Arsenic (As) pollution in soils is a major health and environmental threat to water resources. Arsenic bioavailability in the environment is directly influenced by communities of As(III) oxidizing and As(V) reducing bacteria that catalyse the transformation of As(III) to As(V), a form easily precipitated, and the transformation of As(V) into As(III), the more mobile and toxic form, respectively. The objective of our work is to define molecular bio-indicators of arsenic mobility, in parallel with existing physic-chemical methods, and to evaluate their ability to predict As behaviour in the environment. Incubations of two soil samples from an industrial waste land containing different amounts of As (about 500 and 1600 ppm) showed that in aerobic conditions, favorable to As(III) oxidation, the microbial community stabilizes the arsenic by maintaining it as As(V). In anaerobic conditions, with the addition of a source of exogenic carbon to stimulate heterotrophic bacteria, the decrease in redox potential induces conditions in favor of As(V) reduction: 70% of the initial As(V) is solubilized. Without carbon addition, As solubilisation and As(V) reduction occur simultaneously in conditions where the redox potential is not favorable for As(V) respiration. The different As(III) oxidizing and As(V) reducing-bacterial communities issue from these incubations were studied, aiming in particular an As(III)-oxidase gene, aioB and an As(V)-reductase, coded by arrA. The pattern of these genes was also investigated in a set of 10 soil samples from the same waste land, covering a broader range of total As content (100 to 10000 ppm). Overall, a link between the evolution of these functional genes (quantity, diversity, ratio) and content, speciation and behavior of inorganic As in soil will be discussed.
Towards defining bio-indicateurs of arsenic mobility in soils