Ocean study shows most deep-sea microbes are nearly inactive

In situ mass leucine incorporation rates normalized to rates obtained under atmospheric pressure conditions. The symbols correspond to the different research expeditions. The regression equation is a power law function: P_{in situ} = 494z^−0.321 (not= 56, number of samples incubated in situ), where P_{in situ}is the percentage of leucine incorporation rate in situ normalized to the average leucine incorporation rate under atmospheric pressure (Atm.) and zis the depth (m). The shaded area indicates a 95% confidence interval for the regression. Note that the data points at 0 m (not= 4) correspond to instrumental tests in which epi- to bathypelagic waters were incubated with ISMI under atmospheric pressure conditions and compared to bottle incubations used for atmospheric pressure incubations to assess potential bias associated with the instrument. These points are excluded from the calculation of the regression line. Credit: nature geoscience(2022). DOI: 10.1038/s41561-022-01081-3

A team of researchers from the University of Vienna, together with a colleague from the University of Tsukuba, found that the vast majority of deep-sea microbes are mostly inactive. The group describes its long-term study of microbes at great depths in the Atlantic and Pacific Oceans and the Mediterranean Sea in the journal nature geoscience.

For many years, ocean scientists assumed that the world’s oceans were teeming with microbes that would eat anything that died in their waters. And while that turned out to be true, it’s not true in the way it was thought to be, at least not according to the results of this new research.

Previous research has suggested that at least some parts of the deep ocean either lack microbes or harbor microbes that do not consume biomatter. A bologna sandwich, for example, found in a sunken submersible with its hatch open, had barely deteriorated, even after 10 months deep underwater. Clearly, the underwater ecosystem is large and varied, with some areas more prone to biomolecular degradation than others.

Such observations have led other researchers to suggest that certain types of algae could extract carbon from the air, sink to death, and carry the carbon with it, sequestering it in very deep, cold waters. But that doesn’t seem likely as new evidence suggests microbes in the ocean aren’t the same everywhere.

To learn more about the microbes in the deep oceans, the researchers sailed around the world to collect samples – but rather than hoist them aboard their ship, where they could change due to pressure differences, the researchers used fluorescent probes – microbes with higher respiration rates glowed more, which the researchers used as a form of measurement. They also took a few of the different types of microbes on board their boat to conduct genetic studies.

Looking at their data, the researchers were surprised to find that only 3% of the microbes they studied converted oxygen into CO₂. The rest were mostly inactive, due, the researchers said, to pressure. They also found that greenhouse gas emissions that end up in the ocean are very unevenly distributed, suggesting trying to use the oceans to store more CO₂is currently impractical, and perhaps reckless. It also shows that ocean scientists still have a very limited understanding of ocean chemistry.

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