ALBERT

Description: Microbes are intimately involved in the iron cycle. First, acquisition of iron by microorganisms for biochemical requirements is a key process in the iron cycle in oxygenated, circumneutral pH environments, where the solubility of Fe(III) (oxyhydr)oxides is extremely low. Second, a number of aerobic (using O2) and anaerobic (living in the absence of O2) autotrophic bacteria gain energy for growth from the oxidation of dissolved and solid-phase Fe(II) compounds to Fe(III) (oxyhydr)oxides. Third, heterotrophic Fe(III)-reducing bacteria close the chemical loop by reducing solid-phase Fe(III) minerals back to dissolved and solid-phase Fe(II). Together these metabolic processes control the partitioning of the Earth's fourth most abundant crustal element, and they are additionally tied to the cycling of several major nutrients (e.g. carbon, oxygen, nitrogen, sulfur) and trace elements (e.g. phosphorus, nickel) in modern and ancient environments.

Description: Iron plaque on rice roots represents a sink and source of iron in paddy fields. However, the extent of iron plaque in impacting paddy field iron cycling is not yet fully deciphered. Here, we followed iron plaque formation during plant growth in laboratory-controlled setups containing a transparent soil matrix. Using image analysis, microsensor measurements, and mineral extractions, we demonstrate that radial oxygen loss (ROL) is the main driver for rhizosphere iron oxidation. While O2 was restricted to the vicinity of roots, root tips showed highest spatio-temporal variation in ROL (30% of the total root surface corresponding to 60–180 mg Fe(III) per gram dried root and gradually transformed from low-crystalline minerals (e.g., ferrihydrite) on root tips, to 〉20% higher-crystalline minerals (e.g., goethite) within 40 days. Iron plaque exposed to an Fe(III)-reducing Geobacter spp. culture resulted in 30% Fe(II) remobilization and 〉50% microbial transformation to Fe(II) minerals (e.g., siderite, vivianite, and Fe–S phases) or persisted by 〉15% as Fe(III) minerals. Based on the collected data, we estimated that iron plaque formation and reductive dissolution can impact more than 5% of the rhizosphere iron budget which has consequences for the (im)mobilization of soil contaminants and nutrients.

Description: Chloroform (CF) is an environmental contaminant that can be naturally formed in various environments ranging from forest soils to salt lakes. Here we investigated CF removal potential in sediments obtained from hypersaline lakes in Western Australia. Reductive dechlorination of CF to dichloromethane (DCM) was observed in enrichment cultures derived from sediments of Lake Strawbridge, which has been reported as a natural source of CF. No CF removal was observed in abiotic control cultures without artificial electron donors, indicating biotic CF dechlorination in the enrichment cultures. Increasing vitamin B12 concentration from 0.04 to 4 µM in enrichment cultures enhanced CF removal and reduced DCM formation. In cultures amended with 4 µM vitamin B12 and 13C labelled CF, formation of 13CO2 was detected. Known organohalide-respiring bacteria and reductive dehalogenase genes were neither detected using quantitative PCR nor metagenomic analysis of the enrichment cultures. Rather, members of the order Clostridiales, known to co-metabolically transform CF to DCM and CO2, were detected. Accordingly, metagenome-assembled genomes of Clostridiales encoded enzymatic repertoires for the Wood-Ljungdahl pathway and cobalamin biosynthesis, which are known to be involved in fortuitous and nonspecific CF transformation. This study indicates that hypersaline lake microbiomes may act as a filter to reduce CF emission to the atmosphere.

Description: As iron sulfide mineral phases are important sedimentary sinks for naturally occurring or contaminant metals, it is important to know the fate of metals during the diagenetic transformation of primary sulfide minerals into more stable phases, such as pyrite (FeS2). Furthermore, the trace metal content of pyrite has been proposed as a marine paleoredox proxy. Given the diverse low-temperature diagenetic formation pathways for pyrite, this use of pyrite requires validation. We, therefore, studied nickel (Ni) and cobalt (Co) incorporation into freshly precipitated mackinawite (FeSm), and after experimental diagenesis to pyrite (FeS2) using S0 as an oxidant at 65 °C. Metal incorporation was quantified on bulk digests using ICP-OES or ICP-AES. Bulk mineralogy was characterized with micro-X-ray diffraction (micro-XRD), documenting the transformation of mackinawite to pyrite. Epoxy grain mounts were made anoxically of mackinawite and pyrite grains. We used synchrotron-based micro-X-ray fluorescence (μXRF) to map the distribution of Co and Ni, as well as to collect multiple energy maps throughout the sulfur (S) K-edge. Iron (Fe) and S K-edge micro-X-ray absorption near edge spectroscopy (μXANES) was used to identify the oxidation state and mineralogy within the experimentally synthesized and diagenetically transformed minerals, and map end-member solid phases within the grain mounts using the multiple energy maps. Metal-free FeSm transformed to pyrite, with residual FeSm detectable. Co- and Ni-containing FeSm also transformed to pyrite, but with multiple techniques detecting FeSm as well as S0, implying less complete transformation to pyrite as compared to metal-free FeSm. These results indicate that Co and Ni may inhibit transformation for FeSm to pyrite, or slow it down. Cobalt concentrations in the solid diminished by 30% during pyrite transformation, indicating that pyrite Co may be a conservative tracer of seawater or porewater Co concentrations. Nickel concentrations increased several-fold after pyrite formation, suggesting that pyrite may have scavenged Ni from the dissolution of primary FeSm grains. Nickel in pyrites thus may not be a reliable proxy for seawater or porewater metal concentrations.