Microaerophilic Oxidation of Fe(II) Coupled with Simultaneous Carbon Fixation and As(III) Oxidation and Sequestration in Karstic Paddy Soil

Abstract

Microaerophilic Fe(II)-oxidizing bacteria are often chemolithoautotrophs, and the Fe(III) (oxyhydr)oxides they form could immobilize arsenic (As). If such microbes are active in karstic paddy soils, their activity would help increase soil organic carbon and mitigate As contamination. We therefore used gel-stabilized gradient systems to cultivate microaerophilic Fe(II)-oxidizing bacteria from karstic paddy soil to investigate their capacity for Fe(II) oxidation, carbon fixation, and As sequestration. Stable isotope probing (SIP) demonstrated the assimilation of inorganic carbon at a maximum rate of 8.02 mmol C m -2 d -1 . Sequencing revealed that Bradyrhizobium, Cupriavidus, Hyphomicrobium, Kaistobacter, Mesorhizobium, Rhizobium, unclassified Phycisphaerales, and unclassified Opitutaceas, were fixing carbon. Fe(II) oxidation produced Fe(III) (oxyhydr)oxides, which can absorb and/or co-precipitate As. Adding As(III) decreased the diversity of functional bacteria involved in carbon fixation, the relative abundance of predicted carbon fixation genes, and the amount of carbon fixed. Although the rate of Fe(II) oxidation was also lower in the presence of As(III), over 90% of the As(III) was sequestered after oxidation. The potential for microbially mediated As(III) oxidation was revealed by the presence of arsenite oxidase gene (aioA), denoting the potential of the Fe(II) oxidizing and autotrophic microbial community to also oxidize As(III). The results of this study demonstrate that carbon fixation coupled to Fe(II) oxidation can increase the carbon content in soils by microaerophilic Fe(II)-oxidizing bacteria, as well as accelerate As(III) oxidation and sequester it in association with Fe(III) (oxyhydr)oxides.

Carbon fixation pathways across the bacterial and archaeal tree of life

“…These MAGs have the genetic potential to perform the sedoheptulose-1,7-bisphosphate (SBPase) variant of the cycle, where erythrose 4-phosphate and dihydroxyacetone phosphate are condensed into sedoheptulose 1,7-biphosphate by an aldolase ( 45 ). We also found here first evidence for its presence in members of the families Beijerinckiaceae, Methyloligellaceae, Pseudoxanthobacteraceae , and UBA4765 , which all belong to the order Rhizobiales that has recently been shown to contain autotrophic organisms ( 46 ).…”

Vertical variations of soil carbon under different land uses in a karst critical zone observatory (CZO), SW China

“…In karst regions, macroaggregates in topsoil can recover rapidly (Liu et al, 2020)and the lost calcium caused by long-term tillage can also be rapidly replenished from the limestone dissolution after farming cessation, stabilising SOC (Li et al, 2017). The functional bacteria involved in C fixation might be more active and diverse in karst soils (Tong et al, 2021). Additionally, dissolved organic C in surface runoff may be partially withheld or adsorbed during percolation processes, contributing further to the SOC pool (Qin et al, 2021).…”

Microbial autotrophy explains large‐scale soil CO2 fixation

“…Members in Thiobacillaceae and Acidiferrobacteraceae, in particular, can utilize CO 2 as the sole C source by oxidation of sulfur using ferric iron as an electron acceptor under anaerobic conditions and oxidation of ferrous iron using oxygen as an electron acceptor in aerobic environment (Issotta et al, 2018; Monachon et al, 2019). The frequent transformation of iron and sulfur at the oxic–anoxic interface in paddy ecosystem may facilitate the enrichment of autotrophic iron and sulfur oxidizers and accelerate the chemolithoautotrophic fixation of CO 2 (Tong et al, 2021).…”