Marine and freshwater microbial ecology, with emphasis on the trophic relationships between protists (microalgae and protozoa) and other planktonic and benthic microorganisms. Recent research programs have focused on the distribution, feeding ecology, respiration and nutrient regeneration of bacterivorous and herbivorous protozoa, the ecology of harmful algae, the physiology of Antarctic protists, feeding and growth of phagotrophic (mixotrophic) microalgae, and the development of molecular biological approaches for studying the ecology of free-living microorganisms.