Genetic diversity and distribution of Karenia in the eastern coastal seas of China and implications for the trends in Karenia blooms under global environmental changes.

Species of Karenia G. Hansen & Moestrup gen. nov. frequently contribute to intense harmful algal blooms on a global scale, resulting in detrimental effects on fisheries, aquatic ecosystems, and human health. Over the past two decades, there has been noticeable increase in the reporting of Karenia blooms, with outbreaks attributed to newly recorded species, sometimes with multiple causative species. This trend highlights an insufficient understanding of species diversity and geographical distribution of Karenia and related key environment drivers under global environmental change. In this study, we employed a tailored barcode for genus Karenia combined with high-throughput sequencing to examine the species diversity and geographical dispersion of Karenia, as well as their relationships with environment factors in the eastern Chinese coastal seas (ECCS) in spring and autumn. Our findings revealed an unprecedented presence of both described and unrecorded Karenia species in the ECCS. K. mikimotoi was predominantly observed in the cold waters of the ECCS north of 35°N in autumn and the major waters of the ECCS in spring, while various Karenia species tend to co-inhabit in the warmer waters of autumn in the East China Sea. Sea temperatures were significantly correlated to the distribution patterns of Karenia species in the ECCS. In contrast, concentrations of inorganic nitrogen and phosphorus were not identified as major correlates to Karenia distribution. In light of the findings of this study and the understanding that Karenia species exhibit strong mixotrophic capabilities, it is suggested that ocean warming and increased coastal eutrophication, particularly the rise of dissolved and particulate organic substances, may contribute to the proliferation of Karenia blooms associated with undocumented species and/or multiple causative species.