Dissolved organic matter (DOM) can be degraded by sunlight into a
variety of photoproducts which stimulate the growth and activity of
microorganisms in aquatic environments. All biologically labile
photoproducts identified to date fall into one of four categories: i) low
molecular weight (MW) organic compounds (carbonyl compounds with MW < 200);
ii) carbon gases (primarily CO); iii) unidentified bleached organic matter;
and iv) nitrogen- and phosphorus-rich compounds (including NH4+ and PO43-).
A number of laboratory studies using bacterial bioassay approaches have shown
that the photochemical breakdown of DOM can stimulate biomass production or
activity by 1.5- to 6-fold. Results of photochemical studies, extrapolated to
estimate formation rates of biologically available photoproducts from DOM in
surface waters, also predict important biological roles for these compounds.
For surface seawater from the continental shelf of the southeastern United
States fully exposed to sunlight for one summer day, it is estimated that at
least 20% of the bacterial carbon demand can be met by DOM photodegradation
products. Likewise, 30% of the bacterial nitrogen demand can be met by
photodegradation of the nitrogen components of DOM, a process likely to be
of particular importance in nitrogen-limited systems. When considered on a
depth-integrated basis around the globe, at least 1.0 x 1015 g C and
0.15 x 1015 g N are estimated to be available annually for utilization by
planktonic microorganisms through the conversion of light-absorbing fractions of
DOM to more biologically labile compounds. By comparison, direct photochemical
mineralization of DOM is estimated to convert 12 - 16 x 1015 g C to CO2 annually.