Planktonic protist diversity across contrasting Subtropical and Subantarctic waters of the southwest Pacific
Authors: Gutiérrez-Rodríguez Andres , dos Santos Adriana Lopes , Safi Karl , Probert Ian , Not Fabrice , Fernández Denise , Gourvil Priscillia , Bilewitch Jaret , Hulston Debbie , Pinkerton Matt , Nodder Scott D 
Published: September 2021

ABSTRACTPlanktonic protists are an essential component of marine pelagic ecosystems where they mediate important trophic and biogeochemical functions. Although these functions are largely influenced by their taxonomic affiliation, the composition and spatial variability of planktonic protist communities remain poorly characterized in vast areas of the ocean. Here, we investigated the diversity of these communities in contrasting oceanographic conditions of the southwest Pacific sector (33-58°S) using DNA metabarcoding of the 18S rRNA gene. Seawater samples collected during twelve cruises (n = 482, 0-2000 m) conducted east of New Zealand were used to characterize protist communities in Subtropical (STW) and Subantarctic (SAW) water masses and the Subtropical Front (STF) that separates them. Diversity decreased with latitude and temperature but tended to be lowest in the STF. Sample ordination resulting from the abundance of amplicon single variants (ASVs) corresponded to the different water masses. Overall,Dinophyceae(34% of standardized total number of reads) and Chlorophyta (27%) co-dominated the euphotic zone, but their relative abundance and composition at class and lower taxonomic levels varied consistently between water masses. Among Chlorophyta, several picoplanktonic algae species of theMamiellophyceaeclass includingOstreococcus lucimarinusdominated in STW, while theChloropicophyceaespeciesChloroparvula pacificawas most abundant in SAW.Bacillariophyta(7%),Prymnesiophyceae(5%), andPelagophyceae(3%) classes were less abundant but showed analogous water mass specificity at class and finer taxonomic levels. Protist community composition in the STF had mixed characteristics and showed regional differences with the southern STF (50°S) having more resemblance with subantarctic communities than the STF over the Chatham Rise region (42-44°S). Below the euphotic zone, Radiolaria sequences dominated the dataset (52%) followed byDinophyceae(27%) and other heterotrophic groups like Marine Stramenopiles and ciliates (3%). Among Radiolaria, several unidentified ASVs assigned toSpumellaridawere most abundant, but showed significantly different distribution between STW and SAW highlighting the need to further investigate the taxonomy and ecology of this group. This study represents a significant step forward towards characterizing protistan communities composition in relation to major water masses and fronts in the South Pacific providing new insights about the biogeography and ecological preferences of different taxa from class to species and genotypic level.HighlightsWater-mass preference of different taxa emerged at class, species and genotypic level.Mamiellophyceaegreen algae dominated in subtropical waters.DinophyceaeandChloropicophyceaegreen algae dominated in subantarctic waters.A diverse assemblage of Radiolaria dominated the mesopelagic zone.Small rather than large taxa dominated phytoplankton blooms in subtropical waters.