Manganese Oxide Biominerals from Freshwater Environments in Quadrilatero Ferrifero, Minas Gerais, Brazil

Abstract

Biogeochemical Mn cycling in aquatic environments is driven mostly by microbes, and includes reductive dissolution in anaerobic sediments as well as oxidation in aerobic regions. Oxidation is followed by precipitation, which occurs mainly on the extracellular structures of microorganisms. In this work, we studied the minerals precipitated on extracellular structures of native microorganisms from freshwater environments in Quadrilatero Ferrifero (Iron Quadrangle), Minas Gerais, Brazil, known to contain high levels of manganese. Light microscopy of biofilms and floating material showed diverse biomineralized structures. The most conspicuous were identified as the holdfasts of algae from the genus Ulothrix. Diatom frustules associated to manganese oxide precipitates were relatively common. In addition, both filaments and holdfasts produced by bacteria of the genus Leptothrix were found, as well as structures similar to those described as Siderocapsa and Metallogenium. Some previously unknown structures were also observed. Transmission electron microscopy of most of these structures showed the ‘crumpled tissue-paper’ morphology common in biomineralised manganese oxides. Energy-dispersive X-ray analysis (EDXA) showed that manganese and oxygen were the main components, along with minor amounts of Al, P, S, K, Ca and/or Ba. Our results bring new perspectives to the study of biomineralized manganese oxide structures from the environment. Moreover, they add information about the background of present-day microbial structures needed to better interpret fossilized microbial biominerals.