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Microbiology

Under Pressure: Novel Experiments in Microbiology on the Seafloor

Citation: Fortunato, C. S., Butterfield, D. A., Larson, B., Lawrence-Slavas, N., Algar, C. K., Allen, L. Z., Holden, J. F., Proskurowski, G., Reddington, E., Stewart, L. C., Topcuoglu, B. D., Vallino, J. J., & Huber, J. A. (2021). Seafloor incubation experiment with deep-sea hydrothermal vent fluid reveals effect of pressure and lag time on autotrophic microbial communities. Applied and Environmental Microbiology, 87(9). https://doi.org/10.1128/AEM.00078-21

If you’ve ever been in an airplane or driven to the mountains, you’ve experienced a change in pressure. At higher elevations, there’s less pressure because there are fewer air molecules pressing down on you from above. As you descend, the number of overhead air molecules accumulates, and so does the pressure. But how does this work if you descend below sea level, and into the ocean?

Pressure accumulates much faster underwater than on land. (Image Source: Cristian Palmer on Unsplash)

Because water is denser and heavier than air, pressure accumulates much faster underwater than it does on land. At 10 meters (33 feet) below the surface of the ocean, the pressure is already double what it was at the surface. At the deepest point in the ocean – the Mariana Trench – the pressure is more than 1000 times greater than at the surface.

Humans could not survive in the deep ocean; without the protection of a submarine, we’d be crushed by the weight. But somehow, microorganisms (“microbes”) thrive. Deep-sea microbes are specially adapted to high pressures, darkness, and cold temperatures, and their neat adaptations are one of the reasons scientists are so interested in studying them.

Under Pressure to Study Life Under Pressure

While marine microbiologists have gotten very good at replicating most aspects of a microbe’s environment in the laboratory (such as temperature and light levels), fewer techniques exist to replicate deep-sea pressure. To accurately simulate the environment of a microorganism collected from 1 kilometer below the ocean surface, for instance, the pressure inside vials and test tubes would need to be 100 times greater than in the rest of the laboratory. This pressure difference is dangerous, and could lead to broken equipment and injuries if replicated. Furthermore, to do such an experiment accurately, a researcher would need to keep the deep-sea microbes in a pressurized environment constantly – including throughout their entire journey from the sampling location, up to the ship, and to the lab. Due to these technical difficulties, pressure is often ignored by marine microbiologists in their research.

But knowing how much pressure affects humans, can we be certain it doesn’t also affect microbes?

Recently, a team of scientists from the United States and Canada decided to tackle this problem, and to ask: do deep-sea microorganisms really behave similarly at sea level than they do under natural conditions (i.e. pressures tens to hundreds of times greater)? Rather than conduct potentially dangerous, pressurized experiments in the lab, however, they chose to conduct their experiments right on the seafloor!

Remote Operated Science

An image of the ROV Jason II being deployed off the side of a ship. (Image Source: Wikimedia Commons)

With the help of a remotely operated vehicle (or ROV) called Jason II, the team traveled to a hydrothermal vent off the coast of Washington state and conducted two experiments to test the effect of pressure on deep-sea microbes. One was a traditional “shipboard” experiment. The team collected water from the deep-sea sample site (1,500 meters below the surface), brought it back up to the ship, and let the microbes inside grow for 12 hours. Afterward, the team passed those microbes through a filter and collected the RNA inside. By sequencing the RNA, the researchers were able to create a profile of the genes that the microbes were copying, and a census of which microbes were present.

The team also conducted a second, “seafloor” experiment. Rather than bring the seawater up to the surface and process it on the ship, however, the team conducted the entire experiment on the seafloor. The seawater was collected and isolated in a chamber-like instrument attached to the ROV, and after 12 hours at the seafloor, the microbes inside were filtered, and the RNA was collected and preserved.

Depressurization Stresses Microorganisms

By comparing the census results of the nearly-identical experiments, shipboard and seafloor, the research team noticed several significant differences. In the shipboard experiment, a smaller percentage of the microbes were using oxygen to power their activities, suggesting that oxygen had been lost from the shipboard samples as they were transported up to the ship. (This also suggests that aerobic processes would’ve been underestimated without the seafloor experiment to provide a sanity check.) Furthermore, the shipboard microbes were furiously making proteins to help them deal with heat shock and other stressors. It was clear to the researchers that depressurization was a significant source of stress for the microbes.

This experiment is an important step toward future technologies that can safely evaluate the effect of pressure on microorganisms. In the meantime, the results of the study also give marine microbiologists clues about what biases they’re introducing by ignoring the effect of pressure (and sample transport). By understanding these biases, researchers can make far more accurate estimates of the environmental relevance of microorganisms, including their effect on the Earth’s climate.

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