Phylogenomic analysis of metagenome-assembled genomes indicates new taxa in the order Spirochaetales and proposal of Thalassospirochaeta sargassi gen. nov. sp. nov. from seaweeds.

Six metagenome-assembled genomes (JB008, JB007, JB015, JB003, JB004, and JB002) belonging to the order Spirochaetales were generated from seaweed samples collected from the Gulf of Mannar, India. The binned genomes JB008 and JB007 shared highest 16S rRNA gene identity of 94.9 % and 92.2-93.4 %, respectively with uncultivated Spirochaetaceae family members, and < 90 % identity with Marispirochaeta aestuari JC444. While, the bin JB015 showed 99.1 % identity with Pleomorphochaeta naphthae SEBR 4209. The phylogenomic and 16S rRNA gene-based phylogenetic analysis of the binned genomes JB007 and JB008 confirmed that these members belong to the family Spirochaetaceae and bins JB015, JB002, JB003, and JB004 belong to the genus Pleomorphochaeta within the family Sphaerochaetaceae. The AAI values of the binned genomes JB007 and JB008 compared to other members of the Spirochaetaceae family were between 53.9- 56.8 % and 53.8-57.1 %, respectively. Furthermore, the comparison of ANI, dDDH, and POCP metrics of the binned genomes JB007 and JB008, both among themselves and with the members of Spirochaetaceae, was also below the suggested thresholds for genera delineation. Consequently, the binned genome JB008 is proposed as a new genus according to the guidelines of code of nomenclature of prokaryotes described from sequence data (SeqCode) with the name Thalassospirochaeta sargassi gen. nov. sp. nov., in the family Spirochaetaceae while the bin JB007 could not be proposed as novel taxa due to low-quality estimates. The bin JB015 and its additional genomes form a distinct clade, but their taxonomic status remains ambiguous due to the absence of genomic evidence from other Pleomorphochaeta members.