Invited Review 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 variety of photoproducts that stimulate the growth and activity of microorganisms in aquatic environments. All biologically labile photoproducts identified to date fall into one of four categories: (1) low-molecular-weight (MW) organic compounds (carbonyl compounds with MW of <200); (2) carbon gases (primarily CO); (3) unidentified bleached organic matter; and (4) nitrogenand phosphorus-rich compounds (including NH,+ and POb3-). A number of laboratory studies using bacterial bioassay approaches have shown that the photochemical breakdown of DOM can stimulate biomass production or activity by 1.5to 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. In a continental shelf system, for example, full exposure of surface seawater to sunlight for one summer day can produce DOM photodegradation products equivalent to >20% of the bacterial carbon demand. 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 lOI g C and 0.15 X 1O1’ 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 lOI g C to CO2 annually. When sunlight is absorbed by dissolved organic matter (DOM) of either freshwater or marine origin, the average molecular weight (MW) is reduced and a variety of photoproducts are formed (Zepp et al. 1995). Some of these pho-