbon photoproducts, currently considered to be the compounds responsible for stimulating bacterial activity following photodegradation of DOM, were insufficient to account for the enhanced bacterial production observed. Thus, higher molecular weight, chemically uncharacterized fractions of DOM may also be modified to more biologically available forms during exposure to natural sunlight.