Tsukuba, Japan—It was thought, until recently, that the carbon demand of deep-sea microbes was much higher than the amount of available organic carbon particles floating down to the depths. Now, researchers from Japan have discovered why this isn't the case.
In a recently published study, a research group led by the University of Tsukuba has revealed that high pressure conditions in the deep sea have considerable limiting effects on the metabolic activity of deep-sea microbes.
The deep sea is a place of cold, dark, and high-pressure conditions. In the bathypelagic zone (1,000-4,000 meters), no natural light from above the surface reaches these depths, and the temperature ranges from 0°C to 4°C. Some microbes living in this zone are piezophilic (i.e., they have maximum growth rates at a hydrostatic pressure (the force created by a non-moving fluid) of more than 10 megapascals (MPa), which corresponds to a depth of 1,000 meters). Others are piezotolerant or piezosensitive, exhibiting maximum growth at less than 10 MPa and at atmospheric pressure (0.1 MPa), respectively. All three types can still grow at higher hydrostatic pressures, but at lower rates.
"How these microbes respond to the high-pressure conditions of the deep sea is not well understood," says co-author Professor Motoo Utsumi. "In this study, we investigated whether the metabolic activity of most microbial community members is suppressed, or if only a small proportion respond to depressurization with heightened activity."
The research group measured single-cell activity using the in situ microbial incubator (ISMI) that was lowered into oceanic basins. They found that the higher the hydrostatic pressure, the greater the inhibition of bulk activity by heterotrophic prokaryotes (single-celled organisms that lack a nucleus and other membrane-bound cell compartments, and cannot produce their own food—instead obtaining their nutrition from other sources of organic carbon).
At a depth of 4,000 meters, the bulk microbial activity was about a third of the amount measured in the same community at atmospheric pressure. In the bathypelagic zone, ~85% of the prokaryotic community was piezotolerant, and ~5% was piezophilic. Despite piezosensitive prokaryotes making up only ~10% of the deep-sea prokaryotic community, the more than 100-fold increase in their metabolic activity after depressurization contributed to a high apparent proportion of metabolic activity.
"Overall, the activity of heterotrophic prokaryotes in the deep sea is probably considerably lower than has been previously assumed—with major effects on oceanic carbon cycling," explains Professor Utsumi.
The group's findings could have important consequences for geoengineering strategies to address climate change, such as the transfer of organic carbon to the deep sea to reduce increased atmospheric carbon dioxide levels.
This study was supported by JSPS KAKENHI Grant (23651004) to M.U., the Austrian Science Fund (FWF) project I486-B09, Z194, P28781-B21 and P35587-B to G.J.H., P27696-B22 to E.S. and P23221-B11 to T.R., and the European Research Council under the European Community's Seventh Framework Program (FP7/2007- 2013)/ERC grant agreement (MEDEA project 268595) to G.J.H. C.A. was supported by JSPS Postdoctoral Fellowships for Research Abroad (H26-168), the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie no. 701324 and ERC Advanced Grant (TACKLE project, 695192).
The article, "Limited carbon cycling due to high-pressure effects on the deep-sea microbiome," was published in Nature Geoscience at DOI: 10.1038/s41561-022-01081-3
Professor UTSUMI Motoo
Faculty of Life and Environmental Sciences, University of Tsukuba
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