The photo of a mint-green, pristine pool in Lechuguilla cave circulating on social media resulted in expressions of speculation, wonder, and a desire to visit. For many reasons, the cave area must remain off-limits and isolated.

On May 31, 2020, Carlsbad Caverns National Park Facebook page featured a photo of a light green pool surrounded by milky white mineralization. It is highly unlikely the location was previously observed by human eyes and never before photographed. The story was picked up and spread by some news outlets. The photo was taken by geoscientist and photographer Max Wisshak, author of the Facebook post, during a carefully planned expedition to Lechuguilla cave in October 2019. Wisshak noted that the pristine pool was a “wondrous sight” and the team “took special precautions to ensure there were no contaminants introduced to these pools of water”.

Photo Max Wisshak, Carlsbad Caverns National Park

Lechuguilla cave and its nearby sister system, Carlsbad Caverns, in Carlsbad Caverns National Park, New Mexico, were created four to six million years ago when the limestone reef formation was uplifted and openings were etched out by hydrothermal fluids containing sulfuric acid. This created massive deposits of gypsum and deposition of yellow sulfur. Most of the cave system is more than 300 meters below the surface. Not all caves in the system are connected to a surface opening and can be completely isolated from the atmosphere. Lechuguilla, first explored in 1986, is the third-longest cave in the US and the deepest. Because of the geologic history, the National Park Service notes that there is still “an exceptional potential for additional cave discovery, significant exploration and research.”

Access to the cave is strictly limited to prevent destruction and further contamination from surface microbes.

The Green Pool

Many observers were fascinated by the “eerie pool” picture which some said looked like a fairy fountain or suggesting the water perhaps had magical qualities. A story by the Kansas City Star on June 4 added a few details on the pool that were not evident by the photo alone. For example, the water was not visibly milky green but described by Wisshak as “crystal clear”. It appears that the flash from the camera reflected off the water that is saturated with minerals, producing the opaqueness and weird color.

As the photo contained no scale (likely due to the conditions and the need to not contaminate the area), it was impossible to tell the size. The pool itself is small – estimated at one foot wide, two feet long, and less than a foot deep. The source of the water is likely from precipitation that made the long journey from the surface, 700 feet down through the rock formations. Water molecules within the rock condense into droplets that drip along the walls and collect into ponds. Long-term evaporation and concentration can cause the pool to become super-saturated, with minerals. The original post noted that this pool could contain bacterial colonies – life even under these extreme conditions.

The Lechuguilla system contains many cave pools that have been studied and sampled in the past. The water contains calcium, magnesium, and some sodium and potassium, with dominant ions of carbonate, bicarbonate and sulfate, but also chloride and nitrate. Reflecting the sand and silt of the Chihuahuan desert surface deposits and bedrock, the water is also high in dissolved silica.

Infiltrating water entering the cave releases its dissolved carbon dioxide due to the difference in atmospheric pressure. Thus, the water tends to become supersaturated with respect to carbonate minerals. One of the pools is named the Dilithium Pool for the spectacular crystals (of selenite-gypsum) that grow in the saturated solution.

Isolated bacteria and their extraordinary features

The many pools of Lechuguilla are alkaline, between pH 7-9, and are generally nutrient-poor because almost no organic material finds its way into the system. Yet, an array of very interesting bacteria exist here. The isolation of the cave allows researchers to study how microbes evolved without the influence of human activity. The bacteria exist in the water and as biofilm along the surfaces forming their own food chain. Without a typical food source, the bacteria metabolize carbon dioxide and utilize the trace minerals. In the process, the cave walls may be further eroded and additional residual deposits are formed. In other words, the bacteria can be physically changing the cave formation.

Other studies reveal these bacteria can yield important clues to antibiotic resistance. Surveys of bacterial strains in the cave system have shown that the species here conserve the traits that make them resistant to antibiotics. Because portions of Lechuguilla Cave have been isolated from the surface for over 4 million years, there is no way that this resistance is a result of human overuse of antibiotics. Therefore, the implication is that pervasive and diverse antibiotic resistance may be an ancient and natural feature.

The potential explanation for this arises as a result of the extreme isolation and fierce competition for the scarce nutrients in the cave system. Microbes protect themselves against the waste products of competitors. The resistance mechanisms are similar to those found in terrestrial bacteria. Other studies in extreme and isolated habitats also support the idea that the ability to form resistance is hardwired into the foundational bacterial genome. Populations utilize this potential and survive, evolving complex resistance.

Obviously, releasing such antibiotic-resistant bacteria into the environment could cause a serious issue for us surface dwellers. So, continued isolation of the system is important. Researchers who do enter the system must take strict precautions not only to not introduce surface contaminants to the system but also not bring any back.

Map of cave system using 3-D cave software.

Contamination

Unfortunately, some contamination occurred from the surface. Coliform bacteria was first discovered in the cave in 1995, sourced from toilet disposal facilities around the caves. Drinking water sources were found to be contaminated with unacceptably high levels of human fecal coliform. Currently, the land above the cave areas is a protected wilderness area and bathroom facilities are controlled. As you go deeper into a cave system, it becomes more likely that surface contaminants will be filled out and not make it that far.

The geological setting has created and preserved a unique and incredible environment and ecology in Lechuguilla cave. A cool picture circulating on social media provides an opportunity to learn far more about this complex and incredible system.

References

Hunter, A.J., D. E. Northup, C. N. Dahm, and P. J. Boston. (2004). Persistent coliform contamination in Lechuguilla cave pools. Journal of Cave and Karst Studies, v. 66, no. 3, p. 102-110.

Pawlowski, A., Wang, W., Koteva, K. et al. (2016). A diverse intrinsic antibiotic resistome from a cave bacterium. Nat Commun 7, 13803. https://www.nature.com/articles/ncomms13803

Turin, H.J. and M.A. Plummer (2000). Lechuguilla Cave pool chemistry, 1986-1999. Journal of Cave and Karst Studies 62(2): 135-143.

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