Arsenic Accumulation in Benthic Periphytic Biofilms
Arsenic (As) is a highly toxic metalloid that occurs naturally in the Earth’s crust and can be mobilized as a result of anthropogenic activities. Arsenic can exist in two different oxidation states, As(III) and As(V). These different chemical forms have varying mobilities and bioavailabilities. Understanding the biogeochemical controls on As speciation is therefore necessary to predict how As will accumulate in aquatic food webs. Benthic periphytic biofilms form on streambeds and have been shown to accumulate high concentrations of As. Furthermore, periphytic biofilms are at the base of the food chain in aquatic environments and thus could transfer accumulated As to organisms at higher trophic levels. Lopez et al. sought to determine how inorganic As(III) and As(V) were taken up by periphytic biofilms, how these biofilms modified As speciation, and whether these biofilms were a source of As to higher trophic levels.
In this study, Lopez et al. examined the uptake of As(III) versus As(V) by periphytic biofilms. They also fed these biofilms to larval mayflies. Both oxidation states of As were taken up by the biofilms, leading to ~6,000 times higher As concentrations relative to the initial solutions. However, the concentrations present in the larval mayflies were diluted relative to the biofilms (for both oxidation states), showing that As was not biomagnified in this food chain and suggesting that As was sequestered in the biofilm in a form that was not bioavailable. The authors used X-ray fluorescence imaging performed on beamline 2-3 at the Stanford Synchrotron Radiation Lightsource (SSRL) to examine the speciation of As in periphytic biofilms under their different experimental conditions. This showed that (1) As was associated with iron regardless of its initial oxidation state and (2) As(III) was oxidized to As(V) when taken up by the periphyton biofilm. The authors t concluded that the periphyton biofilm sequestered As by oxidizing As(III) to As(V), possibly via redox active iron or manganese oxides present within the biofilm. The As(V) then adsorbed strongly to iron oxides.
In summary, Lopez et al. have demonstrated the ability of benthic periphytic biofilms to accumulate As(V) and As(III) efficiently, without transferring As to higher trophic levels. This likely was because As was sequestered in a strongly adsorbed As(V) complex on iron oxide surfaces.
Lopez, A., et al. 2018. “Periphyton and Abiotic Factors Influencing Arsenic Speciation in Aquatic Environments,” Environmental Toxicology 37(3), 903–13. [DOI:10.1002/etc.4025]