Niche Differentiation of Arsenic-Transforming Microbial Groups in the Rice Rhizosphere Compartments as Impacted by Water Management and Soil-Arsenic Concentrations

Functional gene array and non-target soil microorganisms in nanopesticides captan@ZnO35-45nm and captan@SiO2 20-30nm environmental risk assessment

Currently there are no widely recognized standards for assessing the environmental risk of nanopesticides. Therefore, whether they are safer than conventional pesticide products remains open to discussion. We used non-target soil microorganisms as indicators of environmental change and applied functional gene arrays (FGAs) using GeoChip 5.0S targeting 151 000 genes involved in ecologically relevant functions. We synthesized nanofungicides in which the active substance captan was bound to inorganic nanocarriers (ZnO and SiO2) to examine the functional capabilities of microbial communities. During a 100-day microcosm study, changes in soil were compared to the effect of pesticide and nanocarriers alone. Based on 72 functional gene diversity profiles, we conducted environmental risk assessment of nanopesticides. Nanoagrochemicals affected the alpha and beta diversity of microbial functional genes, and the most profound negative effect was detected for captan, impacting carbon cycling and the organic remediation process from day 30. Additionally, the effect of nanopesticides changed during the experiment. On day 42, the effect was nanocarrier-dependent, and an increase of genes involved in denitrification (nirS, norB), archaeal conversion of N2 to NH3 and fungal N-assimilation was observed, especially for SiO2-treated set-ups. After 100 days, the negative effect was mainly related to the active substance released from the nanocarrier impacting the nitrate reductase gene (narG) and genes from the denitrification and nitrogen fixation subcategory. Analysis of microarrays did not indicate a recovery process for carbon cycling. Moreover, pesticide and nanoagrochemicals affected arsenic detoxification (aoxB, arsM, arsC), which may lead to an elevation of toxic As(III) availability. Our study indicated that FGAs are a sensitive method, revealing long-term changes previously undetected by other methods, including next-generation sequencing (NGS). This is the first study to confirm the usefulness of GeoChips for the evaluation of microbial redundancy as an important factor for fair environmental risk assessment of nanopesticides.

Influence of irrigation methods on arsenic speciation in rice grain

Although the bioaccumulation of arsenic (As) in rice grains is a global health issue, its speciation is not less worrying. Despite the ascertained effectiveness of the intermittent irrigation methods in minimizing the amount of total As in rice, knowledge of its influence on the As speciation has been insufficient so far. Hence, this contribution was aimed to measure the concentrations of As(III), As(V), dimethylarsinic acid (DMA), and monomethylarsonic acid (MMA) in grains from twenty-six different rice genotypes irrigated either with continuous flooding (CF), periodic saturation (SA) or sprinkler irrigation (SP). In CF-irrigated rice, As(III) and DMA prevailed in roughly equal amounts, only As(III) was found in SA-irrigated rice, whereas As(V) was largely predominant on As(III) in SP-irrigated rice. Organoarsenic species were below the limits of detection (LoD) in rice irrigated by intermittent methods. Principal component analysis (PCA) explained the influence of the irrigation method on the total amount of As, its chemical species, and their correlation. Furthermore, PCA showed also significant differences in As speciation as a function of the rice genotype, whereas no differences were found among Indica and Japonica subspecies.