About half a kilometer beneath the surface of the ocean, currents move large masses of water slowly throughout the world’s oceans. Physical oceanographers have characterized ocean water masses by their salinity, temperature, age, and a variety of other physical and chemical features. Now, researchers have a map of how ocean microbes associate with these water masses too.
Adapted from Figure 4 in Kolody BC, et al. Overturning circulation structures the microbial functional seascape of the South Pacific. Science. 2025;389(6756):176-182.; © istock.com, alejomiranda, Tomacco, pijama61; designed by erin lemieux
1) Surface Water
At the surface, wind and waves mix the ocean water. Given the easy access to sunlight at the surface, microbial species with genes for iron acquisition, photosynthesis, and heme biosynthesis thrive in this layer.
2) Antarctic Intermediate Water
The Antarctic Intermediate Water mass sits just below the surface waters in the mesopelagic or “twilight” zone of the ocean. Here, microbes with genes that metabolize certain carbon and aromatic compounds are enriched. Some also have genes that can oxidize nutrients produced by algae.
3) Upper Circumpolar Deep Water
Analysis of the cohort of microbes that clustered in the Upper Circumpolar Deep Water mass revealed that they had genes for nitric oxide reduction, metal ion homeostasis, microaerophilic metabolism. This group also had many archaeal markers.
4) Pacific Deep Water
The oldest water in the world resides in the Pacific Deep Water mass. Microbes here have adapted to the nutrient- and oxygen-poor environment by possessing genes for anaerobic metabolism, degrading recalcitrant carbon, and metabolizing sulfur compounds. The researchers found that these microbes were also enriched with trehalose biosynthesis genes, which help stabilize biomolecules from environmental stress.
5) Antarctic Bottom Water
Some of the youngest water in the ocean forms just off the coast of Antarctica. The Antarctic Bottom Water mass is very salty and cold. Microbes that live in this extreme environment have genes that help maintain membrane fluidity, promote osmotic regulation, promote bacteriostasis in stressful situations, and transposases.
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