Bacterial endosymbionts of the family Ca. Azoamicaceae obligately associate with anaerobic ciliates belonging to the class Plagiopylea. The symbionts' unique role for their host involves anaerobic respiration of nitrate and generation of ATP, analogous to the role of mitochondria in aerobic eukaryotes. As this symbiosis remains so far uncultured, insights into its functioning have been mainly inferred from environmental metagenomes. Here we investigated the distribution and environmental role of this symbiosis in the anoxic basins of two freshwater lakes Zug and Lugano (Switzerland) over a course of several years. We found that the environmental niche of the ciliate host is defined by the combined effects of sulfide, oxygen and nitrate, the latter of which is essential for the symbiont's respiratory function. Moreover, the distribution and abundances of ciliates with denitrifying endosymbionts in the water column suggests that they may substantially contribute to nitrate consumption in Lake Zug. Our microscopic analyses further demonstrated a coordinated division of the Candidatus Azoamicus ciliaticola symbionts and their ciliate hosts, implying a vertical inheritance of denitrifying symbionts. These observations offer new insights into the evolution of ciliates with denitrifying endosymbionts and their ecological role in oxygen-depleted lakewaters.
Zeller et al. (Fri,) studied this question.