"Biofilms, which are complex layered communities of sulfur-consuming microbes, increase the rate of cave formation, but may also shed light on other biofilms, including those that grow on teeth and those that corrode steel ships hulls, according to a team of geologists.
The researchers investigated the Frasassi cave system located north of Rome and south of Venice in Italy. These limestone caves are like New Mexico's Carlsbad Caverns and Lechuguilla
Cave, but in those caves, sulfur entered the caves from oil and gas reserves, while in Italy, the sulfur source is a thick gypsum layer below. Having sulfur in the environment allows these biofilms to grow.
Most limestone caves form when rainwater and runoff permeate the caves from above. Water and carbon dioxide mix to form carbonic acid, a very weak acid, that erodes the limestone cave walls. In sulfidic caves, water enters the caves from below, carrying hydrogen sulfide. Microbes in the biofilms use the sulfur for energy and produce sulfuric acid, a very strong acid.
"One type of biofilm, called a snottite because of its appearance, has a pH of zero or one," says Daniel S. Jones, graduate student in geosciences. "This is very, very acidic."
Carbonic acid cave systems lose about a third of an inch of wall every thousand years, while sulfuric acid cave systems lose about two and a third inches or six times as much in the same time. The researchers are interested in the make-up of the biofilms and how they cycle sulfur.
Biofilms are made up of thin layers of microbe species that can be very different. All require water, but some biofilms live in the pools, lakes and streams in caves and others live on the damp walls. The layers against the rock surface use oxygen and hydrogen sulfide for energy and produce sulfuric acid. The layer on the outside does as well, but, because middle layers exist, there is an opportunity for microbes that find oxygen poisonous to thrive. These middle layers may convert the sulfuric acid to hydrogen sulfide, creating a complete sulfur cycle in a few microns.
In dental biofilms, the microbes on the teeth are the ones that produce the acids that cause cavities, while the ones on the top create the right conditions for the acid-producing microbes to survive. Cave biofilm layers also fulfill different niches in their very tiny environment. " (...)