Search

The bacterial phylum Chlamydiae is so far composed of obligate symbionts of eukaryotic hosts. Well known for Chlamydiaceae, pathogens of humans and other animals, Chlamydiae also include so-called environmental lineages that primarily infect microbial eukaryotes. Environmental surveys indicate that Chlamydiae are found in a wider range of environments than anticipated previously. However, the vast majority of this chlamydial diversity has been underexplored, biasing our current understanding of their biology, ecological importance, and evolution. Here, we report that previously undetected and active chlamydial lineages dominate microbial communities in deep anoxic marine sediments taken from the Arctic Mid-Ocean Ridge. Reaching relative abundances of up to 43% of the bacterial community, and a maximum diversity of 163 different species-level taxonomic units, these Chlamydiae represent important community members. Using genome-resolved metagenomics, we reconstructed 24 draft chlamydial genomes, expanding by over a third the known genomic diversity in this phylum. Phylogenomic analyses revealed several novel clades across the phylum, including a previously unknown sister lineage of the Chlamydiaceae, providing new insights into the origin of pathogenicity in this family. We were unable to identify putative eukaryotic hosts for these marine sediment chlamydiae, despite identifying genomic features that may be indicative of host-association. The high abundance and genomic diversity of Chlamydiae in these anoxic marine sediments indicate that some members could play an important, and thus far overlooked, ecological role in such environments and may indicate alternate lifestyle strategies., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 Elsevier Inc. All rights reserved.)

The origin of the eukaryotic cell remains one of the most contentious puzzles in modern biology. Recent studies have provided support for the emergence of the eukaryotic host cell from within the archaeal domain of life, but the identity and nature of the putative archaeal ancestor remain a subject of debate. Here we describe the discovery of 'Lokiarchaeota', a novel candidate archaeal phylum, which forms a monophyletic group with eukaryotes in phylogenomic analyses, and whose genomes encode an expanded repertoire of eukaryotic signature proteins that are suggestive of sophisticated membrane remodelling capabilities. Our results provide strong support for hypotheses in which the eukaryotic host evolved from a bona fide archaeon, and demonstrate that many components that underpin eukaryote-specific features were already present in that ancestor. This provided the host with a rich genomic 'starter-kit' to support the increase in the cellular and genomic complexity that is characteristic of eukaryotes.