Microbial Fingerprinting of Marine Water Masses in an Antarctic and Hydrographically Complex Area

Microorganisms are ubiquitous components of marine ecosystems, yet the role of water masses in their distribution remains underexplored, particularly in remote and extreme environments such as Antarctica. This work studies the microbial communities of the Gerlache-Bismarck Strait, an area with complex hydrography. In this area, we measured microbial abundances and diversity (both prokaryotic and eukaryotic), and we recorded multiple oceanographic and biogeochemical variables. Water samples covered from 0 to 400 m depth, and we included 3 size fractions: pico- (0.2-3.0µm), nano- (3.0–20 µm) and microparticles (20–200 µm). The results revealed that the water mass was the main driver determining the assembly of microbial communities and was more relevant than location, depth, size fraction, and local environmental conditions. Water masses detected in this area included AASW, GMW and TBW (in surface waters), as well as TWW, and CDW (in intermediate and deep waters). To examine in detail the links between water masses and microbes, we defined the microbial fingerprint of water masses, which includes different descriptors, such as the core microbiome and tracer microbes. Each water mass had a characteristic microbial fingerprint, which revealed ecological roles of the water masses linked to biogeochemical cycles (e.g., the ammonia-oxidizing archaea Nitrosopumilaceae in the CDW) and food webs (e.g., the phototroph Chrysochromulina simplex in the GMW). This study provides direct evidence of the strong links between microbes and water masses, revealing that microbial communities found in seawater are not only shaped by direct local conditions but also by water masses characteristics and transport. We thereby develop the concept of “microbial fingerprinting of water masses”, where microbial communities emerge as ecological indicators, an approach that can be explored and applied in other marine systems.