Global and local DNA (meta)barcoding reveal new biogeography patterns in tintinnid ciliates

Tintinnid ciliates are suitable models to study the diversity and biogeography of microbial plankton. In addition to morphological data accumulated over two centuries, most known families and common genera have been linked to DNA sequences in relatively recent barcoding efforts. This backbone of morphologically identified sequences is used here to classify environmental sequences in order to study global and local spatial trends. Analyses of tintinnid SSU rDNA data collected worldwide (about 900 sequences available in NCBI GenBank) and in the northwest Atlantic Ocean (about 500,000 sequences obtained by metabarcoding) support distribution patterns related to salinity, bathymetry and climate/ latitude. In addition to the marine–freshwater dichotomy and a pattern of coastal-only taxa known for tintinnids, there is a global trend of phylotypes restricted to brackish or open waters. Local alpha and beta diversity analyses show that assemblage differences among estuarine, coastal and open waters are not significant regarding richness, but are significant in terms of phylogenetic composition. We also confirm spatial restriction of boreal and austral taxa, and stress that cosmopolitanism cannot be assessed by molecular methods that lump data from potentially endemic and commonly widespread taxa. Heterogeneous diversity, biogeography and phylogenetic resolution within and among tintinnid lineages raise questions about the processes that promote their diversification and determine their spatial distributions.