At deep-sea hot springs, the hot, chemical-rich fluids produced by geothermal processes in the ocean crust mix with cold seawater, providing conditions that support thriving communities of microbes. These organisms survive by chemosynthesis, converting the chemicals in the fluids into usable energy and biomass. They serve as the foundation of a strange ecosystem, including tubeworms, crustaceans, and octopuses, which flourishes in the darkness of the deep sea. Microbes live below the seafloor of the vents as well, but while scientists have investigated the identity of these subseafloor organisms, measuring their productivity has been far more challenging.
In a new paper in Proceedings of the National Academy of Sciences , researchers have discovered that the microbes living below hydrothermal vents systems are surprisingly active. DCO Deep Life Community members Jesse McNichol (now at University of Southern California, USA), and Stefan Sievert (Woods Hole Oceanographic Institution (WHOI), USA) and Deep Energy Community member Jeffrey Seewald (WHOI, USA), along with colleagues at the Helmholtz Centre for Environmental Research (Germany), measured the activity levels of subsurface microbes while maintaining their local pressure and temperature conditions with specially designed gas-tight samplers. These experiments allowed them to make comprehensive measurements of microbial growth, identify dominant organisms, and estimate the global production of organic carbon by microbes living beneath deep-sea hot springs.
“We knew that there was quite a bit of carbon potentially being produced down there, but of course it’s hard to obtain fluid samples and to incubate them under natural conditions to measure the microbes’ activities,” said Sievert. “We were quite surprised to see how actively microbes were growing. The subseafloor biosphere is just as productive as the vent community above the seafloor.”
The researchers made these productivity measurements on microbes from a well-studied hydrothermal vent system named Crab Spa, which lies on the East Pacific Rise, off the coast of Central America. They sampled the microbes from beneath the vent using isobaric gas-tight cylinders (IGTs) that could maintain the local pressure inside the chamber. Seewald initially developed the IGT in 2002, so that he could analyze the chemistry of vent fluids without the dissolved gases escaping as he brought them to the surface.
For these experiments, Sievert, Seewald, and McNichol devised ways to modify the IGT for cultivating microbes. After bringing the samples to the surface under pressure, they maintained them at the same temperature as the deep-sea vent environment, 24 degrees Celsius, which conveniently, is room temperature in the labs of the research vessel, Atlantis. The researchers spiked some the IGTs with different chemicals that microbes might use for growth, such as nitrate, hydrogen, and oxygen, while leaving others unaltered. They also added inorganic carbon that carried an isotopic tracer, so that they could track the carbon as the microbes converted it into microbial biomass.
From the incubations, the scientists were able to calculate a new parameter they called “chemosynthetic growth efficiency,” which quantifies how efficiently the microbes convert available energy into biomass. Using this parameter in combination with fluid flow and depletions of chemicals in the vent fluids, the researchers calculated that for every liter of Crab Spa hydrothermal fluid, microbes produced up to 250 micrograms of carbon. That doesn’t sound like much, but when they scaled up their calculations to include all of the fluids flowing from seafloor hot springs worldwide, they estimate that microbes living beneath hydrothermal vent systems contribute 1.4 teragrams (or about 1,400,000 tons) of organic carbon to the seafloor each year. They could also infer that the residence time of the subseafloor microbes at Crab Spa is 17 to 41 hours, within the range of doubling times of chemosynthetic microbes growing under ideal conditions in the laboratory.
The WHOI researchers also teamed up with Niculina Musat at the Helmholtz Centre for Environmental Research to use a modern instrument called NanoSIMS to detect how much carbon individual cells assimilated. In combination with a technique that identified individual cells, the analysis showed that the most active microbes in the IGTs belonged to a group called Campylobacteria, which dominate the natural community at Crab Spa. They also observed that the presence or absence of oxygen changed the community composition of these organisms, implying oxygen could be an important factor that controls the distribution of different groups of microbes in the subseafloor.
“I like to think about it as a scientific dream team – we had all these people with incredible expertise,” said McNichol. “Because of that, we were able to tie all these things together and get a comprehensive picture of the ecosystem.”
Moving forward, the researchers have developed new equipment that will let them incubate the microbes directly at the seafloor. They also want to investigate the fate of all that ‘deep’ carbon and how these productive subsurface microbes are contributing to the deep ocean environment.
Main image: The manipulator arm on the remotely operated, deep-sea vehicle Jason uses an isobaric gas-tight (IGT) sampler to collect samples of fluids and microbes spewing from hydrothermal vents surrounded by a community of tubeworms at a site called "Crab Spa" on the East Pacific Rise. Credit: Photo courtesy of Stefan Sievert, WHOI/NSF/ROVJason, © Woods Hole Oceanographic Institution