Diversity and temporal pattern of Pseudo-nitzschia species (Bacillariophyceae) through the molecular lens

Abstract

In diatoms, as in other organisms, many genetically distinct and reproductively isolated species may show identical or highly similar morphological features. Such groups of species are defined as cryptic and pseudo-cryptic species, respectively. The difficulty of discriminating them with optical means impairs the study of their temporal patterns and geographic ranges. This is also the case for Pseudo-nitzschia, a worldwide distributed planktonic diatom genus which includes several toxigenic species. Using a Pseudo-nitzschia-specific pair of large sub-units ribosomal DNA (LSU rDNA) primers, we generated clone libraries from 19 samples collected at the Long Term Ecological Station MareChiara (LTER-MC) in the Gulf of Naples (GoN) from 2009 to 2010 and compared sequence records with light microscopy (LM) counts from the same samples. Our aim was to elucidate the diversity and the seasonal patterns of taxa within Pseudo-nitzschia. Most of the Pseudo-nitzschia species already known from the GoN were identified within the 1643 obtained sequences. In addition, two species known from elsewhere and three un-described ribotypes were detected. Several cryptic species showed distinct temporal patterns of occurrence, with most species confined to restricted periods and only a few present year-round. Microscopic and molecular results generally concurred for species recognizable using LM, while clone libraries tended to overestimate the relative abundance of some of the species. Due to its high resolution and detection power, the DNA-barcoding approach used in our study is an optimal tool to trace the distribution of cryptic and toxigenic Pseudo-nitzschia species and the diversity of this key diatom genus in the natural environment.