No abstract

Fe(III) (oxyhydr)oxide minerals exhibit a high sorption affinity for arsenic (As) and the reductive dissolution of As-bearing Fe(III) (oxyhydr)oxides is considered to be the primary mechanism for As release into groundwater. To date, research has focused on the reactivity of abiogenic Fe(III) (oxyhydr)oxides, yet in nature biogenic Fe(III) (oxyhydr)oxides, precipitated by Fe(II)-oxidizing bacteria are also present. These biominerals contain cell-derived organic matter (CDOM), leading to different properties than their abiogenic counterparts. Here, we follow Fe mineralogy and As mobility during the reduction of As-loaded biogenic and abiogenic Fe(III) minerals by Shewanella oneidensis MR-1. We found that microbial reduction of As(III)-bearing biogenic Fe(III) (oxyhydr)oxides released more As than reduction of abiogenic Fe(III) (oxyhydr)oxides. In contrast, As was immobilized more effectively during reduction of As(V)-loaded biogenic than abiogenic Fe(III) (oxyhydr)oxides during secondary Fe mineral formation. During sterile incubation of minerals and after microbial Fe(III) reduction stopped, As(V) was mobilized from biogenic Fe(III) (oxyhydr)oxides probably by sorption competition with phosphate and CDOM. Our data show that the presence of CDOM significantly influences As mobility during reduction of Fe(III) minerals and we suggest that it is essential to consider both biogenic and abiogenic Fe(III) (oxyhydr)oxides to further understand the environmental fate of As.

Rice is one of the most important staple foods worldwide, but it often contains inorganic arsenic, which is toxic and gives rise to severe health problems. Rice plants take up arsenate As(V) via the phosphate transport pathways, though it is not known how As(V), as compared to phosphate, modifies the expression of phosphate transporters (PTs). Therefore, the impact of As(V) or phosphate (Pi) on the gene expression of PTs and several Pi signaling regulators was investigated. Rice plants were grown on medium containing different As(V) or Pi concentrations. Growth was evaluated and the expression of tested genes was quantified at different time points, using quantitative RT-PCR (qPCR). The As and P content in plants was determined using inductively coupled plasma mass spectrometry (ICP-MS). As(V) elicited diverse and opposite responses of different PTs in roots and shoots, while Pi triggered a more shallow and uniform transcriptional response in several tested genes. Only a restricted set of genes, including PT2, PT3, PT5 and PT13 and two SPX-MFS family members, was particularly responsive to As(V). Despite some common reactions, the responses of the analyzed genes were predominantly ion-specific. The possible reasons and consequences are discussed.

No abstract

The dissolution of arsenic-bearing iron(III) (oxyhydr)oxides during combined microbial iron(III) and arsenate(V) reduction is thought to be the main mechanism responsible for arsenic mobilization in reducing environments. Besides its mobilization during bioreduction, arsenic is often resequestered by newly forming secondary iron(II)-bearing mineral phases. In phosphate-bearing environments, iron(II) inputs generally lead to vivianite precipitation. In fact, in a previous study we observed that during bioreduction of arsenate(V)-bearing biogenic iron(III) (oxyhydr)oxides in phosphate-containing growth media, arsenate(V) was immobilized by the newly forming secondary iron(II) and iron(II)/iron(III)mineral phases, including vivianite. In the present study, changes in arsenic redox state and binding environment in these experiments were analyzed. We found that arsenate(V) partly replaced phosphate in vivianite, thus forming a vivianite-symplesite solid solution identified as Fe3(PO4)1.7(AsO4)0.3·8H2O. Our data suggests that in order to predict the fate of arsenic during the bioreduction of abiogenic and biogenic iron(III) (oxyhydr)oxides in arsenic-contaminated environments, the formation of symplesite-vivianite minerals needs to be considered. Indeed, such mineral phases could contribute to a delayed and slow release of arsenic in phosphate-bearing surface and groundwater environments.