Genomic evidence reveals temperature-associated ecotypic divergence of Phaeocystis globosa in Chinese coastal waters

Phaeocystis globosa, a cosmopolitan species notorious for triggering harmful algal blooms (HABs), displays substantial genetic variation across geographically isolated populations. This study analyzed the morphological and genetic characteristics of four P. globosa strains isolated from distinct coastal regions of China, encompassing the Beibu Gulf - Guangxi (strain BB172), Mirs Bay - Shenzhen (strain SZ211), Wuyuan Gulf - Xiamen (strain XM-1), and Tangdao Bay - South Yellow Sea near Qingdao (strain SYS01). These strains share similar morphological characteristics and cannot be reliably distinguished based on morphology. Phylogenetic analysis based on multiple molecular markers including nuclear 18S rDNA, 28S rDNA D1-D2 regions, mitochondrial COX1, and chloroplast pgcp1 sequences demonstrated a high level of genetic diversity (comprising over 156 sequences) of P. globosa populations. Phylogenetic population structure analysis delineated two evolutionarily distinct ecotypes: warm-adapted (Beibu Gulf and Mirs Bay strains) and cold-adapted lineages (Tangdao Bay and Wuyuan Gulf strains). Notably, southern populations were represented in both ecotypes and exhibited higher genetic diversity (South vs. North sequences: 69:27), suggesting a possible speciation hotspot. In contrast, northern strains were restricted to the cold-adapted clade and showed reduced variation, consistent with an origin from recent colonization or environmental filtering by a limited number of founders, followed by rapid local adaptation to colder temperatures. The clear temperature-associated genomic partitioning underscores the role of thermal adaptation in driving ecotypic diversification. These findings provide a genomic framework for understanding the biogeography and bloom dynamics of P. globosa across China's coastal temperature gradients, with implications for predicting HABs responses to climate change.