Diversity and seasonal dynamics of the marine diatom family Chaetocerotaceae (Bacillariophyta, Mediophyceae) in Catalan coastal waters (northwest Mediterranean).

The family Chaetocerotaceae includes the genera Chaetoceros and Bacteriastrum, with Chaetoceros being one of the most cosmopolitan, abundant, and diverse diatom genera in the oceans. This study investigated the diversity and seasonal dynamics of the Chaetocerotaceae in surface waters of two coastal sites along the Catalan coast (Barcelona and Blanes Bay), from 2004 to 2019. A combination of field sample observations, morphological (light and scanning electron microscopy), and molecular (SSU and LSU rDNA gene sequences) analyses of monoclonal cultures, and V4 18S rDNA metabarcoding time-series data from Blanes was used for species identification. The seasonal dynamics of both genera were investigated during two annual cycles at both sites through monthly light microscopy observations and, at Blanes Bay, also through the metabarcoding dataset. A total of 60 species of Chaetoceros and 10 of Bacteriastrum were detected. Based on cell ultramorphology and SSU and LSU rDNA gene phylogenies, two species within the genus Chaetoceros are putatively new to science, although they are not formally described here. The most abundant and frequent taxa were C. tenuissimus, C. socialis, C. vixvisibilis, and the C. curvisetus complex. These species presented similar seasonal trends at both sites, with C. tenuissimus and C. vixvisibilis peaking in late spring and summer, whereas C. socialis was more prevalent in winter. The C. curvisetus complex did not show a clear seasonality although its components were less abundant in summer. These results highlight the value of integrating morphological and molecular approaches to assess the diversity and ecological dynamics of the dominant Chaetoceros species.