Abstract:

Ostreococcus, the smallest known free-living eukaryote, plays a pivotal role in global primary production and marine biogeochemical cycles. This study presents the first comprehensive characterization of Ostreococcus clade E through the isolation and analysis of strain SK1113. Using PacBio-Illumina hybrid genome sequencing and assembly, we revealed a highly streamlined genome comprising 20 chromosomes, including two specialized outlier chromosomes, which contain distinct gene clusters encoding stress responses, cellular defense mechanisms, and viral interactions, indicating their crucial role in environmental adaptation. Physiological studies showed clade E thrives under phosphate limitation but struggles with nitrogen scarcity. Ecological monitoring showed that clade E exhibits thermal plasticity, thriving across a wide temperature range (3°C–27°C) and dominating during spring in Hong Kong coastal waters. Furthermore, we successfully isolated and characterized three novel prasinoviruses (OE-V1, OE-V2, OE-V3) specific to clade E, expanding our understanding of marine virus-host interactions. These viruses exhibited diverse infection strategies, ranging from rapid lytic cycles to more complex temperate lysogenic phases. Climate variability and environmental fluctuations are likely to amplify the ecological importance of Ostreococcus clade E in nutrient cycling and carbon dynamics. Combining these findings. This study advances our understanding of picoeukaryote diversity and host-virus interactions in marine systems, while establishing a foundation for investigating the ecological roles of Ostreococcus in a rapidly changing ocean. Our findings emphasize the importance of this newly characterized clade in marine ecosystem functioning and its potential responses to future environmental changes.
 

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