Formation and occurrence of biogenic iron-rich minerals

Abstract

Iron cycling in the Earth's crust depends on redox reactions, which often trigger the precipitation and dissolution of Fe-rich minerals. Microbial activity is also an integral part of iron cycling, through carbon fixation, respiration and passive sorption reactions. Iron oxides formed in close association with bacteria (either as internal or external precipitates) are referred to as biogenic minerals. They form in several types of environments on Earth, from freshwater to marine systems, aquifers, soils and mining impacted systems. Biogenic iron oxides generally occur as nanocrystals and show a wide range of morphology and mineralogy. These minerals form as a result of the direct metabolic activity of bacteria or as a result of passive sorption and nucleation reactions. The metabolic activity of acidophilic and neutrophilic iron-oxidizing bacteria under oxic conditions promotes the oxidation of Fe(II) to Fe(III) and the precipitation of biogenic iron oxides as extracellular precipitates near or on the bacterial cells. Iron oxidation under anoxic conditions can also occur, as a result of the activity of nitrate-reducers and photoautotrophic bacteria using Fe(II) as an electron donor. Secondary Fe-oxide formation has been reported during the microbial reduction of iron oxides. Passive Fe sorption and nucleation onto bacterial cell walls represents another important mechanism leading to iron oxide formation. The surface reactivity of the bacterial surface under environmental pH conditions confers a net negative charge to the cell wall, which leads to the binding of soluble iron and eventually to the precipitation of iron oxides under saturation conditions. Extracellular polymers produced by bacteria can act as a template for iron sorption and Fe-oxide nucleation. Intracellular iron oxide formation has been observed in natural environments. Magnetotactic bacteria produce intracellular magnetosomes, occurring as chains of magnetite crystals within the cells, and an unidentified iron-rich mineral phase forms inside Shewanella cells during the anaerobic reduction of ferrihydrite. Several studies have clearly shown that biogenic iron oxides form in present-day environments, but they might also be important components of ancient geological formations, such as banded-iron formations (BIF). BIF formation is still being debated, but there is now strong evidence that bacteria, more specifically, phototrophic iron oxidizers and possibly iron reducers might have been involved. Biogenic iron oxides represent a potential tool in the search for past and present life on Earth and other planetary systems. Despite the promising use of Fe-isotopes and magnetosomes, there is still no clear proof that they can form only as a result of biological activity. In fact, Fe isotope fractionation of abiotic iron oxides is often similar to that of biogenic oxides and the specific mineralogical characteristics of magnetite crystals present inside magnetotactic bacteria can be reproduced under abiotic conditions. In summary, the role of bacteria in iron cycling has been the focus of several studies in the last few decades, but clearly, more research is needed in order to fully assess the role of microorganisms in their formation.