Microbes in the deep biosphere exist at pressures that would crush most surface organisms. At one kilometer beneath the ocean’s surface, for example, a person would experience about 100 times the pressure at sea level, which is equivalent to a cow standing on every square inch of the body. And even deeper into the ocean, and the underlying seafloor sediments, and ocean crust, the pressure only continues to grow. These seemingly extreme pressures are part of everyday life for deep microbes. To obtain a complete picture of what’s going on in the subsurface, scientists need to study microbes under these native pressure conditions.
In a new paper  in Frontiers in Earth Science, DCO Deep Life Community members Anaïs Cario, Gina Oliver, and Karyn Rogers (all at Rensselaer Polytechnic Institute, USA) discuss the challenges of sampling and culturing microbes under constant high pressure, and also highlight recent technological advances. They describe the Pressurized Underwater Sampler Handler (PUSH50), supported by the DCO, which is designed to maintain in situ pressure during sampling and throughout the transfer and culturing process in the lab. By applying this technology to conduct experiments that avoid decompression entirely, the researchers hope to refine our understanding of deep life and its contributions to global biogeochemical cycles. The article is part of a Frontiers special collection on deep carbon science.
“We’re just scratching the surface with our current understanding of the deep biosphere,” said Rogers. “The DCO has done an extraordinary job of shedding light on the extent and importance of the subsurface biosphere, but there’s still a lot we don’t know, and part of that is because of technological problems we’re still trying to solve.”
The deep ocean and subseafloor sediments represent Earth’s largest and least explored ecosystem. Deep ocean sampling and drilling expeditions have enabled scientists to culture a subset of deep microbes and to use DNA sequencing to estimate what lives down there and how they survive. Much of this work has been performed at surface pressures, however, which can yield substantially different results compared to experiments performed under the high pressures to which subsurface microbes are accustomed.
“These communities are a really big chunk of the global biosphere but we understand very little about how they persist at pressure, and the pressure probably matters,” said Rogers. “Doing experiments and sampling life at high pressure is really difficult but we need to keep improving our technologies if we’re going to unravel this mystery.”
Accurately estimating microbial growth and the movement of carbon and energy through the ecosystem is especially challenging. Some studies have found that microbes from the subsurface are more active when maintained at high-pressure conditions, suggesting that we may be underestimating the pace of life in the deep marine subsurface when we study deep organisms at surface pressures.
Rogers, working with Isabelle Daniel (Université Claude Bernard Lyon1, France), commissioned the PUSH50 to promote DCO objectives in understanding the deep biosphere. This 50-milliliter sampling container is now ready to be deployed on an HOV or ROV submersible that has a robotic arm to collect high-pressure fluids. The sampler maintains the fluids at in situ local pressures up to almost 1000 times the pressure at sea level in transport back to the lab and during incubation. Researchers can even take samples from the system without depressurizing the entire culture.
In January 2020, Rogers and her colleagues will take the PUSH50 to sea for the first time to test its performance with an ROV, but also to assess the importance of maintaining pressure during subsurface sampling. Combining the PUSH50 with other high-pressure transporters and samplers, such as those developed and used for many years by Jeffrey Seewald and colleagues (Woods Hole Oceanographic Institution, USA), will significantly expand our understanding of the effects of decompression on samples retrieved from the deep marine biosphere.
Due to the expense of developing and implementing high-pressure technology, Rogers proposes that Deep Life researchers share high-pressure technology, as well as the expertise needed to use it. Because of the inherent challenges, such collaboration is essential to successfully advance the entire field of subsurface microbiology. “We really need to develop a new model for collaboration when it comes to high-pressure research, because we have far more questions than we have the people and equipment to answer them,” said Rogers.
Through advances in affordable tools needed to study microbes under extreme pressures, the authors hope to open a window into the deep biosphere and to yield insights into microbial communities from Earth’s past as well as potential subsurface habitats on other planets.