Variations of stable isotope fractionation during bacterial chromium reduction processes and their implications

Abstract

Many chemical processes generate subtle but readily measured changes in isotope compositions of elements across the periodic table. The elements involved therefore carry diagnostic information about their chemical histories in complex geochemical or biochemical environments. Distinctive Cr isotope signatures can be used to identify immobilization processes of Cr in the environment, such as microbial Cr(VI) reduction, abiotic Cr(VI) reduction, and adsorption. Here we demonstrate that under well-controlled conditions, Cr isotopes can also be used to distinguish between different biological Cr(VI) reduction pathways. The reduction of Cr(VI) by two facultative anaerobic bacteria, Pseudomonas fluorescens LB 300 and Shewanella oneidensis MR 1, was investigated to determine the conditions under which Cr(VI) is reduced and to quantify the corresponding isotope signatures. The present study considers the effects of a broad range of parameters on Cr isotope fractionation, including bacterial species, electron donors, pH, and respiration pathways (aerobic vs. anaerobic) that must be considered for understanding Cr isotope variations under different experimental and environmental conditions. In the bacterial Cr(VI) reduction experiments, the 53 Cr/ 52 Cr isotope ratio of the remaining Cr(VI) increased by up to + 8‰, indicating that lighter isotopes of Cr were preferentially reduced. In aerobic experiments, although Cr reduction rates increased as pH increased from 4 to 8, the fractionation factor did not vary significantly (ε =-3.21±0.18‰). Experiments using different electron donors demonstrated that citrate promoted the greatest Cr reduction rate compared with glucose, acetate, and propionate. Under aerobic conditions, although the Cr(VI) reduction rates varied substantially between different experimental settings, the isotope fractionation factors were indistinguishable between all the environmental conditions examined (ε =-3.1 ‰), with the January 30, 2018 exception of when citrate was the electron donor (ε =-4.3 ‰). Cr reduction rates were generally much faster under anaerobic conditions for both bacteria investigated. The utilisation of different electron donors resulted in the same Cr reduction rates by the bacteria, but fractionated Cr with a broad range of isotope fractionation factors, from-1.58±0.16‰ to-4.93±0.36‰. Although it has been proposed in many previous studies that there is an inverse relationship between reduction rates and the fractionation factors, no clear relationship between the reduction rates and fractionation factors was observed in this study. The Cr isotope fractionation factors ε were insensitive to pH and electron donor concentration, but dependent on the type of electron donors and redox conditions in the cultures. This indicates that isotope variations may be used to identify when different biological pathways are involved, and so to investigate metabolic processes. The ε value from all experimental conditions examined ranged between-1.58 to-4.93 ‰, with a mean value at-3.3 ‰. While Cr isot...

Microbially induced chromium isotope fractionation and trace elements behavior in lower Cambrian microbialites from the Jaíba Member, Bambuí Basin, Brazil

“…Regardless if abiotic or biotic, the reduced Cr compounds would no longer be water‐soluble, thus precipitating as oxide or hydroxide phases encrusting cell surfaces and partially trapped in EPS matrix (Smith & Gadd, 2000; Wielinga et al., 2001), or retained as a hydrocarbonate coating on calcite surface with the following general stoichiometry, Cr 2 O 3 xH 2 OyCO 2 (García‐Sánchez & Álvarez‐Ayuso, 2002). Abiotic and biotic Cr(VI)‐Cr(III) reactions are commonly synchronous within a microbial mat and can induce large negative Cr isotope fractionation in both aerobic and anaerobic conditions, with ε values from −0.4 to −5.0‰ (e.g., Basu et al., 2014; Døssing et al., 2011; Ellis et al., 2002; Han et al., 2012; Pereira et al., 2016; Sikora et al., 2008; Wei et al., 2020; Xu et al., 2015; Zhang et al., 2018). Environmental changes in pH and electron donor concentrations, although determinant for bacterial metabolic rates, do not substantially impact the fractionation factors of Cr by microbial reduction.…”

Chemical-Assisted Microbially Mediated Chromium (Cr) (VI) Reduction Under the Influence of Various Electron Donors, Redox Mediators, and Other Additives: An Outlook on Enhanced Cr(VI) Removal

“…The utilization of electron donors for Cr(VI) reduction is species-dependent, and the reduction rate in specific microorganisms also varies for different sources ( Das et al, 2014 ; Zhang et al, 2018 ). The electron donor that is effective in catalyzing Cr(VI) reduction for one microorganism might not be as useful for other species.…”

Monitoring Cr toxicity and remediation processes - combining a whole-cell bioreporter and Cr isotope techniques

“…The supernatant was filtered (0.22 µm) and then transferred to a clean Eppendorf tube and stored at -80 • C before further treatment. Cr(VI) concentrations were measured by HPLC (Thermo (Zhang et al, 2018).…”

“…To assess the effectiveness of remedial interventions of Cr(VI) contamination, and to better understand the mechanism of Cr(VI) reduction, a more efficient method that can identify which are the dominant processes is required. Such a technique involves monitoring Cr isotope fractionation (Zhang et al, 2018).…”

“…where has not yet been studied, although our previous study has identified the relationships between several other important environmental conditions and Cr isotope signatures (Zhang et al, 2018(Zhang et al, , 2019. Therefore, this study focuses on determining the levels of Cr toxicity and their impact on Cr isotope fractionation by bacteria.…”