Comparison of Deep-Sea Picoeukaryotic Composition Estimated from the V4 and V9 Regions of 18S rRNA Gene with a Focus on the Hadal Zone of the Mariana Trench.

Diversity of microbial eukaryotes is estimated largely based on sequencing analysis of the hypervariable regions of 18S rRNA genes. But the use of different regions of 18S rRNA genes as molecular markers may generate bias in diversity estimation. Here, we compared the differences between the two most widely used markers, V4 and V9 regions of the 18S rRNA gene, in describing the diversity of epipelagic, bathypelagic, and hadal picoeukaryotes in the Challenger Deep of the Mariana Trench, which is a unique and little explored environment. Generally, the V9 region identified more OTUs in deeper waters than V4, while the V4 region provided greater Shannon diversity than V9. In the epipelagic zone, where Alveolata was the dominant group, picoeukaryotic community compositions identified by V4 and V9 markers are similar at different taxonomic levels. However, in the deep waters, the results of the two datasets show clear differences. These differences were mainly contributed by Retaria, Fungi, and Bicosoecida. The primer targeting the V9 region has an advantage in amplifying Bicosoecids in the bathypelagic and hadal zone of the Mariana Trench, and its high abundance in V9 dataset pointed out the possibility of Bicosoecids as a dominant group in this environment. Chrysophyceae, Fungi, MALV-I, and Retaria were identified as the dominant picoeukaryotes in the bathypelagic and hadal zone and potentially play important roles in deep-sea microbial food webs and biogeochemical cycling by their phagotrophic, saprotrophic, and parasitic life styles. Overall, the use of different markers of 18S rRNA gene allows a better assessment and understanding of the picoeukaryotic diversity in deep-sea environments.