Spectroscopic evidence of microbial organic matter in secondary mineral deposits at Naica Underground System (NUS) and the biological role in its mineralization

Abstract

The Naica Underground System (NUS) hosts the largest gypsum crystals (>10 m in length and >1 m in width) ever found in natural caves; these are growing from conspicuous Fe-oxyhydroxide deposits that presumably were formed in processes controlled by microorganisms. In contrast to other studies where microbial participation is elucidated only from morphological textural characterization and total DNA sequencing, here, we report a comprehensive FTIR characterization of the conspicuous secondary mineral deposits in the NUS that provides physicochemical evidence suggesting that the NUS microbial communities contributed to the mineralization of the Fe-oxyhydroxides and gypsum at the NUS.FTIR analyses of gypsum and Fe-oxyhydroxides, as well as the mineral fraction dissolved in aqueous samples collected from different sites at NUS, reveal that such minerals are intimately associated with organic material, such as polysaccharides, phospholipids, proteins, and, to a minor extent, nucleic acids; suggesting that the formation of gypsum and Fe-oxyhydroxides at NUS was microbially mediated. Our results also provide compelling evidence that FTIR is a valuable tool for the characterization of biomineralization processes and should be used as a complement to morphological and massive DNA analyses.