Making New Friends in the Frozen Depths

Paloma Salmeron-O’Brien

Life is tough; that’s a lesson that deep sea creatures know better than most. Sea creatures living beyond the reach of the sun have to survive in some of the harshest conditions known to planet Earth. They must contend with intense pressure, frigid temperatures, and near complete darkness. Fortunately for some of these organisms, they’ve found that cooperation tends to alleviate some of the stressors of living down deep. At the bottom of the ocean, symbiosis makes the world go round.

One of the best spots to see these organismal partnerships at work en mass are the oases of the deep: hydrothermal vents and methane seeps. Across the globe, the ocean floor is spotted with cracks and faults where chemicals and superheated water rise up through the rock. These seeps and vents release nutrients and other useful substances that can be used to support life, which are rare commodities in the largely barren deep. As such, these deep-sea oases host thriving communities of chemosynthetic life supported by symbiotic relationships. Chemosynthesis is the use of certain inorganic chemicals to produce sugar for food as opposed to using sunlight in photosynthesis. Typically, scientists have observed symbiotic relationships between sulfide-oxidizing bacteria (which make use of available sulfide) and other organisms. However, owing to the existence of methane seeps, they thought it plausible that there are also relationships between methane-oxidizing bacteria and other deep-sea organisms.

Enter worms, stage right. The ocean as a whole is full of different species of worms. Their body plans are relatively simple and evolutionarily versatile, so it isn’t much of a surprise that some make the depths their home. One of the most common organisms seen around hydrothermal vents and seeps are several species of tube-dwelling annelids called “fan worms”. They get their name from the feathery appendages they use to swipe up bacteria to eat. Fan worms have a long history of living around seeps, having been found fossilized around methane deposits indicating an ancient seep. These fossilized worms have been found to be up to 200 million years old. Just like they are seen to do in the present, these fossils were found in rings around ancient vents, almost certainly thriving off their chemical resources. These fossils suggest that fan worms are a well-instated member of chemosynthetic communities and, considering their history with methane seeps, could well have developed symbiotic relationships with methane-oxidizing bacteria.

This idea excited a lot of scientists (symbiosis is pretty neat after all), and so a collaboration between universities such as Washington State, Rhode Island, Costa Rica, and the research vessel Atlantis was forged. The team employed a range of different research techniques to see if fan worms utilized methane as a source of nutrition. They profiled the microbes found in fan worm communities and used microscopes to closely view the worms’ internal structures. Researchers also conducted stable isotope tracer experiments in which they provided fan worms with methane composed of special carbon isotopes. Isotopes are versions of an element with different numbers of neutrons (in this case, scientists used C13). Since we can detect the presence of this isotope, if the worms took up the special methane, then the scientists would know based on the presence of this isotope in the worms. Finally, scientists also incorporated seafloor surveys to get a better understanding of the layout of methane seep communities. Data was collected from a site named Jaco Scar, an area rife with active seepage and fan worms. The site exists off the shores of Costa Rica where the depth of the seafloor ranges from 1768 to 1887 meters (roughly 5801 to 6191 feet).

Going into this research process, the scientists had two main hypotheses. They already suspected that the fan worms did have some relationship with the methanotrophic bacteria. Given this, they figured that the worms either fed off the bacteria or had a mutualistic relationship with them (a symbiotic relationship in which both parties benefit). The worms’ successful uptake of tagged methane indicated that both these hypotheses could be plausible, however, further investigation yielded more support for the latter idea. Before the study had commenced, previous research had found that some fan worm species do selectively feed on bacteria. However, the methane seeps surveyed did not maintain sufficient concentrations of methanotrophic bacteria to feed the large worm colonies present. Furthermore, no bacterial remains were found in the digestive systems of the worms. Scientists found abundant bacteria on or embedded in the tissue of worm samples but no bacteria in the surrounding water column. This suggested that the worms were hosting the bacteria and providing them with enhanced growth conditions. The worms were also found to be able to oxidize and sequester methane, a feat they could not accomplish without the methanotrophic bacteria. All of this evidence supports the existence of a symbiotic relationship between methanotrophic bacteria and fan worms.

But so what? What's the big deal about worms living at the bottom of the ocean making microscopic friends? Well, something to note is the fact that methane is a potent greenhouse gas. In fact, it is significantly better at trapping heat than CO2. A lot of organisms, including livestock, produce methane which then gets released into the atmosphere to intensify the greenhouse effect. Having an organic means to reduce or otherwise make use of this released methane may help subvert the warming trends observed as part of climate change. It might seem like a long shot, but science has a long history of hijacking bacteria to accomplish large-scale feats. Perhaps it could be possible to culture special strains of methanotrophic bacteria to reduce agricultural methane production.

Whether or not that is a possibility, learning more about deep sea life is always important. While the dark depths might seem out of reach of most of the world, it too is under threat from human activity. These slow-growing, extremely fragile deep-sea ecosystems can be destroyed by trawling and oil drilling operations. Knowing more about them can help convince the public that they are worth protecting. Learning more about the farthest, coldest corners of our planet can do a lot to save them.




Sources:

Goffredi, Shana K, Ekin Tilic, Sean W Mullin, Katherine S Dawson, Abigail Keller, Raymond W Lee, Fabal Wu, et al. “Methanotrophic Bacterial Symbionts Fuel Dense Populations of ... - Science.” Science Advances, April 3, 2020. https://www.science.org/doi/10.1126/sciadv.aay8562.


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