Genomic characterisation and ecological distribution of : a novel Mamiellophycean green alga from the Western Pacific.

Mamiellophyceae are dominant marine algae in much of the ocean, the most prevalent genera belonging to the order Mamiellales: , and , whose genetics and global distributions have been extensively studied. Conversely, the genus , despite its potential ecological importance, remains relatively under-characterised. In this study, we isolated and characterised a novel species of Mamiellophyceae, , from subtropical coastal waters in the South China Sea. Morphologically, it resembles other species; however, a comparative analysis of the 18S and ITS2 marker genes revealed its genetic distinctiveness. Furthermore, we sequenced and assembled the first genome of , uncovering significant differences from previously studied Mamiellophyceae species. Notably, the genome lacked any detectable outlier chromosomes and exhibited numerous unique orthogroups. We explored gene groups associated with meiosis, scale and flagella formation, shedding light on species divergence, yet further investigation is warranted. To elucidate the biogeography of , we conducted a comprehensive analysis using global metagenomic read mapping to the newly sequenced genome. Our findings indicate this species exhibits a cosmopolitan distribution with a low-level prevalence worldwide. Understanding the intricate dynamics between Mamiellophyceae and the environment is crucial for comprehending their impact on the ocean ecosystem and accurately predicting their response to forthcoming environmental changes.