Primary data of thiotrophic free-living and ectosymbiotic genomes

Thiotrophic symbioses between sulfur-oxidizing bacteria and various unicellular and metazoan eukaryotes are widespread in reducing marine environments. The giant colonial ciliate Zoothamnium niveum and its vertically transmitted ectosymbiont Candidatus Thiobius zoothamnicola (short Thiobius), however, is the only thiotrophic mutualism that has been cultivated so far. Because theoretical predictions posit a smaller genome in vertically transmitted endosymbionts compared to free-living relatives, we investigated whether this is true also for an ectosymbiont. We used metagenomics to recover the high-quality draft genome of this bacterial symbiont. For comparison, we have also sequenced a closely related free-living cultured but not formally described strain Milos ODIII6 (short ODIII6). We then performed comparative genomics to assess the functional capabilities at the gene, metabolic pathway, and trait levels. 16S rRNA gene trees and average amino acid identity confirmed the close phylogen...