Iron oxide-rich filaments: Possible fossil bacteria in Lechuguilla Cave, New Mexico

Reddish filaments in two fragments of unusual iron oxide bearing stalactite s, "the Rusticles" from Lechuguilla Cave, New Mexico, are found only within the central canals of the Rusticles. The curved, helical, and/or vibrioida l filaments vary from 1 to 6 mum in outer diameter and 10 to >50 mum in len gth. SEM and TEM show the filaments have 0.5-mum diameter central tubes, wi th goethite crystals radiating outwardly along their lengths. The diameter of the central tubes is consistent with the diameter of many iron-oxidizing filamentous bacteria. Although most iron oxide depositing bacteria do not deposit well-crystallized radiating goethite, we propose thick hydrous iron oxide was slowly crystallized from amorphous material to goethite, in plac e, over a relatively long period of time. From the gross morphology and the particular setting, we suggest this represents an occurrence of fossilized , acidophilic iron-oxidizing bacteria.