Unveil of the role of fungal taxa in iron(III) reduction in paddy soil

Abstract

Hitherto, research on iron(III)-reduction has mainly focused on bacteria rather than fungal communities. To acquire insight into fungi involved in iron(III) reduction, typical organic matters (containing cellulose, glucose, lactate, and acetate) and ferrihydrite were used as electron donors and acceptors, respectively, in the presence of antibiotics. After antibiotic addition, microbial iron(III) reduction was still detected at quite high rates. In comparison, rates of iron(III) reduction were significantly lower in cellulose-amended groups than those with glucose, lactate, and acetate under the antibiotic-added condition. Patterns of intermediate (e.g., acetate, pyruvate, glucose) turnover were markedly different between treatments with and without antibiotics during organic degradation. A total of 20 genera of potential respiratory and fermentative iron(III)-reducing fungi were discovered based on ITS sequencing and genome annotation. This study provided an insight into the diversity of iron(III)-reducing fungi, indicating the underestimated contribution of fungi to iron and the coupled carbon biogeochemical cycling in environments.

Keywords: fermentative iron(III) reducers; fungal taxa; iron(III) reduction; organic metabolism; respiratory iron(III) reducers.

Figure 1

Dynamics of Fe(II) concentrations in treatments amended with cellulose (A), glucose (B), lactate (C), and acetate (D) during days of incubation with or without antibiotics. Error bars represent standard deviations of three replications.

Figure 2

Time course of metabolite concentrations in treatments amended with cellulose (A–D), glucose (E–H), lactate (I–K), and acetate (A) during days of incubation with or without antibiotics. Error bars represent standard deviations of three replications.

Figure 3

Pattern of fungal community of top 10 taxa at the family level in treatments amended with cellulose (A), glucose (B), lactate (C), and acetate (D) during days of incubation with or without antibiotics.

Figure 4

Heatmap of relative abundances of fungal taxa with significant difference (p < 0.05) in the genus level detected in treatments amended with cellulose, glucose, lactate and acetate during days of incubation without antibiotics. ☆represented abundances of taxa increased during the incubation, which were denote as active taxa in this study.

Figure 5

Metabolic pathway of cellulose, glucose, lactate and acetate in active fungal taxa. (A) Gene homologs in active fungal MAGs. (B) Proposed active pathway in fungal taxa. Pathways were constructed based on fungal MAGs (Supplementary Table S1). ☆indicates that ITS2 sequences were not extracted from the fungal MAGs. (C) Iron(III)-reducing potential fungi including fermentative and four organic matters metabolism with the reduction of Fe(III) to Fe(II) in cellulose, glucose, lactate and acetate amended setups. The gray arrows represented that cellulose, glucose, lactate and acetate degraded into CO₂ by fungal members and the orange arrows showed the products of glucose, lactate and acetate in the process of cellulose decomposition.