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 hr 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-um 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 non-irradiated 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.