Although bacteria are unicellular and may be freely swimming in water or crawling on sediments, in many environments they live in clumps called biofilms. Biofilms are groups of bacteria that grow on a surface and huddle together by sticky carbohydrates that they produce. Similar to how emperor penguins pack tightly to escape wind and conserve warmth, or football players congregate around the coach, biofilms protect bacteria from environmental stresses and predators. Studying the architecture of biofilms and the microbial interactions within them is important for understanding what makes cells coordinate activities together, which is of interest for many practical applications. In the medical field, for example, this knowledge helps understand why many infections are more resistant to treatment when pathogens form biofilms in the host’s tissues.
The way the biofilm looks and the way bacteria are positioned within the biofilm will depend on the conditions around them. One particular type of biofilm, located in ceilings of caves, produces extensive hanging mucous structures of about 15 cm known as “snottites” — like stalagmites, but phlegm-like.
Not many people would be willing to get inside of a snotty cave, but these scientists were really interested in figuring out what made these biofilms so massive, occupying large portions of the cave ceiling. Typically, snottites have been found in very acidic and high sulfur conditions (excreting highly concentrated sulfuric acid), but this time scientists Clemens Karwautz, Günter Kus, Michael Stöckl, Thomas R Neu and Tillmann Lueders encountered a massive biofilm hanging from the ceiling of a cave in Sulzbrunn that contains a neutral medicinal spring, making this biofilm even more intriguing. Clues on how snottite microbes may be making a living inside this cave could come from looking at the nutrients from the incoming water. The medicinal cave is semi artificial and always kept half-full. The water that circulates brings in high concentrations of iodine (almost a thousand times more than typical natural concentrations) and salt, and brings along hydrocarbons and organics as it passes by sediment deposits.
The scientists had observed that the caves produced significant biogas (methane). This led them to formulate the hypothesis that the food source for this biofilm is the methane produced by the organic material as it is degraded by microbes during the transportation process. Methane can be used as a food source by many microorganisms that breathe oxygen. In some cases, scientists have observed that methane-eating microbes produce an excess of polysaccharides, which could explain why the biofilm in this cave was so snotty. Could this be the case in this cave as well? The scientists set out to answer this question.
To test their hypothesis, scientists went into the cave and took samples of the gases, liquids and solids in the cave. They measured the composition of gasses both in the air and dissolved in the water, the salts dissolved in the water, and the number and species of bacteria that were both in the biofilm as well as in the water. They also put some biofilm samples in bottles, fed them with methane, and measured how much methane was consumed over time.
What they found was that the waters were typically cold and had very little organic carbon and nitrogen, which means that there was not much organic material to feed the microbes, which persisted at relatively high abundance. The concentrations of methane and iodine were high, as expected, and incubation of biofilms fed with methane confirmed the suspicion that methane was serving as the food source.
The role of iodine has yet to be understood. The biofilms could have resulted from eating methane, but the biofilms may have formed as a way for bacteria to protect themselves from a very reactive form of iodine. Another hypothesis is that organisms in the biofilm are using a compound made of methane and iodine called methyliodine produced from organisms in the cave water. As you can see, this interesting system still contains many mysteries to be solved!