Chromium transformation pathways in metal-reducing bacteria
Creators: name: University Of California Office Of The President
Publisher: name: University of California Office of the President
publisherIdentifier: https://ror.org/00dmfq477
publisherIdentifierScheme: ROR
Year: 2024

description: **Introduction** Chromium (Cr) is a common pollutant found in the environment and poses a serious public heath concern. Major sources of Cr pollution include effluents from leather tanning, chromium plating, wood preservation, alloy preparation, evaporative cooling systems, and nuclear wastes from Department of Energy (DOE). Because of its strong oxidizing nature, hexavalent Cr(VI) is extremely toxic and exhibits mutagenic and carcinogenic effects on biological systems. The reduced form of chromium (Cr(III)) is less toxic and bioavailable than Cr(VI), as it readily forms insoluble oxides and hydroxides under cirumneutral pH. Reduction of Cr(VI) and the precipitation of Cr(III) is therefore a potentially useful process for detoxification and/or removal of Cr(VI) from contaminated natural water, soil, and waste streams. **Project Overview** The proposed work will investigate Cr(VI) reduction under advective flow conditions. We will investigate Cr(VI) reduction of two model organism Arthrobacter oxydans and Shewanella sp. ANA-3. The two organism are proposed to carry out Cr(VI) reduction by direct reduction or indirectly by the production of iron(II) via iron(III) oxide reduction, respectively. We hypothesize that: 1. Cr(VI) reduction by iron-reducing bacteria is mediated indirectly by the iron reduction metabolic pathway (mtr/omc genes), which encode for outermembrane cytochromes. 2. Under aerobic conditions Cr(VI) reduction is carried out by a direct enzymatic mechanism, represented by Arhtrobacter. 3. Under anaerobic conditions, Cr(VI) reduction occurs indirectly by Fe(II) produced by Fe(III) reducing bacteria such as Shewanella. To test these hypotheses, the following aims will be carried out: (i) determine the molecular basis of Cr(VI) reduction mediated by the iron-reducing bacterium Shewanella sp. ANA-3; and (ii) determine effects of oxygen (or lack thereof) on Cr sequestration under hydrodynamic conditions. **Experiments** In aim 1, we will generate genetic mutations in the mtr/omc genes of Shewanella and test the strain’s ability to reduce Cr(VI) and Fe(III) within iron-oxide coated sand (IOCS) columns. Column effluent will be monitored for Cr(VI), Cr(III), and Fe(II) production. The second aim will test the effects of aerobic vs. anaerobic conditions on Cr(VI reduction by Arthrobacter and Shewanella inoculated in IOCS columns. A suite of state-of-the-art microscopy and spectroscopic methods will be used to evaluate microbe-mineral interactions and Cr-Fe biomineralization products. The column experiments will be done in the laboratory of Dr. Chad Saltikov by a UCSC graduate student. The spectroscopy and mineralogy work will be conducted at LLNL with a UCSC graduate student and Dr. Yongqin Jiao under the supervision and guidance of Dr. Alexander Malkin. **Project Impacts** This project will evaluate if microbial processes can be an efficient and effective way for Cr(VI) removal from the aqueous environments. The experiments will combine both genetic and geochemical approaches to address questions about the mechanisms of microbial processes known to promote Cr(VI) reduction. Using the advective flow columns to investigate Cr(VI) reduction represents a more relevant simulation of the potential mechanisms of Cr redox cycling that might occur in natural settings. The results will likely define the feasibility of using microbial assisted Cr sequestration as a means of Cr bioremediation.