Freeze–Thaw Cycling Accelerates Microbial Reduction and Immobilization of Vanadium(V) in Groundwater

As global warming intensifies, freeze-thaw cycling has become more pronounced, significantly impacting microbial metabolic processes. In this study, microbial vanadium(V) [V(V)] reduction under freeze-thaw conditions was investigated. Temperature fluctuations between -4 and +4 °C accelerated V(V) reduction, together with the faster decrease in organic matter and total nitrogen in the sediment, compared to constant-temperature incubations at either -4 or +4 °C. V(V) was bioreduced to vanadium(IV) precipitates, increasing the acid-soluble, reducible, and oxidizable fractions of vanadium in the sediment. Freeze-thaw cycling enhanced microbial carbon and nitrogen utilization, as well as the richness and diversity of the microbial community. Metagenomics, real-time quantitative polymerase chain reaction quantification, and electron-transfer measurements revealed the underlying mechanisms. During thawing, microorganisms such as Flavobacterium sp., Stutzerimonas sp., and Giebergeria sp. reduced V(V) via denitrification genes (narG, nirS, and nosZ). Under freezing conditions, enriched Pseudomonas sp. and Acinetobacter sp. achieved V(V) reduction through the electron-transfer gene (mtrC), with increased cytochrome c concentration. Key cold-adaptation genes, including cspA, HSP, SOD, katG, and SCD, were prevalent in these microorganisms, supporting antifreeze activities. This study offers insights into microbial V(V) reduction under freeze-thaw cycling scenarios and is helpful in developing pertinent remediation strategies.