Diversity and temporal patterns of planktonic protist assemblages at a Mediterranean Long Term Ecological Research site

We tracked temporal changes in protist diversity at the Long Term Ecological Research (LTER)-MC site in the Gulf of Naples (Mediterranean Sea) on eight dates in 2011 using a metabarcoding approach. The ILLUMINA analysis of the V4 and V9 fragments of the 18S rDNA produced 869,522 and 1,410,071 sequences resulting in 6,517 and 6,519 OTUs, respectively. Marked compositional variations were recorded across the year, with less than 2% of OTUs shared among all samples and similar patterns for the two marker tags. Alveolata, Stramenopiles and Rhizaria were the most represented groups. A comparison with light microscopy data indicated an over-representation of Dinophyta in the sequence dataset, whereas Bacillariophyta showed comparable taxonomic patterns between sequences and light microscopy data. Shannon diversity values were stable from February to September, increasing thereafter with a peak in December. Community variance was mainly explained by seasonality (as temperature), trophic status (as chlorophyll a), and influence of coastal waters (as salinity). Overall, the background knowledge of the system provided a sound context for the result interpretation, showing that LTER sites provide an ideal setting for the HTS-metabarcoding characterization of protist assemblages and their relationships with environmental variations.