The Ross Sea is one of the most productive marginal seas in the Southern Ocean and plays a pivotal role in global ocean circulation and carbon cycling. Its hydrographic structure is shaped by distinct water masses, including Antarctic Surface Water (AASW), Shelf Water (SW), and modified Circumpolar Deep Water (mCDW), each characterized by contrasting physicochemical characteristics. Surface AASW is strongly influenced by seasonal sea-ice melt, whereas deeper SW and mCDW remain cold, saline, and nutrient-rich throughout the year. These vertically structured water masses create pronounced environmental gradients that may regulate microbial habitats and ecosystem processes. However, how water-mass stratification influences active microbial communities and their cross-domain ecological interactions remains poorly understood.
Here, we investigated the composition and potential ecological interactions of active microbial eukaryotic and prokaryotic communities across contrasting water masses in the Ross Sea. During the austral summer, seawater samples were collected across horizontal and vertical gradients from the surface to 596 m depth. RNA-based high-throughput sequencing, environmental analyses, and cross-domain co-occurrence network analysis were combined to characterize active microbial communities and their potential interactions among water masses.
Distinct water masses supported contrasting active microbial assemblages and cross-domain interaction patterns. Surface AASW was characterized by diatom-dominated communities closely associated with heterotrophic bacterial groups, suggesting strong coupling between primary production and organic matter remineralization. In contrast, deeper mCDW harbored higher portions of heterotrophic and parasitic protists, including Syndiniales, together with deep-water bacterial and archaeal lineages. These communities formed more complex interaction networks dominated by putative parasitic, predatory, and heterotrophic associations.
Our results highlighted water-mass stratification as a key ecological factor structuring active microbial biodiversity and cross-domain interactions in the Ross Sea. By linking hydrographic structure with microbial food-web organization, this study provides new insights into microbial contributions to ecosystem functioning and carbon cycling in Antarctic marginal seas under ongoing climate change.