Contrary to popular belief, caves contain many different forms of life. For most people, bats and fish come to mind, but actually most cave life is microbial. Some of these microorganisms break down rocks and minerals to use as their energy source in the same way that we break down our food for energy. In doing so, they take advantage of the unusual energy sources that caves can contain.
Sulfuric acid caves have remarkable microbial communities. These cave systems have rivers running through them that contain hydrogen sulfide, the form of sulfur that gives that rotten egg smell. When hydrogen sulfide evaporates into the cave atmosphere, it reacts with oxygen to create sulfuric acid that dissolves the rock of the cave walls. This type of cave makes up less than 5% of all caves in the world, so the forms of life found there are usually very different than in other environments.
In general, the organisms who live in these caves use sulfur as their energy source, and are uniquely adapted to acidic environments. Because these caves are a harsh environment with very different life forms than what we see above ground, these caves can help inform scientists about what life might look like on other planets with similarly harsh environments. Scientists interested in finding life on other planets are called astrobiologists.
Researchers went into a sulfuric acid cave in southern Italy called Fetida Cave to collect microbial biofilms. A biofilm is a collection of microorganisms that live together in a community held together by a sticky, mucus-like material. They collected samples from the water, and from two different kinds of biofilm on the cave walls. These scientists wanted to know what kinds of microorganisms were living in the cave because, unlike most sulfuric acid caves, Fetida Cave is connected to the ocean. The ocean alters the cave’s environment and changes the potential energy sources for microbes in the cave. The researchers wanted to know if the ocean influenced the type of life that would be found in Fetida Cave compared to land-locked sulfuric acid caves.
From DNA extracted from the biofilms, the researchers sequenced specific genes and then identified the microorganisms based on their genetic sequences. They found that the majority of the organisms in this cave can probably use sulfur for energy, but most microorganisms are different from those found in other known sulfuric acid caves. Most of the organisms living in these biofilms are bacteria.
The authors are among the first to describe organisms found living in gypsum, an acidic mineral deposit found on the cave walls. The majority of the microbes in the gypsum likely use sulfur as their energy source, but were different from those found in the other samples. Furthermore, the microorganisms in the gypsum were mostly archaea, another kind of microorganism, rather than bacteria like the other samples.
The results from this study allow astrobiologists and cave scientists to understand what kinds of organisms live in these unusual sulfuric acid environments. These microbial communities can help astrobiologists envision how to look for life on other planets. These cave systems help scientists understand what kinds of life forms live in these caves and how they relate to other organisms in similar environments.