Reticulated filaments in cave pool speleothems: Microbe or mineral?
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We report on a reticulated filament found in modern and fossil cave samples that cannot be correlated to any known microorganism or organism part. These filaments were found in moist environments in five limestone caves (four in New Mexico, U.S.A., one in Tabasco, Mexico), and a basalt lava tube in the Cape Verde Islands. Most of the filaments are fossils revealed by etching into calcitic speleothems but two are on the surface of samples. One hundred eighty individual reticulated filaments were imaged from 16 different samples using scanning electron microscopy. The filaments are up to 75 mm (average 12 mm) long, but all filaments appear broken. These reticulated filaments are elongate, commonly hollow, tubes with an open mesh reminiscent of a fish net or honeycomb. Two different cross-hatched patterns occur; 77% of filaments have hexagonal chambers aligned parallel to the filament and 23% of filaments have diamond-shaped chambers that spiral along the filament. The filaments range from 300 nm to 1000 nm in diameter, but there are two somewhat overlapping populations; one 200-400 nm in size and the other 500-700 nm. Individual chambers range from 40 to 100 nm with 30-40 nm thick walls. Similar morphologies to the cave reticulated filaments do exist in the microbial world, but all can be ruled out due to the absence of silica (diatoms), different size (diatoms, S-layers), or the presence of iron (Leptothrix sp.). Given the wide range of locations that contain reticulated filaments, we speculate that they are a significant cave microorganism albeit with unknown living habits.