Metabarcoding reveals high species diversity of Chaetoceros, Pseudo-nitzschia, and Thalassiosira in Hong Kong coastal waters, a typical subtropical region

Chaetoceros, Pseudo-nitzschia, and Thalassiosira are ecologically important genera which formed blooms frequently in Hong Kong coastal waters in past decades. However, species identification based on microscopic observation for diatoms in these genera is difficult. In this study, we investigated species diversity of Chaetoceros, Pseudo-nitzschia, and Thalassiosira in Hong Kong coastal waters using metabarcoding approach. Based on the analysis of SSU rRNA gene (Small subunit of ribosome), LSU rRNA gene (Large subunit of ribosome), and ITS region (Internal transcribed spacer), ITS region was the best gene marker for Pseudo-nitzschia while LSU rRNA gene was better to reveal the species diversity of Chaetoceros and Thalassiosira than other two gene markers. We detected seventeen, thirteen, and seventeen species of Chaetoceros, Pseudo-nitzschia, and Thalassiosira, respectively, in Hong Kong coastal waters. Twelve Chaetoceros species, six Pseudo-nitzschia species, and five Thalassiosira species were recorded for the first time in this study. Taking consideration of previous studies together, there are at least 31, 22 and 38 species of Chaetoceros, Pseudo-nitzschia, and Thalassiosira in Hong Kong coastal waters. In addition, according to previous studies and local records, multiple species of three genera were capable to form blooms and ten Pseudo-nitzschia species were potentially toxic. The bloom-forming species C. tenuissimus, P. micropora, P. cuspidata, and T. lundiana as well as non-bloom forming species T. tenera were the main dominant species. The remarkably high abundance of Pseudo-nitzschia in summer was potentially mainly contributed by the P. cuspidata, which is capable to produce domoic acid locally.