Genetic diversity and distribution patterns of Phaeocystis globosa in the eastern Chinese coastal seas: Significance of a global intraspecific re-examination.

Phaeocystis globosa is broadly distributed across tropical and temperate oceans, exhibiting substantial genetic and phenotypic diversity. This species plays a critical role in the global carbon and sulfur biogeochemical cycles, and is associated with harmful algal blooms (HABs) in eutrophic waters, particularly in Southeast Asia and the North Sea. Although extensively studied in the South China Sea due to recurrent HAB events, its genetic diversity, geographic distribution and environmental adaptations remain poorly understood in other coastal regions of China. In this study, we investigated the genetic diversity and distribution patterns of P. globosa in the Bohai Sea (BS), Yellow Sea (YS), and East China Sea (ECS) during mid-March to late April and mid-October to early November 2021, using a high-resolution chloroplast marker rbcS-rpl27. Metabarcoding analyses identified seven distinct genotypes (Types I-VII), with Type IV corresponding to the "giant-colony" ecotype responsible for HABs along the Chinese coasts. Spatial-temporal distribution patterns indicated that Types III and VII dominated the BS and YS during spring under low-temperature conditions, while Type I prevailed in the warmer ECS during autumn. Notably, Type IV exhibited eurythermal adaptation, thriving in both seasons across a wide salinity range. Seawater temperature emerged as the primary driver of genotype distribution, while nutrient factors and salinity also contributed to niche partitioning during non-bloom periods. These findings suggest that the genetic diversity of P. globosa is structured by environmental variables, particularly temperature gradients, and underscore the importance of re-examining this organism and its blooms from an intraspecific perspective on a global scale.