Diversity patterns and isolation of Planctomycetes in metalliferous deposits from hydrothermal vent fields along the Valu Fa Ridge

The microbial diversity associated with diffuse venting deep-sea hydrothermal deposits is tightly coupled to the geochemistry of the hydrothermal fluids. Previous analyses of marine iron-hydroxide deposits along the Arctic Mid Ocean Ridge, using 16S rRNA amplicon sequencing (metabarcoding), revealed the presence of a diverse bacterial community at these sites. One of the most abundant and diverse groups detected were the enigmatic Planctomycetes. Here we report on the comparative analyses of the diversity and distribution patterns of Planctomycetes associated with metalliferous deposits from two diffuse flow hydrothermal vent fields (Mariner and Vai Lili) from the Valu Fa Ridge (VFR) in South West Pacific. Amplicon pyrosequencing showed that the major prokaryotic phyla were Proteobacteria (51 – 73% of all 16S rRNA gene reads), Epsilonbacteria (0.5 – 19%). Bacterioidetes (5-17%), Planctomycetes (0.4 – 11%), Candidatus Latescibacteria (0 – 5%) and Marine Benthic Group E (Hydrothermarchaeota (0-5%). The two different sampling sites varied significant in overall community composition. The abundance of Planctomycetes also varied substantially between the samples and the sites, and the majority of the sequences affiliated with uncultivated members of the classes Planctomycetacia and Physciphaerae, as well as other deep branching lineages. Seven different strains affiliated with the Planctomycetales were isolated, mostly from the Vai Lili samples where also the highest Planctomycetales diversity was seen. Most of the isolates affiliated with the genera Gimesia, Rhodopirellula and Blastopirellula. One isolate was only distantly related to known cultured, but uncharacterized species within the Pir4 group. Another strain was related to members within the Planctomycetes_1 clade which are known to be able to sequester iron. This study shows that the Planctomycetes in these environments represents a highly heterogeneous group with a high genetic diversity and a substantial genetic potential for novel organism discovery in these deep ocean environments. However, it is still poorly understood if the Planctomycetes are closely connected to the geochemical conditions or if their distribution is driven by other factors such as interactions within the microbial community and degradation of organic compounds.