Pioneering new study explores the structure and function of microbial communities at expired hydrothermal vent sites
Undiscovered lifeforms abound in Earth’s most seemingly inhospitable environments, as demonstrated by the recent discovery of bacteria living deep underneath the seafloor. An equally extreme environment can be found in the vicinity of hydrothermal vents, where water is expelled from the Earth’s crust at temperatures exceeding 400°C, after it percolates down through cracks formed at the intersection of tectonic plates. We now know that active vents teem with life, yet little is known about these habitats after venting eventually stops.
A study recently published in the Open Access journal mBio explored the inactive mineral deposits left behind by expired vents along the floor of the deep sea, showing they serve as long-term microhabitats for a succession of unique bacterial communities with potentially important roles within the broader marine food chain. This work is the first to demonstrate that life continues even as vent activity drops off.
Active hydrothermal vents and their distinctive chimneys (source: Wikipedia)
Searching for life at inactive vent sites: a genetic approach
The study, co-authored by researchers at the University of Southern California and the University of Minnesota, characterised microbial communities using advanced genetic sequencing techniques. The scientists obtained samples from expired vent chimneys on the East Pacific Rise, a tectonic plate boundary that runs along the Pacific Ocean, using the US Navy deep-sea submersible, Alvin — famous for its use in the exploration of the Titanic in 1986.
The genetic sequences provide a snapshot of the bacterial community at each sampling location. These data include the type of species present and, based on similarities with described species, their potential role within the local microbial food chain; in other words, what contribution their uptake of nutrients could make to the wider marine ecosystem. By obtaining similar data from active vents, the researchers could predict how relative counts of bacterial species change as venting ceases and the environment cools.
Microbes vital for marine ecosystems
The chimneys are formed by minerals carried by the vent emissions as they emerge from deep within the hot crust and collide with seawater. In the context of the study now published, they are of particular interest as long-term habitats for microbes because they are both widespread and resilient, remaining for up to 20,000 years following the expiry of a vent.
As venting ceases, the microbial community composition within chimneys changes drastically and the remaining bacterial biomass, the size of the community, grows by up to five times. Furthermore, the new community comprises several species that can take up and distribute essential elements back into their global cycles. In other words, not only does this study show that inactive vent sites contain unique communities of microbes, but the results also suggest these species are particularly important for the healthy functioning of the marine ecosystem.
Microbial communities serve as an important link in the global cycling of elements vital for life, such as carbon and nitrogen. Bacteria break down dead plant and animal matter, taking in carbon and thereby reintroducing it to the food chain when they are in turn consumed by larger organisms. Through a process known as nitrogen fixation, some bacteria also create – or ‘fix’ – nitrogen, an element necessary for the growth of plants.
Understanding these processes in the deep sea gives us unprecedented insight into how entire marine ecosystems function. “There are all these organisms down there making biomass, and that’s not at all accounted for in our carbon cycle,” commented senior author, Katrina Edwards, in an interview with OurAmazingPlanet.
Bacterial lifestyle shift
Apart from demonstrating the important ecological role of life at expired vents, this study also complements previous work by illustrating how the drastic environmental change that accompanies vent expiry favours organisms with an entirely different lifestyle. At active vent sites, bacteria get the energy they need to survive from the heat and content of the fluids coming from deep within the Earth. At inactive vents, the two most commonly found bacterial groups stay alive using a different mechanism, generating energy from the minerals freed up by the natural weathering of the chimneys. “Seeing the shift in the microbial population – seeing who actually came and left, was fairly illuminating for me,” said Edwards.
This exploratory study will be followed up by more work on the mechanisms of bacterial community succession, both at hydrothermal vent sites and also, as Edwards explains to AstroBio Magazine, in microbes existing beneath rock surfaces. The authors conclude by explaining that studies focusing on other microscopic species have yet to be undertaken, but may hold equally great potential in understanding the vital ecological contribution of deep-sea microorganisms.
By Edvard Glücksman, EGU Science Communications Fellow