The diversity and abundance of As(III) oxidizers on root iron plaque is critical for arsenic bioavailability to rice.

Min Hu,Chuanping Liu,Weijian Wu,Fangbai Li

doi:10.1038/srep13611

Abstract

Iron plaque is a strong adsorbent on rice roots, acting as a barrier to prevent metal uptake by rice. However, the role of root iron plaque microbes in governing metal redox cycling and metal bioavailability is unknown. In this study, the microbial community structure on the iron plaque of rice roots from an arsenic-contaminated paddy soil was explored using high-throughput next-generation sequencing. The microbial composition and diversity of the root iron plaque were significantly different from those of the bulk and rhizosphere soils. Using the aoxB gene as an identifying marker, we determined that the arsenite-oxidizing microbiota on the iron plaque was dominated by Acidovorax and Hydrogenophaga-affiliated bacteria. More importantly, the abundance of arsenite-oxidizing bacteria (AsOB) on the root iron plaque was significantly negatively correlated with the arsenic concentration in the rice root, straw and grain, indicating that the microbes on the iron plaque, particularly the AsOB, were actively catalyzing arsenic transformation and greatly influencing metal uptake by rice. This exploratory research represents a preliminary examination of the microbial community structure of the root iron plaque formed under arsenic pollution and emphasizes the importance of the root iron plaque environment in arsenic biogeochemical cycling compared with the soil-rhizosphere biotope.

Highlights

Rice is the world’s single most important food crop and a primary food source for more than a third of the world’s population[1]
Bacterial populations are associated with As(III) oxidation in soil ecosystems[9,10], and many autotrophic or heterotrophic Arsenite-oxidizing bacteria (AsOB) have been isolated from soil environments[11,12]
Oxygen and soluble ferrous iron are key factors in controlling iron plaque generation on the roots of aquatic plants[15]; significant numbers of bacteria are associated with iron oxidation within the rhizosphere, which suggests that the microbial community may play a role in iron plaque formation[16,17]

Summary

Introduction

Rice is the world’s single most important food crop and a primary food source for more than a third of the world’s population[1]. Because the mobility of As is redox-sensitive, this redox change has a significant impact on the behavior of As in the soil particles and pore water of paddy fields, eventually affecting As accumulation in rice plants[5]. Microbe-generated iron plaque may impact As speciation at the soil-root interface and reduce As uptake by rice plants. The microbial reduction of As(V) to As(III) increases the mobility of As due to the lower sorption strength of Fe(III) (hydr)oxides, resulting in the predominance of As(III) among the As species in reducing paddy soil environments. In As-contaminated flooding soil, anoxic conditions leading to the microbial reduction of As(V) and Fe(III) may enhance the mobility of As, posing a threat to rice plant As transport. Reduced As(III) on soil particles or in solution may be transferred to the root surface and oxidized to As(V) chemically or biologically under anaerobic conditions

Methods

Results

Conclusion