Light, in conjunction with nutrient supply, is the primary abiotic resource structuring life in the sea. With the exception of various behavioral phenomena such as phototaxis and bioluminescence, light has not been thought to directly influence marine zooplankton energetics. In recent research, we have found evidence for the direct use of light energy in the digestion of phytoplankton by three heterotrophic (non?photosynthetic) protists. We hypothesize that light aids in the digestion of phytoplankton prey through the photosensitized formation of reactive oxygen in grazer food vacuoles. This reactive oxygen may then degrade lipids and proteins, supplementing digestion both qualitatively (by yielding smaller, more readily assimilated compounds) and quantitatively (by providing an energy supplement). Our data indicate that light?aided digestion can result in 2? to 10- fold enhancements of ingestion rate, and up to 20?fold enhancements of population growth rates. At very low food levels, light?aided digestion can make the difference between survival and severe mortality for heterotrophic protists feeding on phytoplankton. Because protists are the primary consumers of phytoplankton in the sea, light?aided digestion has profound consequences for the regulation of phytoplankton consumption, nutrient regeneration, trophic transfer efficiency, and prey selection. We propose an investigation of the role of light in regulating protist grazing activities. The proposed research will be conducted across multiple biological levels, from molecular events in grazer food vacuoles to regulation of phytoplankton consumption in whole microplankton communities. The proposed research centers on the exciting notion that light, a fundamental abiotic resource, may regulate the activities of the ocean's primary herbivores on the fundamental level of prey digestion. A direct coupling of protist grazing, growth, and growth efficiency to irradiance means that these rates and efficiencies will vary with depth, latitude, and season in a heretofore-unsuspected manner. In addition, the degree of coupling between populations of phytoplankton and their primary consumers, a fundamental determinant of community structure and function, could be strongly mediated by a single abiotic resource, and thus might be much closer than previously believed.
