Bioconstructions in ochreous speleothems from lava tubes on Terceira Island (Azores)

Abstract

The ochreous speleothems examined here were obtained from a lava tube on Terceira Island (Azores) and show compact mineralized areas intermixed with zones in which bacterial structures are evident. Bacterial layers of filament-like structures are common throughout the deposits but differences in microstructure among the different speleothems were observed, reflecting a broad morphological range of deposits. The structures and minerals detected in the speleothems betray their biogenic origin. Gallionella and Leptothrix were the two most frequently observed morphotypes and probably the main contributors to speleothem formation. However, DGGE analysis indicated the presence of another bacterial population (with a predominance of proteobacteria) that could also contribute to iron hydroxide–oxide precipitation. The sheaths of Leptothrix cells and stalks of Gallionella cells were associated with large amounts of extrapolymeric substances (EPS), which play a role in biomineralization processes. Independently of the taxa present, mineral deposits were composed of poorly ordered Si-rich ferrihydrite, a typical mineral phase of biogenic Fe precipitates. In this microscopy study of interrelationships between mineral precipitates and associated microorganisms and their structures, we attribute bacteria to have an important role in constructing these speleothems. Through their metabolic activity, these bacteria cause the precipitation of ferrihydrite but their mineral structures could also act as nucleation points for passive mineral precipitation. Finally, the build-up of bacterial mineral structures and later cementation processes seems responsible for the formation of mineral layers that confer consistency to the speleothem. Our findings point to bacterial activity as the main factor determining speleothem structure and formation.