Molecular characterization of harmful algal blooms in the Bohai Sea using metabarcoding analysis

Although the occurrences of harmful algal blooms (HABs) have been intensifying, many HABs in coastal waters may have been neglected despite their damaging impact directly on ecology and indirectly on human and animal health. The current detection of HABs depends primarily on the water coloration, chlorophyll intensity, cell density, and mortality due to HAB toxicity. Such methods may not be adequately sensitive to detecting HABs that are relatively transient or small scale. The Bohai Sea is the largest inlet of the Yellow Sea located on the northeast coast of China and famous for shipping and marine aquacultures. HABs frequently occur in the Bohai Sea. In this study, we explored the composition, diversity, and distribution of HAB species using the metabarcoding approach. Through sequencing and the analyzing the 18S rDNA V4 region of 15 samples collected from spatially isolated sites in the Bohai Sea during an expedition in the summer of 2019, we identified 74 potential HAB species including 34 that had not been reported in the Bohai Sea in previous studies. This project provided a detailed analysis of phytoplankton composition, and molecular detection of HAB species in the Bohai Sea. In particular, these analyses revealed extremely high relative abundances of the ichthyotoxic phytoplankton species Vicicitus globosus (Dictyochophyceae) at multiple adjacent sampling sites in the Bohai Bay, which were close to the Yellow River Estuary during the expedition. The results revealed the occurrence of a potential HAB event that would be otherwise undetected using conventional methods, highlighting the sensitivity and power of metabarcoding analysis in detecting HABs and HAB species. This research suggested the value for routine and long-term monitoring of HAB species as an approach for monitoring HABs.