Marine organic photochemistry previewed

Abstract Sunlight photolyzes organic compounds in surface waters of the marine environment, destroying some substances and synthesizing others. Such reactions may proceed at high rates and have biological, chemical, and geological implications; the subject is in its infancy. Models are presented that estimate the maximum realistic rates of photochemical transformations at the sea surface and in a 40-meter thick mixed layer; typical rates vary widely and an average cannot be specified. The maximal rates are extremely high at the surface and throughout the layer, corresponding to daylight photodestruction times in the seconds minutes range. Highly photosensitive materials will thus have vanishingly small steady-state concentrations in surface waters. Actual rates ranging from 1 to 10 −8 times the modelled ones may, depending on the rates of competing processes, be significant in the marine environment. The rate of a photoprocess is the product of the excitation rate for light absorption and the chemical efficiency (quantum yield) of ensuing reactions. Mechanistic description of marine photoreactions involves three basic component processes: light absorption, primary photochemical reactions, and secondary reactions. Water, dissolved organic matter, and living and dead particulate matter are the principal absorbers in seawater. The chemical structures of the chromophores involved are not well known, even for the dissolved organic “Gelbstoffe” that color seawater. Most simple organic compounds known in seawater are not effective absorbers. The ensuing reactions of excited states include, beside may specific transformations, general reaction types such as free-radical formation by bond cleavages and by electron transfer redox reactions. Electronic energy transfer also excites other solute molecules. Both of these general primary process types lead to secondary reactions. It is proposed here that secondary reactions are of general importance in marine photochemistry, that they have a net oxidative character in seawater, and that such reactions may provide one sink for both colored and non-absorbing refractory organic molecules in the marine environment. For chromophores, there may be a self-destroying negative feedback effect. Organic radicals produced directly, or indirectly by interaction with inorganic radicals such as O 2 − , CO 3 − , NO, NO 2 , OH, Br 2 − are expected to react semiquantitatively with dissolved oxygen, leading to unstable organoperoxy radicals. One photon may thus result in the oxidation of several carbon atoms by short-chain auto-oxidations. Similarly, electronic energy transfer in seawater should result almost exclusively in formation of electronically excited singlet molecular oxygen, a reactive but selective oxidant. Progress in marine organic photochemistry can be anticipated, but artifacts arising from inappropriate light intensities and wavelengths, trace contaminant effects, and photobiological processes must be avoided. Methods for overcoming these difficulties are available for some solution reactions, but potentially important particulate photochemistry studies are much less tractable. Conceptually, marine photochemistry may develop fruitful ties with solution-free radical and radiation chemistry on the one hand, and optical oceanography and euphotic zone biology and chemistry on the other.