ROLE OF PHOTOREACTIONS IN THE FORMATION OF BIOLOGICALLY LABILE COMPOUNDS FROM DISSOLVED ORGANIC-MATTER

Dissolved organic matter (DOM) can be degraded by sunlight into a vari ety of photoproducts that stimulate the growth and activity of microor ganisms in aquatic environments. All biologically labile photoproducts identified to date fall into one of four categories: (1) low-molecula r-weight (MW) organic compounds (carbonyl compounds with MW of <200); (2) carbon gases (primarily CO); (3) unidentified bleached organic mat ter; and (4) nitrogen- and phosphorus-rich compounds (including NH4+ a nd PO43-). A number of laboratory studies using bacterial bioassay app roaches have shown that the photochemical breakdown of DOM can stimula te biomass production or activity by 1.5- to 6-fold. Results of photoc hemical studies, extrapolated to estimate formation rates of biologica lly available photoproducts from DOM in surface waters, also predict i mportant biological roles for these compounds. In a continental shelf system, for example, full exposure of surface seawater to sunlight for one summer day can produce DOM photodegradation products equivalent t o >20% of the bacterial carbon demand. Likewise, 30% of the bacterial nitrogen demand can be met by photodegradation of the nitrogen compone nts of DOM, a process likely to be of particular importance in nitroge n-limited systems. When considered on a depth-integrated basis around the globe, at least 1.0 x 10(15) g C and 0.15 x 10(15) g N are estimat ed to be available annually for utilization by planktonic microorganis ms through the conversion of light-absorbing fractions of DOM to more biologically labile compounds. By comparison, direct photochemical min eralization of DOM is estimated to convert 12-16 x 10(15) g C to CO2 a nnually.