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 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.