Soil Humic Acid Stimulates Potentially Active Dissimilatory Arsenate-Reducing Bacteria in Flooded Paddy Soil as Revealed by Metagenomic Stable Isotope Probing

Dissimilatory arsenate reduction contributes a large proportion of arsenic flux from flooded paddy soil, which is closely linked to soil organic carbon input and efflux. Humic acid (HA) represents a natural ingredient in soil and is shown to enhance microbial arsenate respiration to promote arsenic mobility. However, the community and function profiles of metabolically active arsenate-respiring bacteria and their interactions with HA in paddy soil remain unclear. To probe this linkage, we performed a genome-centric comparison of potentially active arsenate-respiring bacteria in anaerobic microcosms amended with 13C-lactate and HA by combining stable-isotope probing with genome-resolved metagenomics. Indeed, HA greatly accelerated the microbial reduction of arsenate to arsenite. Enrichment of bacteria that harbor arsenate-respiring reductase genes (arrA) in HA-enriched 13C-DNA was confirmed by metagenomic binning, which are affiliated with Firmicutes (mainly Desulfitobacterium, Bacillus, Brevibacillus, and Clostridia) and Acidobacteria. Characterization of reference extracellular electron transfer (EET)-related genes in these arrA-harboring bacteria supports the presence of EET-like genes, with partial electron-transport chain genes identified. This suggests that Gram-positive Firmicutes- and Acidobacteria-related members may harbor unspecified EET-associated genes involved in metal reduction. Our findings highlight the link between soil HA and potentially active arsenate-respiring bacteria, which can be considered when using HA for arsenic removal

Soil Humic Acid Stimulates Potentially Active Dissimilatory Arsenate-Reducing Bacteria in Flooded Paddy Soil as Revealed by Metagenomic Stable Isotope Probing

Dissimilatory arsenate reduction contributes a large proportion of arsenic flux from flooded paddy soil, which is closely linked to soil organic carbon input and efflux. Humic acid (HA) represents a natural ingredient in soil and is shown to enhance microbial arsenate respiration to promote arsenic mobility. However, the community and function profiles of metabolically active arsenate-respiring bacteria and their interactions with HA in paddy soil remain unclear. To probe this linkage, we performed a genome-centric comparison of potentially active arsenate-respiring bacteria in anaerobic microcosms amended with 13C-lactate and HA by combining stable-isotope probing with genome-resolved metagenomics. Indeed, HA greatly accelerated the microbial reduction of arsenate to arsenite. Enrichment of bacteria that harbor arsenate-respiring reductase genes (arrA) in HA-enriched 13C-DNA was confirmed by metagenomic binning, which are affiliated with Firmicutes (mainly Desulfitobacterium, Bacillus, Brevibacillus, and Clostridia) and Acidobacteria. Characterization of reference extracellular electron transfer (EET)-related genes in these arrA-harboring bacteria supports the presence of EET-like genes, with partial electron-transport chain genes identified. This suggests that Gram-positive Firmicutes- and Acidobacteria-related members may harbor unspecified EET-associated genes involved in metal reduction. Our findings highlight the link between soil HA and potentially active arsenate-respiring bacteria, which can be considered when using HA for arsenic removal

Anaerobic Transformation of DDT Related to Iron(III) Reduction and Microbial Community Structure in Paddy Soils

We studied the mechanisms of microbial transformation in functional bacteria on 1,1,1-trichloro-2,2-bis­(<i>p</i>-chlorophenyl)­ethane (DDT) in two different field soils, Haiyan (HY) and Chenghai (CH). The results showed that microbial activities had a steady dechlorination effect on DDT and its metabolites (DDx). Adding lactate or glucose as carbon sources increased the amount of <i>Desulfuromonas</i>, <i>Sedimentibacter</i>, and <i>Clostridium</i> bacteria, which led to an increase in adsorbed Fe­(II) and resulted in increased DDT transformation rates. The electron shuttle of anthraquinone-2,6-disulfonic disodium salt resulted in an increase in the negative potential of soil by mediating the electron transfer from the bacteria to the DDT. Moreover, the DDT-degrading bacteria in the CH soil were more abundant than those in the HY soil, which led to higher DDT transformation rates in the CH soil. The most stable compound of DDx was 1,1-dichloro-2,2-bis­(<i>p</i>-chloro-phenyl)­ethane, which also was the major dechlorination metabolite of DDT, and 1-chloro-2,2-bis-(<i>p</i>-chlorophenyl)­ethane and 4,4′-dichlorobenzo-phenone were found to be the terminal metabolites in the anaerobic soils

Biostimulation of Indigenous Microbial Communities for Anaerobic Transformation of Pentachlorophenol in Paddy Soils of Southern China

This study explored biostimulation mechanisms with an electron donor and a shuttle for accelerating pentachlorophenol (PCP) transformation in iron-rich soils. The results indicated that indigenous microbial communities are important for PCP transformation in soils. Biostimulation of indigenous microbial communities by the addition of lactate and anthraquinone-2,6-disulfonate (AQDS) led to the enhanced rates of PCP dechlorination by the dechlorinating- and iron-reducing bacteria in soils. The electrochemical studies using cyclic voltammograms and microbial current measurements confirmed the high reduction potential and the large amount of electrons generated under biostimulation conditions, which were responsible for the higher rates of PCP transformation. After biostimulation treatments by the additions of lactate and/or AQDS during PCP dechlorination processes, microbial community analysis by the terminal restriction fragment length polymorphism (T-RFLP) method showed the abundance terminal restricted fragments (T-RFs), an indicator of bacterial abundance, which represents the dechlorinating- and iron-reducing bacteria, suggesting their critical roles in PCP dechlorination in soils