Candidatus Megaira are diverse symbionts of algae and ciliates with the potential for defensive symbiosis. Candidatus Megaira species extracted from algae and ciliate SRA genomes

Symbiotic microbes from the Rickettsiales genus Candidatus Megaira are known to be common associates of algae and ciliates. However genomic resources for these bacteria are scarce, limiting our understanding of their diversity and biology. We therefore utilized SRA and metagenomic assemblies to explore the diversity of this genus. We successfully extracted four draft Ca. Megaira genomes including one complete scaffold for a Ca. Megaira SRA data and identified an additional 14 draft genomes from uncategorised environmental Metagenome-Assembled Genomes. We use this information to resolve phylogeny in the hyper-diverse Ca. Megaira, with hosts broadly spanning ciliates, micro- and macro-algae, and find that current single genus Ca. Megaira significantly underestimates their diversity. We also evaluate the metabolic potential of these new Ca. Megaira from their genomic data and find no clear evidence of nutritional symbiosis. In contrast, we hypothesize a potential for defensive symbiosis in Ca. Megaira. Intriguingly, one symbiont genome revealed a proliferation of ORFs with ankyrin, tetratricopeptide and Leucine rich repeats akin to those observed in the genus Wolbachia. Onward research should investigate the phenotypic interactions between Ca. Megaira and their various potential hosts, including the economically important Nemacystus decipiens, and target acquisition of genomic information to reflect the diversity of this massively variable group.