ABSTRACT Choanoflagellates offer key insights into bacterial influences on the origin and early evolution of animals. Here we report the isolation and characterization of a new colonial choanoflagellate species, Barroeca monosierra, that, unlike previously characterized species, harbors a microbiome. B. monosierra was isolated from Mono Lake, California and forms large spherical colonies that are more than an order of magnitude larger than those formed by the closely related Salpingoeca rosetta . By designing fluorescence in situ hybridization probes from metagenomic sequences, we found that B. monosierra colonies are colonized by members of the halotolerant and closely related Saccharospirillaceae and Oceanospirillaceae, as well as purple sulfur bacteria ( Ectothiorhodospiraceae ) and non-sulfur Rhodobacteraceae. This relatively simple microbiome in a close relative of animals presents a new experimental model for investigating the evolution of stable interactions among eukaryotes and bacteria. IMPORTANCE The animals and bacteria of Mono Lake (California) have evolved diverse strategies for surviving the hypersaline, alkaline, arsenic-rich environment. We sought to investigate whether the closest living relatives of animals, the choanoflagellates, exist among the relatively limited diversity of organisms in Mono Lake. We repeatedly isolated members of a single species of choanoflagellate, which we have named Barroeca monosierra, suggesting that it is a stable and abundant part of the ecosystem. Characterization of B. monosierra revealed that it forms large spherical colonies that each contain a microbiome, providing an opportunity to investigate the evolution of stable physical associations between eukaryotes and bacteria.

Colonial choanoflagellate isolated from Mono Lake harbors a microbiome