Anaerobic free-living protozoa: growth efficiencies and the structure of anaeorobic communities

Abstract

Most anoxic environments host populations of phagotrophic, eukaryote microorganisms. Many physiological properties of these anaerobic eukaryotes are still incompletely understood and their role in communities of anaerobic microorganisms has so far drawn little attention. Here we present theoretical considerations and experimental evidence to show that the net growth efficiency ([assimilated C]/[assimilated C + dissimilated C]) and gross growth efficiency (yield = [assimilated C]/[consumed C]) of anaerobic protozoa are about 20% and about 25%, respectively of those of their aerobic counterparts. This accords with the observation that the biomass ratios of predators and their prey is about one fourth of that foundin oxic environments. These fiedl data also suggest that bacterial numbers are controlled by protozoa grazing in at least some anoxic environments. Finally, the results explain whe phagotrophic food chains are short and eukaryote diversity is low in anaerobic habitats.