Geology of a Mystery Microbe: Reticulated Filaments in Speleothems

Some carbonate speleothems commonly host fossil microbial filaments. We report herein on a new form of reticulated filament found in a variety of speleothems worldwide. Reticulated filaments are usually found inside speleothems by etching with weak (5-10%) hydrochloric acid before imaging in a scanning electron microscope (SEM). They often show a higher concentration of carbon, which apparently allows them to resist the acid etching. We currently have imaged 134 individual filaments from six different caves and one surface sample. Most come from pool precipitates in Carlsbad Cavern, Hidden, Cotton-wood, and Endless Caves, Guadalupe Mountains, NM. Additional examples are found in cave pearls from Tabasco, Mexico, a lava tube in the Cape Verde Islands and one example in desert varnish from NM. Reticulate filaments are 1-75 mu m in length. Two populations overlap in size, one averaging 0.5 mu m and a second averaging 0.9 mu m in diameter. The reticulated pattern resembles an open fish-net tube. Two varieties occur, one with diamond-shaped chambers that spiral, and a more common one with hexagonal-shaped chambers in a line. Some filaments are hollow, others are solid; a few samples are collapsed or even torn open. We hypothesize that reticulated filaments are the fossilized remains of an unknown microbe that is apparently common in damp cave environments. Efforts to identify living examples have so far failed, but cave microbes are notoriously difficult to culture. We are interested if finding out if anyone else has observed these filaments in living or fossil cave systems.