Ferric Iron Reduction in Extreme Acidophiles

Biochemical processes are a key element of natural cycles occurring in the environment and enabling life on earth. With regard to microbially catalyzed iron transformation, research predominantly has focused on iron oxidation in acidophiles, whereas iron reduction played a minor role. Microbial conversion of ferric to ferrous iron has however become more relevant in recent years. While there are several reviews on neutrophilic iron reducers, this article summarizes the research on extreme acidophilic iron reducers. After the first reports of dissimilatory iron reduction by acidophilic, chemolithoautotrophic Acidithiobacillus strains and heterotrophic Acidiphilium species, many other prokaryotes were shown to reduce iron as part of their metabolism. Still, little is known about the exact mechanisms of iron reduction in extreme acidophiles. Initially, hypotheses and postulations for the occurring mechanisms relied on observations of growth behavior or predictions based on the genome. By comparing genomes of well-studied neutrophilic with acidophilic iron reducers (e.g., Ferroglobus placidus and Sulfolobus spp.), it became clear that the electron transport for iron reduction proceeds differently in acidophiles. Moreover, transcriptomic investigations indicated an enzymatically-mediated process in Acidithiobacillus ferrooxidans using respiratory chain components of the iron oxidation in reverse. Depending on the strain of At. ferrooxidans, further mechanisms were postulated, e.g., indirect iron reduction by hydrogen sulfide, which may form by disproportionation of elemental sulfur. Alternative scenarios include Hip, a high potential iron-sulfur protein, and further cytochromes. Apart from the anaerobic iron reduction mechanisms, sulfur-oxidizing acidithiobacilli have been shown to mediate iron reduction at low pH (< 1.3) under aerobic conditions. This presumably non-enzymatic process may be attributed to intermediates formed during sulfur/tetrathionate and/or hydrogen oxidation and has already been successfully applied for the reductive bioleaching of laterites. The aim of this review is to provide an up-to-date overview on ferric iron reduction by acidophiles. The importance of this process in anaerobic habitats will be demonstrated as well as its potential for application.

Thermodynamic controls on rates of iron oxide reduction by extracellular electron shuttles

Anaerobic microbial respiration in suboxic and anoxic environments often involves particulate ferric iron (oxyhydr-)oxides as terminal electron acceptors. To ensure efficient respiration, a widespread strategy among iron-reducing microorganisms is the use of extracellular electron shuttles (EES) that transfer two electrons from the microbial cell to the iron oxide surface. Yet, a fundamental understanding of how EES–oxide redox thermodynamics affect rates of iron oxide reduction remains elusive. Attempts to rationalize these rates for different EES, solution pH, and iron oxides on the basis of the underlying reaction free energy of the two-electron transfer were unsuccessful. Here, we demonstrate that broadly varying reduction rates determined in this work for different iron oxides and EES at varying solution chemistry as well as previously published data can be reconciled when these rates are instead related to the free energy of the less exergonic (or even endergonic) first of the two electron transfers from the fully, two-electron reduced EES to ferric iron oxide. We show how free energy relationships aid in identifying controls on microbial iron oxide reduction by EES, thereby advancing a more fundamental understanding of anaerobic respiration using iron oxides.

Methaemoglobinaemia and the radical curative efficacy of 8-aminoquinoline antimalarials

Methaemoglobin results from the oxidation of ferrous to ferric iron in the centre of the haem moeity of haemoglobin. The production of dose-dependent methaemoglobinaemia by 8-aminoquinoline antimalarial drugs appears to be associated with, but is not directly linked to therapeutic efficacy against latent vivax and ovale malarias. Iatrogenic methaemoglobinaemia may be a useful pharmacodynamic measure in 8-aminoquinoline drug and dose optimization.

The chromian spinels of the Lyavaraka ultrabasic complex, Serpentinite Belt, Kola Peninsula, Russia: Patterns of zoning, hypermagnesian compositions, and early oxidation

ABSTRACT The maximum value of Mg# [= 100Mg/(Mg + Fe2+ + Mn)] in chromium-bearing spinel-group minerals (Chr) in the Ultrabasic Core Zone (UCZ) of the Lyavaraka orthopyroxenite – harzburgite – dunite complex of the Serpentinite Belt in the Kola Peninsula is 54.5–67.5. Such highly magnesian compositions of spinel are associated with notable enrichments of ferric iron (Fe3+# 58–63). There are two generations of accessory Chr in the UCZ unit. The first generation occurs as inclusions in olivine that is not unusually magnesian (Mg# 90.3), and the second is closely associated with serpentine. The compositional series of Chr at Lyavaraka attains more aluminous compositions than was observed in nearby intrusive bodies. The anomalously high level of Mg in Chr, also manifest in ilmenite, is mainly a result of the high intrinsic fugacity of oxygen attained locally in the melt. A progressive buildup in H2O and increase in fO2 likely resulted from efficient vesiculation and selective loss of H2 from the Al-undepleted komatiitic magma crystallizing in a shallow setting. The chromian spinel forming in such a modified magma is virtually unzoned in Mn, and a minor quantity of Mn is also present in olivine and orthopyroxene. In contrast, zinc is strongly partitioned in the core of Chr, as it is relatively incompatible in the coexisting olivine and orthopyroxene at that stage. Zinc efficiently partitioned into the H2O-enriched melt, which crystallized as the pegmatitic orthopyroxenite near the contacts at Lyavaraka. A high potential of oxidation appears to be characteristic of all orthopyroxenite – harzburgite – dunite suites of the Serpentinite Belt formed from a primitive melt of komatiitic composition.

Altered Pseudomonas Strategies to Inhibit Surface Aspergillus Colonies

Pseudomonas aeruginosa and Aspergillus fumigatus infections frequently co-localize in lungs of immunocompromised patients and individuals with cystic fibrosis (CF). The antifungal activity of P. aeruginosa has been described for its filtrates. Pyoverdine and pyocyanin are the principal antifungal P. aeruginosa molecules active against A. fumigatus biofilm metabolism present in iron-limited or iron-replete planktonic P. aeruginosa culture filtrates, respectively. Using various P. aeruginosa laboratory wild-type strains (PA14, PAO1, PAK), we found antifungal activity against Aspergillus colonies on agar. Comparing 36 PA14 and 7 PAO1 mutants, we found that mutants lacking both major siderophores, pyoverdine and pyochelin, display higher antifungal activity on agar than their wild types, while quorum sensing mutants lost antifungal activity. Addition of ferric iron, but not calcium or magnesium, reduced the antifungal effects of P. aeruginosa on agar, whereas iron-poor agar enhanced antifungal effects. Antifungal activity on agar was mediated by PQS and HHQ, via MvfR. Among the MvfR downstream factors, rhamnolipids and elastase were produced in larger quantities by pyoverdine–pyochelin double mutants and showed antifungal activity on agar. In summary, antifungal factors produced by P. aeruginosa on agar differ from those produced by bacteria grown in liquid cultures, are dependent on quorum sensing, and are downregulated by the availability of ferric iron. Rhamnolipids and elastase seem to be major mediators of Pseudomonas’ antifungal activity on a solid surface.

Low-spin ferric iron in primordial bridgmanite crystallized from a deep magma ocean

The crystallization of the magma ocean resulted in the present layered structure of the Earth’s mantle. An open question is the electronic spin state of iron in bridgmanite (the most abundant mineral on Earth) crystallized from a deep magma ocean, which has been neglected in the crystallization history of the entire magma ocean. Here, we performed energy-domain synchrotron Mössbauer spectroscopy measurements on two bridgmanite samples synthesized at different pressures using the same starting material (Mg0.78Fe0.13Al0.11Si0.94O3). The obtained Mössbauer spectra showed no evidence of low-spin ferric iron (Fe3+) from the bridgmanite sample synthesized at relatively low pressure of 25 gigapascals, while that directly synthesized at a higher pressure of 80 gigapascals contained a relatively large amount. This difference ought to derive from the large kinetic barrier of Fe3+ rearranging from pseudo-dodecahedral to octahedral sites with the high-spin to low-spin transition in experiments. Our results indicate a certain amount of low-spin Fe3+ in the lower mantle bridgmanite crystallized from an ancient magma ocean. We therefore conclude that primordial bridgmanite with low-spin Fe3+ dominated the deeper part of an ancient lower mantle, which would contribute to lower mantle heterogeneity preservation and call for modification of the terrestrial mantle thermal evolution scenarios.