This review is focused on the biogeochemistry of arsenic in freshwaters and, especially, on the key role that benthic microalgae and prokaryotic communities from biofilms play together in through speciation, distribution, and cycling. These microorganisms incorporate the dominant iAs (inorganic arsenic) form and may transform it to other arsenic forms through metabolic or detoxifying processes. These transformations have a big impact on the environmental behavior of arsenic because different chemical forms exhibit differences in mobility and toxicity. Moreover, exposure to toxicants may alter the physiology and structure of biofilms, leading to changes in ecosystem function and trophic relations. In this review we also explain how microorganisms (i.e., biofilms) can influence the effects of arsenic exposure on other key constituents of aquatic ecosystems such as fish. At the end, we present two real cases of fluvial systems with different origins of arsenic exposure (natural vs. anthropogenic) that have improved our comprehension of arsenic biogeochemistry and toxicity in freshwaters, the Pampean streams (Argentina) and the Anllóns River (Galicia, Spain). We finish with a briefly discussion of what we consider as future research needs on this topic. This work especially contributes to the general understanding of biofilms influencing arsenic biogeochemistry and highlights the strong impact of nutrient availability on arsenic toxicity for freshwater (micro) organisms.

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Chemical pollution is one of the main issues globally threatening the enormous biodiversity of freshwater ecosystems. The toxicity of substances depends on many factors such as the chemical itself, the species affected, environmental conditions, exposure duration, and concentration. We used the random forest technique to examine the factors that mediate toxicity in a set of widespread fishes and analyses of covariance to further assess the importance of differential sensitivity among fish species. Among 13 variables, the 5 most important predictors of toxicity with random forests were, by order of importance, the chemical substance itself (i.e., Chemical Abstracts Service number considered as a categorical factor), octanol-water partition coefficient (log P), pollutant prioritization, ecological structure-activity relationship (ECOSAR) classification, and fish species for 50% lethal concentrations (LC) and the chemical substance, fish species, log P, ECOSAR classification, and water temperature for no observed effect concentrations (NOECs). Fish species was a very important predictor for both endpoints and with the two contrasting statistical techniques used. Different fish species displayed very different relationships with log P, often with different slopes and with as much importance as the partition coefficient. Therefore, caution should be exercised when extrapolating toxicological results or relationships among species. In addition, further research is needed to determine species-specific sensitivities and unravel the mechanisms behind them.

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