Microbial anaerobic Fe(II) oxidation – Ecology, mechanisms and environmental implications

Abstract

Iron is the most abundant redox-active metal in the Earth's crust. The one electron transfer between the two most common redox states, Fe(II) and Fe(III), plays a role in a huge range of environmental processes from mineral formation and dissolution to contaminant remediation and global biogeochemical cycling. It has been appreciated for more than a century that microorganisms can harness the energy of this Fe redox transformation for their metabolic benefit. However, this is most widely understood for anaerobic Fe(III)-reducing or aerobic and microaerophilic Fe(II)-oxidizing bacteria. Only in the past few decades have we come to appreciate that bacteria also play a role in the anaerobic oxidation of ferrous iron, Fe(II), and thus can act to form Fe(III) minerals in anoxic settings. Since this discovery, our understanding of the ecology of these organisms, their mechanisms of Fe(II) oxidation and their role in environmental processes has been increasing rapidly. In this article, we bring these new discoveries together to review the current knowledge on these environmentally important bacteria, and reveal knowledge gaps for future research.

Cryptic Cycling of Complexes Containing Fe(III) and Organic Matter by Phototrophic Fe(II)-Oxidizing Bacteria

“…It is likely that the photic cryptic Fe cycle we describe here also exists and is widespread in many natural habitats. First, microbial Fe(II) oxidation is widespread, and phototrophic Fe(II)-oxidizing bacteria were commonly found in many environments, including lakes, soils, and freshwater and marine sediments (36,(51)(52)(53)(54). Second, Fe-OM complexes are abundant in the environment, and many biologically produced Fe-chelating siderophores contain alphahydroxy carboxylate groups which can reduce Fe(III) to Fe(II) in the light (11,28,(55)(56)(57)(58).…”

Genome reconstruction reveals distinct assemblages of Gallionellaceae in surface and subsurface redox transition zones

“…However, NO can react abiotically with Fe 2+ to produce N 2 O and Fe 3+ , thus allowing the full reduction of NO 3 to N 2 . These MAGs could represent real autotrophic nitrate-reducing iron oxidizing bacteria as defined by Bryce et al (2018). They also possess a full repertoire of genes to oxidize sulfide into sulfate.…”

Seasonal blooms of neutrophilic Betaproteobacterial Fe(II) oxidizers and Chlorobi in iron-rich coal mine drainage sediments

“…If this is the case, the temporal transition from the putative microaerophilic to an anoxygenic photoferrotrophic metabolism could represent a phenomenon that has yet to be described in circumneutral mine drainage environments. Currently, most photoferrotrophic isolates have originated from either freshwater sediment and lakes or marine sediments (Bryce et al 2018). The occurrence of Chlorobi has been reported in 16S rRNA gene sequencing surveys of acid mine drainage (Volant et al 2014;Mesa et al 2017) and soils impacted by circumneutral mine drainage (Pereira, Vicentini and Ottoboni 2014) though at low abundance (<1%).…”

“…Microbially mediated Fe(III) reduction involves reduction of Fe(III) by microorganisms that can use either H 2 or organic carbon as an electron donor (Lovley and Phillips 1988;Lovley 1997). At circumneutral pH, microorganisms capable of Fe(II) oxidation can be divided into three physiological groups: (i) anoxygenic nitrate-reducing, (Kappler, Schink and Newman 2005;Laufer et al 2016c), (ii) anoxygenic phototrophic (Widdel et al 1993) and (iii) microaerophilic (Emerson and Moyer 1997), the activities of which lead to the production of a variety of biogenic Fe(III) minerals (Bryce et al 2018).…”