Rates and possible mechanism of light-dependent degradation of pigments in detritus derived from phytoplankton

Rates of degradation of phytoplankton chlorophylls and carotenoids under visible light exposure were determined in laboratory experiments. Killed phytoplankton cells and copepod fecal pellets were used as sources of pigments. Chlorophylls, pheopigments and carotenoids were analyzed by HPLC. Experiments included aerated and N2 bubbled treatments (light and dark), and in one experiment, killed phytoplankton cells were aged in darkness for 5 days in the presence of an active bacterial population prior to exposure to light. Pigment bleaching in solution was also examined. Pigment degradation was light- and oxygen-dependent and showed apparent first-order kinetics with respect to light exposure. The chlorophylls, pheopigments and major carotenoids bleached at similar rates. Pigments in killed cells bleached faster than the pigments contained in copepod fecal pellets, perhaps due to a higher effective light dose for the smaller particles and to differences in the structural organization of pigments in killed cells versus fecal pellets. Light-dependent degradation of pigments in aerated organic solutions varied with solvent composition. In pure acetone, carotenoids and chlorophylls bleached at similar rates. In acetone:water mixtures, carotenoids were considerably more stable than chlorophylls. A sensitized mechanism of pigment photooxidation could account for the apparent coupling of light-dependent degradation of different pigment types in detrital particles and in acetone solution. In detrital particles such as fecal pellets, sensitized photooxidations may be enhanced by the localized concentration of detrital pigments and other lipophilic material within hydrophobic microenvironments