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Microbiology

Toxic living: finding the right home for sulfur-oxidizing bacteria

 

Meier DV, Pjevac P, Bach W, Hourdez S, Girguis PR, Vidoudez C, Amann R, Meyerdierks A. 2017. Niche partitioning of diverse sulfur-oxidizing bacteria at hydrothermal vents. ISME Journal 11:1545-58. doi:10.1038/ismej.2017.37.

 

Hydrothermal vents are pretty weird environments. In the middle of the cold, deep ocean, 750°F (400 °C) water shoots out from brittle rock chimneys like miniature volcanoes in constant eruption. This water is filled with the toxic compound hydrogen sulfide – the same toxin whose pungent smell warns you away from eating rotten eggs – and it streams out into the water as a plume of black smoke. And of course, all of this happens in pitch black since no sunlight reaches the bottom of the ocean.

A hydrothermal vent spewing a plume of hot, black, hydrogen sulfide-rich water into the deep ocean. (Image: NOAA/PMEL)

 

Despite this apparent inhabitability, hydrothermal vents are also hotspots for life on the deep ocean floor. Sure, that life is correspondingly weird – bright red tubeworms that can grow up to 7 feet (2.4 meters) long are prominent around hydrothermal vents – but the fact that it can occur at all is astounding. Not only does this life have to deal with heavy doses of deadly toxins, but it is also cut off almost entirely from the sun. That’s a big deal, since almost all other life on earth depends on the sun – either directly, in the case of photosynthetic plants and algae, or indirectly in the case of animals and fish that depend on plants and algae at the base of the food chain.

 

The key to this dark and toxic living situation is a special type of bacteria known as sulfur-oxidizing bacteria. These bacteria pair the hydrogen sulfide coming out of hydrothermal vents with oxygen, which is readily available in seawater, to produce energy. In a single process, sulfur-oxidizing bacteria both detoxify the hydrothermal vent emissions and produce energy in the absence of sunlight. It’s no wonder then that sulfur-oxidizing bacteria are everywhere around hydrothermal vents. On the rock chimneys themselves, floating in the water, and even lining the intestines of tubeworms, crabs, and clams that call hydrothermal vents home.

Sulfur oxidizing bacteria also live inside the guts of these tubeworms! (Image: NOAA)

But what has become increasingly clear to researchers interested in how sulfur-oxidizing bacteria support life at hydrothermal vents is that sulfur-oxidizing bacteria is a catch-all term more than a specific bacterium. In fact, there are more than three different groups of bacteria than can oxidize hydrogen sulfide – and they’re about as related to each other as humans are to birds or fish. And just like humans and fish have very different impacts on the world around them, different types of sulfur-oxidizing bacteria may have critical differences in the way they clean up hydrogen sulfide and make energy available to animal life at hydrothermal vents.

 

The first step to understanding these potential impacts, however, is simply figuring out which types of sulfur-oxidizing bacteria are found in different areas around hydrothermal vents. To answer that question, an international team of researchers sailed to a highly active set of hydrothermal vents offshore of Papua New Guinea. There, the researchers sent a remotely operated submersible – best described as a remote control submarine – almost 6,000 feet (1,829 meters) below the surface of the ocean to shine a light on these dark hydrothermal vents. They were able to collect rock samples from the hydrothermal vent chimneys using the submersible’s controllable arm, as well as samples of the vent fluids and the surrounding seawater using sealable bottles attached to its frame. After visiting several different hydrothermal vents, the researchers returned to the lab to investigate which types of sulfur-oxidizing bacteria were present in all the different samples they collected.

The remotely operated submersible Hercules, using its mechanical arm to collect samples in the deep sea. (Image: NOAA/Ocean Explorer)

What they found was that three different types of sulfur-oxidizing bacteria dominated the area around the vents and assorted themselves primarily according to whether they are attached to the rock chimneys or floating freely in the water. The first type, known as Sulfurovum, was the most abundant sulfur-oxidizing bacteria found on the rock chimneys in addition to being found in water samples. However, the other two types, known as SUP05 and Aquificae, were found almost exclusively floating in the water around the hydrothermal fluids and not found on the rocks. Interestingly, this pattern was consistent across all of the vents that the researchers sampled and the sets of sulfur-oxidizing bacteria present did not change from vent to vent.

 

The researchers also dug deeper into the composition of sulfur-oxidizing bacteria in the water samples to see how these bacteria deal with the transition from hot and hydrogen sulfide-rich vent fluids to cold and oxygen-rich seawater. After measuring the concentrations of hydrogen sulfide and oxygen in each of their water samples, they found that Sulfurovum, SUP05, and Aquificae also had highly different preferences for their living spaces in the water around hydrothermal vents. Aquificae was well-adapted to life in the extremely hot and hydrogen sulfide-rich water directly over the hydrothermal vent chimneys, while Sulfurovum appeared to flourish in waters just a little further away from the chimneys where hydrogen sulfide concentrations were high but the temperature had cooled to below 100°F (40°C). Meanwhile, SUP05 preferred more diluted water samples with less hydrogen sulfide and more oxygen, at the edge of the hydrothermal vent plumes.

 

These results provide a tremendous amount of insight into how different types of sulfur-oxidizing bacteria assort themselves around hydrothermal vents and lays a framework for understanding what controls the distribution of these critically important organisms. Although much work remains to test whether gradients in water temperature and hydrogen sulfide and oxygen concentrations affect sulfur-oxidizing bacteria similarly at hydrothermal vents around the world, this is an important first step in better understanding the links between sulfur-oxidizing bacteria and the animal community that depends on them. Hydrothermal vents may still be weird, but thanks to this research they’re becoming a bit less mysterious.

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