In a Cav­ern, in a Canyon….

by Elio

Cer­tain large un­der­ground cav­erns owe their majesty to the bio­chem­i­cal ac­tion of tiny bac­te­ria. In many parts of the world, cav­erns are formed by the dis­so­lu­tion of cal­cium car­bon­ate de­posits by sul­fu­ric acid (this is known in the trade as "sul­fu­ric acid speleo­ge­n­e­sis"). Such mi­cro­bial min­ing formed the Lechuguilla Cave in New Mex­ico, the deep­est cave in the con­ti­nen­tal USA with 184 Km-worth of pas­sages, and so was the most cel­e­brated of our caves, Carls­bad Cav­erns.

Un­til re­cently, it was thought that sul­fu­ric acid arose spon­ta­neously in caves from the seep­age of hy­dro­gen sul­fide and its ox­i­da­tion in air...

Carls­bad Cav­erns from Wikipedia.

It turns out that not much H₂S seeps through freely. In­stead, most of it is con­verted to sul­fu­ric acid by the me­tabolic ac­tiv­ity of sul­fide-ox­i­diz­ing bac­te­ria. They do this: H₂S+2O₂→H₂SO₄, and CaCO₃+H₂SO₄→CaSO₄+H₂CO₃. Cal­cium sul­fate-gyp­sum-is more sol­u­ble in wa­ter than cal­cium car­bon­ate and is thus more read­ily washed away, leav­ing a hole in the ground. This find­ing was first re­ported by En­gel, Stern, and Bennet in the jour­nal Geo­logy. They noted that in the out­flow springs dis­played mi­cro­bial mats that were up to 17 m long. One thing led to an­other, and they found that the loss of sul­fide from the wa­ter could best be ac­counted for by mi­cro­bial oxi­dation. Us­ing flu­o­res­cence in situ hy­bridiza­tion (FISH), they found an abun­dance of fil­a­men­tous ε‑proteobacteria and γ‑proteobacteria.

Fol­low­ing up on such work, Macal­ady and col­leagues re­port con­spic­u­ous white biofilms on the sur­faces of streams in a cave in Italy, be­tween Venice and Rome. Us­ing 16S rDNA li­braries, they showed that pro­teobac­te­ria were again the pre­dom­i­nant or­gan­isms, with most of the bio­mass rep­re­sented by the sul­fur in­clu­sion-con­tain­ing fil­a­men­tous Beg­gia­toa and Thio­thrix. The domi­nance by these species may be due to their pref­er­ence for the rel­a­tively high oxy­gen concentra­tion found in the cave streams. The au­thors es­ti­mate the cor­ro­sion rate of the cave walls to be about 5 cm/1000 years, which is for­mi­da­ble over ge­o­logic times. The sites of col­lec­tion were ac­cessible via "tech­ni­cal cav­ing routes," which sounds to me like se­ri­ous spelunk­ing. The au­thors point out that ac­tively form­ing caves of this sort are rel­a­tively rare and, be­ing iso­lated ecosys­tems, present unique op­por­tu­ni­ties for in­ves­ti­ga­tion. Here life does not de­pend on sun­light but on the ox­i­da­tion of in­or­ganic chem­i­cals, in this case sul­fur com­pounds.

As a mi­cro­bi­o­log­i­cal chau­vin­ist, it does my heart good to re­al­ize how promi­nently mi­crobes fig­ure in ge­o­log­i­cal cir­cles. Pa­pers on mi­crobes are com­mon at the meet­ing of the Amer­i­can Ge­o­log­i­cal Union (AGU) and in their jour­nals. Ge­ol­o­gists are well aware of the role of mi­crobes in shap­ing and re­shap­ing this planet: mi­crobes are min­ers, ma­sons, and mes­sen­gers. I'm not sure that many mi­crobiologists are aware that Mi­cro­bial Geo­chem­istry is such a ro­bust sci­ence. But we may also take plea­sure in the mi­cro­bial ori­gins of grand sights.