In Situ Photochemical Transformation of Hg Species and Associated Isotopic Fractionation in the Water Column of High-Altitude Lakes from the Bolivian Altiplano

Abstract

Photochemical reactions are major pathways for the removal of Hg species from aquatic ecosystems, lowering the concentration of monomethylmercury (MMHg) and its bioaccumulation in foodwebs. Here, we investigated the rates and environmental drivers of MMHg photodegradation and inorganic Hg (IHg) photoreduction in waters of two high-altitude lakes from the Bolivian Altiplano representing meso- to eutrophic conditions. We incubated three contrasting waters in situ at two depths after adding Hg-enriched isotopic species to derive rate constants. We found that transformations mostly occurred in subsurface waters exposed to UV radiation and were mainly modulated by the dissolved organic matter (DOM) level. In parallel, we incubated the same waters after the addition of low concentrations of natural MMHg and followed the stable isotope composition of the remaining Hg species by compound-specific isotope analysis allowing the determination of enrichment factors and mass-independent fractionation (MIF) slopes (Δ199Hg/Δ201Hg) during in situ MMHg photodegradation in natural waters. We found that MIF enrichment factors potentially range from −11 to −19‰ and average −14.3 ± 0.6‰ (1 SE). The MIF slope diverged depending on the DOM level, ranging from 1.24 ± 0.03 to 1.34 ± 0.02 for the low and high DOM waters, respectively, and matched the MMHg MIF slope recorded in fish from the same lake. Our in situ results thus reveal (i) a relatively similar extent of Hg isotopic fractionation during MMHg photodegradation among contrasted natural waters and compared to previous laboratory experiments and (ii) that the MMHg MIF recorded in fish is characteristic for the MMHg bonding environment. They will enable a better assessment of the extent and conditions conducive to MMHg photodegradation in aquatic ecosystems.

Mercury Sources and Fate in a Large Brackish Ecosystem (the Baltic Sea) Depicted by Stable Isotopes

“…27,35 These latter studies have overall evidenced a decrease of the photodegradation extents with increasing DOC concentrations, but the tipping point between enhancement and inhibition remains overall poorly defined. However, it has been suggested that 6−7 mg L −1 of DOC seem to be optimal conditions for MMHg photodegradation in natural waters, 23,34 and here we found it to be at its highest where the more terrestrial and highest concentrations of DOC are found in the Baltic Sea (i.e., averages of 16−18 μg L −1 for humic substances and 3.9 mg L −1 for DOC at Holmoarna and Harufjarden). It gives a new perspective to the negative relationship previously observed between MMHg and humic OM in the northern Baltic Sea, which was hypothesized to be caused by a reduced availability of Hg(II) for microbial MMHg formation.…”

“…Odd Hg-MIF is found in all fish samples, whatever the fish species or subbasins, with Δ 199 Hg ranging from 0.33 to 3.04‰ and Δ 201 Hg from 0.21 to 2.26‰ (Figure and Table SI-3). The Δ 199 Hg/Δ 201 Hg slope is 1.31 ± 0.02 (Figure c, linear fit, no weighting, p -value <0.005, n = 51) when considering all fish samples, which is typical for freshwater fish as it reflects the slope imprinted during MMHg photodegradation in freshwater with excess DOM compared to MMHg. ,,,,,, However, cod from Fladen present a lower Δ 199 Hg/Δ 201 Hg ratio of 1.08 ± 0.22 ( n = 5) compared to the other fish even if their MMHg content is similar (90.9–98.6%). This likely results from both their more pronounced benthic feeding that leads to the accumulation of both pelagic and benthic MMHg that is less or not photodegraded and their migration strategy.…”

“…The maximum Δ 199 Hg values found in the Baltic Sea are relatively low compared to values observed in pelagic fish from other fresh or marine waters (e.g., Lake Baikal, high altitudes Tibetan lakes, North American great Lakes, or the Pacific ocean) 79,93,76,28 that often reach 6−7‰, sometimes up to 9‰, indicating that the extent of MMHg photodegradation in the Baltic Sea is relatively limited. Based on a MIF enrichment factor of 14.3‰, 22,23 we calculated that the extent of MMHg photodegradation before incorporation into the food web averages 4−5% in the central Baltic Sea (Baltic Proper), 6% in the Kattegat, and up to 16% in the northern, less saline basins (Bothnian Sea and Bay). This limited MMHg photodegradation can be explained by a combination of factors, i.e., the amount of incident light, its penetration in the water 28 The light penetration and residence time of Hg (about one year) 45 are limited by relatively high concentrations of DOM and/or POM throughout the entire Baltic Sea compared to marine waters and recurrent "massive" algae blooms that occur especially in the central part of the Baltic Sea, starting with diatoms and dinoflagellates in spring which are then replaced by filamentous cyanobacteria over the summer.…”

Role of phytoplankton in aquatic mercury speciation and transformations

“…For example, low DOM concentration promoted, whereas very high DOM concentration inhibited MeHg photodemethylation by DOM (Klapstein and O'Driscoll 2018). However, using Hg compound-specific stable isotope analysis it was shown in situ that MeHg photodegradation in natural waters mostly occurred in waters exposed to UV radiation and was modulated by the DOM level (Bouchet et al 2022). The photodemethylation of MeHg to iHg was shown to increase in the presence of fulvic acids while this process was limited in the presence of humic acids (Luo et al 2020).…”

“…The development of the Hg stable isotope fractionation approach opens up the possibility to decipher the contribution of interconnected abiotic and biotic transformation (Kritee et al 2013) and to track further the processes controlling origin and cycling of Hg before its incorporation in the foodweb (Bouchet et al 2022). Since the biotic transformations are considered prevailing, the development of the novel -omics approaches would provide key information on the interactions between Hg and phytoplankton species (Beauvais-Flück et al 2016, Beauvais-Flück et al 2018, Mangal et al 2022.…”