Featured Image courtesy Yannis Papanastasopoulos, Unsplash.
Authors: Aurèle Vuillemin, Sergio Vargas, Ömer K. Coskun, Robert Pockalny, Richard W. Murray, David C. Smith, Steven D’Hondt, William D. Orsi
If you, like me, imagine the seafloor to be inhabited by strange, mysterious creatures like vampire squids and goblin sharks, think again: bacteria continue to surprise us with their resilience in the oddest of environments. Scientists have detected microbes living in the mud and rocks on the seafloor, but we don’t know much about them. Are they alive? How do they get energy in such a nutrient-poor environment? Given the inhospitable conditions in the sub-seafloor, scientists have thought that most of these microbes were close to the energy limit for life, which is an estimate of the minimum amount of energy required to sustain life as we know it. For this reason, we’ve assumed that subseafloor microbes die faster than they grow because there simply isn’t enough energy in the deep sea to sustain life long-term.
To understand whether and how microbial life sustains itself under such harsh conditions, Aurèle Vuillemin and colleagues from Germany and the USA examined microbial communities in sea clay at a depth of 5,515 m in the North Atlantic Ocean. Scientists can estimate the age of an ocean sediment by measuring the rate at which sediment deposits on the floor, a number called the sedimentation rate. Using this method, they estimated that this clay sample was formed over 5 million years ago (Myr).
Surprisingly, they found that the number of microbes actually increased as they went deeper into the subseafloor until 10 mbsf (metres below sea floor), and then started to decrease at greater depths. They also found one bacterial group that increased from 5 to 40% of the total community as they went deeper: Candidatus Atribacteria, an uncultured bacterial group whose relatives have previously been identified in deep-sea sediments from other locations like the Ross Sea and both the Pacific and Atlantic Oceans. (Check out Figure 1 here for a snapshot of what Atribacteria may look like.)
To test whether Atribacteria were actively growing in the subseafloor or merely long-dead cells they had just discovered, the team measured the amount of RNA produced by these bacteria and discovered that Atribacteria had the highest rates of gene expression of all bacteria in this sediment, indicating that they were indeed metabolically active.
Further evidence that Atribacteria are actively growing in the subseafloor was provided by their gene expression profiles, which contained RNA from genes we know are involved in cell division (such as the FtsAEKQWZ, MreBC and RodA genes), which are critical to cellular growth and reproduction. The Atribacteria genomes also offer clues to how the organisms survived in such a nutrient-lean environment. They have genes that can produce enzymes to break down dead bacteria in the sediment and convert this necromass to energy, Atribacteria could also have a microcompartment in the cell that recycles toxic by-products of metabolism to help with conserving cellular energy. Perhaps these unusual metabolic activities are an evolutionary advantage that allow Atribacteria to survive in the subseafloor.
This new evidence shows that Atribacteria (and perhaps other ancient microbial lineages) can survive and grow in remote deep sea sediments. Bacteria cling tenaciously to life even in extreme environments, and digging deeper into sub-marine mud and its inhabitants may yield more surprises while helping us understand the limits of life.
Deep Sea Bacteria have Thrived for Millions of Years by Janani Hariharan is licensed under a Creative Commons Attribution 4.0 International License.