Diversity of unclassified Rhizaria in the Indian Ocean hydrothermal vents

Abstract

The submarine hydrothermal vent is widely but sparsely distributed on the seafloor deeper than 1500 m on Earth. Approximately 95% of the hydrothermal vent biota has been unknown taxa. Thus, the deep-sea hydrothermal eukaryote community remains poorly understood. Here, we investigated the benthic eukaryotic community near the Indian Ocean hydrothermal vents known as the Onnuri Vent Field (OVF, station: MC1906, 2019 m in depth) and Invent-B (station: MC1914, 4299 m in depth) using Illumina massive sequencing targeting the V4 and V9 regions of 18S rDNA. Two samples were collected from sediments at >2000 m in depth using a multiple corer, which appeared to be a reliable method capable of taking uncontaminated cores. In total, 505 operational taxonomic units (OTUs) and 680 OTUs were detected using the V4 and V9 primer sets, respectively, at two stations MC1906 and MC1914. Our data showed that Opistokonta, Alveolata, and Rhizaria groups dominated the vent eukaryote community. In particular, the Rhizaria including Foraminifera and Radiolaria, which comprises many of the amoeboid forms with fine pseudopodia, displayed the highest number of OTUs (66 OTUs from MC1906 and 72 OTUs from MC1914) and became a high-abundance group among eukaryotes (ranked second: 34% from MC1906 and 27% from MC1914) in the V9 dataset. Remarkably, at least 8 unclassified Rhizaria clades were newly discovered in the Rhizaria of the Indian Ocean hydrothermal vents (unclassified Rhizaria clade I to VIII). Our finding provides a hint that diverse unclassified Rhizaria are predominant groups around the hydrothermal vents and may play a critical role in one of the most unusual habitats.