Role of photoreactions in the formation of biologically labile compounds from dissolved organic matter

Abstract

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) nitrogen‐ and phosphorus‐rich compounds (including NH 4 + and PO 4 3− ). 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. 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 × 10 15 g C and 0.15 × 10 15 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 × 10 15 g C to CO 2 annually.