Seasonal analysis of protistan community structure and diversity at the USC Microbial Observatory (San Pedro Channel, North Pacific Ocean)

The structure and genetic diversity of marine protistan assemblages were investigated in the upper 500 m of the water column at a Pacific Ocean time-series station off the coast of Southern California. Deoxyribonucleic acid sequence-based microbial eukaryote diversity was examined in January, April, July, and October of 2001 at four depths (5 m, chlorophyll maximum [CM], 150 m, and 500 m). A total of 2956 partial 18S ribosomal ribonucleic acid gene sequences yielded representatives from most of the major eukaryotic lineages. Notable among the taxonomic groups were recently described lineages of stramenopiles, alveolates, and euglenozoa. A large number of polycystine and acantharean sequences were observed at depth. Pairwise sequence analysis was performed to establish operational taxonomic units (OTUs) that were then used to estimate the unsampled protistan diversity by parametric and nonparametric techniques. A total of 2246 protistan sequences grouped into 377 distinct OTUs, with remaining sequences attributed to metazoa. Protistan richness estimates ranged from, 600 to 1500 OTUs when all depths and seasons were combined into a single data set. Seasonal and depth-related trends in the observed protistan diversity were apparent from comparisons of univariate and multivariate analyses. Cluster analysis combined with nonmetric multidimensional scaling and analysis of similarity testing identified distinct protistan assemblages at the shallowest depths (5 m and CM) for each season, which were significantly different (p < 0.03) from assemblages at the two deepest depths (150 and 500 m) where seasonal changes in the protistan assemblage were not apparent.