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.