Wl. Miller et Ma. Moran, INTERACTION OF PHOTOCHEMICAL AND MICROBIAL PROCESSES IN THE DEGRADATION OF REFRACTORY DISSOLVED ORGANIC-MATTER FROM A COASTAL MARINE-ENVIRONMENT, Limnology and oceanography, 42(6), 1997, pp. 1317-1324
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 su
pplemented with humic substances was exposed to alternating cycles of
sunlight (equivalent to 8 h of midday sun) and dark incubations with n
atural bacterial populations (1-2 weeks in length). Photochemical degr
adation of the DOM was monitored during sunlight exposure by direct me
asurements of dissolved inorganic carbon (DIC) and carbon monoxide (CO
) formation in 0.2-mu m filtered seawater. Bacterial degradation was m
onitored during dark incubations by tritiated leucine uptake and chang
es in bacterial numbers in bacterivore-free incubations and by direct
measurements of DOM loss. The alternating cycles of sunlight and micro
bial activity resulted in more complete degradation of bulk DOM and ma
rine 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 photoproduct
s (DIC and CO in a 15:1 ratio) and to enhanced microbial degradation o
f 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 respon
sible for stimulating bacterial activity following photodegradation of
DOM, were insufficient to account for the enhanced bacterial producti
on observed. Thus, higher molecular weight, chemically uncharacterized
fractions of DOM may also be modified to more biologically available
forms during exposure to natural sunlight.