In Thornton’s Cave, an estavelle in west-central Florida, SEM, EDS, and XRD data reveal biofilms that are predominantly comprisedof FeOOH-encrusted hollow sheaths that are overgrown and intercalated with calcite. Fragments of this crystalline biofilm adhereto the walls and ceiling as water levels vary within the cave. Those on the wall have a ‘cornflake’ appearance and those affixed tothe ceiling hang as fibrous membranes. PCR of DNA in the active biofilm, combined with morphologic data from the tubes in SEMmicrographs, point to Leptothrix sp., a common Fe-oxidizing bacteria, as the primary organism in the biofilm. Recent discoveries of‘rusticles’ in other Florida caves suggest that Fe-oxidizing bacteria may reside elsewhere in Florida groundwater and may play a rolein the mobility of trace metals in the Upper Florida aquifer. SEM micrographs from two marble tablets submerged for five months, oneexposed to microbial activity and a second isolated from microbial action, revealed no visible etchings or borings and very limited lossof mass. EDS data from the electron micrographs of the unfiltered tablet document the same FeOOH-encrusted hollow sheaths andsimilar deposits of calcite as seen in the ‘cornflakes’. These results, combined with water chemistry data imply that the biofilm mayfocus or even promote calcite precipitation during low-water level conditions when CO2 degasses from the cave pools
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