Eukaryotic Picophytoplankton of the class Mamiellophyceae are key players in global ocean ecosystems, yet their genomic diversity and environmental adaptations remain poorly understood in many regions. This study presents a comprehensive analysis of Mamiellophyceae in the South China Sea, integrating genomics with environmental data to elucidate their ecological significance. We isolated and sequenced multiple novel strains from Hong Kong coastal waters, producing high-quality genome assemblies that revealed substantial genomic diversity among clades. Comparative genomic analyses, focusing on hypervariable chromosomal regions and gene family evolution, provided insights into potential mechanisms of niche adaptation. By combining our genomic data with global ocean surveys, we mapped the distribution of Mamiellophyceae clades across diverse marine environments, from coastal waters to open oceans. Our comprehensive time-series study, integrating metagenomic and metatranscriptomic data from a subtropical coastal site, unveiled intricate seasonal dynamics of Mamiellophyceae clades. This analysis revealed distinct temporal patterns in both abundance and gene expression, correlating with key environmental variables such as temperature, nutrients, and light availability. Our findings significantly enhance understanding of Mamiellophyceae's genomic diversity, niche adaptations, and spatio-temporal dynamics in marine ecosystems, with particular relevance to the South China Sea region. This research provides crucial insights into the ecological roles and environmental responses of these important marine primary producers, offering valuable resources for predicting their behavior in changing ocean conditions.
 

picoeukaryote, green algae, niche adaptation, phytoplankton diversity