Mercury bioavailability, transformations, and effects on freshwater biofilms

Abstract

Mercury (Hg) compounds represent an important risk to aquatic ecosystems because of their persistence, bioaccumulation, and biomagnification potential. In the present review, we critically examine state-of-the-art studies on the interactions of Hg compounds with freshwater biofilms, with an emphasis on Hg accumulation, transformations, and effects. Freshwater biofilms contain both primary producers (e.g., algae) and decomposers (e.g., bacteria and fungi), which contribute to both aquatic food webs and the microbial loop. Hence they play a central role in shallow water and streams, and also contribute to Hg trophic transfer through their consumption. Both inorganic and methylated mercury compounds accumulate in biofilms, which could transform them mainly by methylation, demethylation, and reduction. Accumulated Hg compounds could induce diverse metabolic and physiological perturbations in the microorganisms embedded in the biofilm matrix and affect their community composition. The bioavailability of Hg compounds, their transformations, and their effects depend on their concentrations and speciation, ambient water characteristics, biofilm matrix composition, and microorganism-specific characteristics. The basic processes governing the interactions of Hg compounds with biofilm constituents are understudied. The development of novel conceptual and methodological approaches allowing an understanding of the chemo-and biodynamic aspects is necessary to improve the knowledge on Hg cycling in shallow water as well as to enable improved use of freshwater biofilms as potential indicators of water quality and to support better informed risk assessment. Environ Toxicol Chem 2017;36:3194-3205. # 2017 SETAC

Long-Term Effects of Mercury on Biofilms Grown in Contaminated Microcosms: A Pilot Study

“…The natural {THg} background measured in the present study is in good agreement with those previously measured in biofilms grown in Geneva Lake, in which {THg} was found to vary between 86 and 214 pmol.gdw -1 depending on the biofilm age [22]. Measured {THg} in biofilms collected in various Hg-impacted rivers range in a wide range between 269 pmol to 1.7 µmol gdw −1 [9]. The extremely high concentration of 1.7 µmol gdw −1 of {THg} was measured in biofilms collected in the Idrijca River (Slovenia), close to a former Hg mine, where high concentrations of dissolved Hg (272 ng L −1 ) were measured [33].…”

“…Indeed, taxonomic composition and biofilm thickness were Comparing Hg accumulation between biofilms to infer conclusions on the quality of their ambient water should, however, be taken with caution. Indeed, Hg accumulation is not only dependent on the concentration of Hg in the ambient water but also of their surrounding environmental parameters (substrata, light, current, temperature), water chemical variables (pH and DOC concentration), and composition [9]. Indeed, taxonomic composition and biofilm thickness were recently shown to modulate Hg uptake kinetics in freshwater biofilms [22].…”

“…Furthermore, Hg is a global pollutant and priority hazardous substance due to its high toxicity and the biomagnification of its methylated forms (MeHg), a potent neurotoxin, along the food chains. The numerous interactions between Hg compounds and biofilms have been recently comprehensively reviewed, with a specific emphasis on Hg availability, transformations and effects towards freshwater biofilms [9]. Biofilm organisms are also known to be one of the pathway entries of inorganic Hg (IHg) and MeHg into aquatic food webs [10,11].…”

Biotic formation of methylmercury: A bio–physico–chemical conundrum

“…Comprehensive evaluations of the chemical and physical processes that govern Hg distribution and fate among the major environmental compartments can be found in the literature (Chételat et al ; Sundseth et al ; Kim et al ; Bjørklund et al ; Dranguet et al ; Paranjape and Hall ; Klapstein and Driscoll ). Briefly, in the water column Hg II can (1) be reduced to Hg 0 and reemitted back to the atmosphere, (2) methylated to the organic form MMHg, or (3) bind to organic matter (OM) as well as inorganic particles and directly deposit to bottom sediments.…”

Biofilms grown in aquatic microcosms affect mercury and selenium accumulation in Daphnia

“…This makes biofilms useful for monitoring metal contamination in aquatic ecosystems (Leguay et al 2016), but may also influence the exposure experienced by higher trophic levels (Stewart et al 2004;Cardoso et al 2013). For example, aquatic biofilms can readily accumulate mercury (Hg) (Dranguet et al 2017), as well as its natural antagonist selenium (Se) (Janz et al 2014). In media and living organisms, selenide ions (Se 2− ) can bind to mercuric ions (Hg 2+ ) to form mercuric selenide (HgSe), a stable and biologically inert complex, thereby reducing the risk of Hg toxic effects.…”