Exploration of protistan diversity from hydrothermal vent on the Central Indian Ridge, Indian Ocean

Abstract

The diverse protists play a critical role in controlling the flux of energy and transfer of materials in marine ecosystems including extreme environments. Deep-sea hydrothermal vents are one of the most unusual and highly dynamic environments, making it difficult to collect samples. To date, almost nothing is known about the diversity of protists inhabiting deep-sea hydrothermal vents based on the combinations of light microscopy and molecular sequencing. To investigate the diversity of the protistan community around the hydrothermal Onnuri Vent Field (OVF), the diverse samples around the OVF were collected by multiple corers, and TV grab on board R/V ISABU from June to July 2019. The sediment samples from the seafloor at a depth of 2020 m were analysed by Illumina high-throughput sequencing using the two V4 and V9 primer sets. Also, the ciliated protozoa were identified by light microscopic observations and molecular phylogeny of 18S rDNA sequences. Based on nextgeneration sequencing, the most abundant sequence reads in both the V4 and V9 regions of 18S rDNA amplicons were the supergroup Opisthokonta (V4: 93%, V9: 62%), and subsequently followed by the supergroups Rhizaria (V4: 2.5%, V9: 30.7%) and Alveolata (V4: 3.5%, V9: 1.0%). However, despite the relatively low sequence read, the supergroup Alveolata including ciliates displayed the highest OTUs (Operational Taxonomic Units) in the V4 and V9. A total of seven species were discovered by microscopical observation of the taxon Ciliophora, which was composed of each one species in Sessilid, Trachelocercid and Discocephalid, two Euplotes spp., Pleuronema sp., and Dysteria sp. Although the Dysteria sp. is considered as a species closed to D. semiluaris, its morphological characteristics of cell sized, shape and ciliary arrangements as well as 18S rDNA sequence differs from those of D. semiluris. Therefore, the Dysteria sp. is regarded as a candidate of new species. For species identification of other ciliate species, morphological and molecular data are needed through further investigations.