Diversity of iron and silica precipitation by microbial mats in hydrothermal waters, Iceland: Implications for Precambrian iron formations

Abstract

Direct examination of microbial mats from Icelandic hot springs with transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed a consortium of bacterial cells in varying stages of mineralization. Differences in observed mineralogy largelyreflectdifferencesinthechemistryofthehydrothermalwaters.Silica-richspheroids formedepicellularlyoncellwallsandsurroundingsheathsandcapsulesofmicroorganisms and,insomecases,intracellularlywhenpresumablythecell(s)hadlysed.Commonly,these precipitateswereobservedcoalescingtoformamatrixofamorphoussilicathatcompletely encapsulated the cells and/or replaced their cytoplasmic material. However, in other cells, the precipitates were composed of amorphous granules made exclusively of iron and silica inapproximatelyequalproportions.Atonelocality,thebacteriaformedseveralepicellular iron minerals, ranging from iron-mineralized capsules tofine-grained spheroids of amorphous ferric hydroxide and acicular aggregates of goethite. The complete encrustation of bacterial cells by silica, iron, or a combination of both may greatly enhance their preservation potential, such that these mineralized microorganisms may conceivably represent future microfossils. Thus, we may be witnessing contemporaneous biomineralization processes that are similar to those of the geologic past, particularly with regard to the origin of some Precambrian banded iron formations.