Diversidade e distribuição de ciliados da Família Strombidiidae (Ciliophora) no oceano global

Abstract

Ciliates dominate microzooplankton and play an essential role in marine food webs. By ingesting flagellates, bacteria and other organisms from pico and nano-plankton, and then being consumed by zooplankton, ciliates are the main pathway of carbon flow that links the microbial loop to the classical trophic chain. However, the diversity and distribution of ciliates in the global ocean is still poorly understood due to the difficulty of observing and classifying these organisms under light microscopy. When fixed, most ciliates are lost or change their morphology, making their identification difficult. Thus, the use of molecular tools represents an opportunity to study the ciliate community in the ocean. Within ciliates, the Strombidiidae family is amongst the most abundant, but there are still few systematic studies at the global scale on the distribution, abundance and phylogeny of this important group in the ocean. This study aims to: 1) evaluate the phylogenetic diversity of Strombidiidae, 2) understand its distribution in the global ocean, 3) determine the ecological niche of the most abundant clades. We used a metabarcoding dataset (region V4 18S rRNA) of 1008 samples from the TARA Oceans and TARA Polar circum-navigation expeditions. We constructed a Strombidiidae reference tree with complete 18S rRNA gene sequences from the literature and inserted the metabarcoding sequences for phylogenetic assignment. Then we related the distribution data of these clades with environmental data to define ecological niches. The phylogeny of Strombidiidae revealed 20 distinct clades, almost all of them widely distributed in most oceanic regions. The most abundant clade (Strombidium_clade2) is cosmopolitan. Furthermore, we obtained 12 environmental clades whose sequences (V4 18S rRNA region) are not identical to those described in the reference tree, and thus, may represent new clades. Niche analysis showed that the different clades of the Strombidiidae family had different distribution patterns in the global ocean, determined by temperature, salinity and oxygen. The Arctic Ocean had greater abundance among the Strombidiidae clades and very characteristic niches were observed for these oceanographic regions. This work represents the first study of the taxonomic and phylogenetic diversity of the Strombidiidae family in the global ocean, thus contributing to the knowledge of the ecology of marine ciliates.