Far below the surface in the Gulf of Mexico, a rich and biodiverse community flourishes on the oil and gas seeps that flow from what are known as asphalt volcanoes. It is now thought that the mussels and sponges that do so well at these depths are only able to achieve their success due to the bacteria that they have co-opted into a symbiotic relationship.
Researchers have found thriving communities of animals living at these asphalt volcanoes before but struggled to explain their existence as they themselves are not able to feed off the oil and gas, and there are precious few other nutrients knocking about. Not only that, but the bacteria that is known to be able to break down the constituents of the hydrocarbons had only ever been found as free-living organisms in the water column.
Called Cycloclasticus, the bacteria are usually known for their ability to crack apart the carbon ring structures found in oil. These polycyclic aromatic hydrocarbons are toxic to most creatures, but not the bacteria, meaning they have an untapped food source. Yet curiously the researchers found that those microorganisms that have opted to live within the sponges and mussels have given up this lucrative market.
This is the first time that Cycloclasticus bacteria have been found that can no longer degrade polycyclic aromatic hydrocarbons, and have instead decided to stick with the much easier breaking down of short-chain alkanes. Yet as it is simpler to do this, they also have to deal with much greater competition from other organisms doing a similar thing.
“We think that they can only afford this ‘luxury’ because of their symbiosis with mussels and sponges,” explains the Max Planck Institute's Nicole Dubilier, co-author of the study published in Nature Microbiology. “These hosts provide the symbiotic Cycloclasticus with a continuous supply of short-chain alkanes through their constant filtering of the surrounding seawater. By living inside animals, these symbionts are well taken care of and do not have to compete with free-living bacteria.”
Until now it was thought that Cycloclasticus could only ever exist on these polycyclic aromatic hydrocarbons. In fact, it was this type of bacteria that bloomed following the massive Gulf of Mexico oil spill, and the researchers now think that the bacteria may at first subsist off of the simpler short chained alkanes before moving onto the more complex constituents of the oil once those are all used up.
They hope to explore these relationships further, and better understand how the bacteria can live off both types of molecules, deep below the waves.