Mercury (Hg2+) bioaccumulation in pico-, nano-, and microplanktonic species of the microbial food web of ultraoligotrophic Andean Patagonian lakes.

SOTO CÁRDENAS, EC; QUEIMALIÑOS, C; RIBEIRO GUEVARA, S; GEREA, M; DIÉGUEZ, MC

Cracovia

Conferencia; 14th International Conference on Mercury as a Global Pollutant; 2019

Dissolved inorganic mercury (Hg2+) is the most common chemical species in aquatic environments. In both marine and freshwater ecosystems, the incorporation of Hg2+ at the base of the pelagic food web can be an important pathway to trophic webs. Basal organisms comprising bacteria and phytoplankton may accumulate up to 105 times more Hg from the aqueous phase than the organisms at higher trophic levels. Then, to understand the incorporation of Hg2+ at the base of pelagic food webs is essential to delineate the pathway of Hg in aquatic ecosystems. Microbial assemblages are typical of deep ultraoligotrophic Andean Patagonian lakes and comprise pico-, nano-, and microplanktonic protists like picocyanobacteria, phytoflagellates and mixotrophic ciliates. These species have a central role in the carbon cycle, primary production, and also in the incorporation of Hg2+ into lake food webs. In this study we evaluated experimentally the incorporation of Hg2+ in different species representative of the picoplankton fraction (Synechococcus sp., 0.2‒2 μm), the nanoplankton (Cryptomonas erosa, 2‒20 μm), and the microplankton (the dinoflagellate Gymnodinium paradoxum and the mixotrophic ciliates, Stentor araucanus and Ophrydium naumanni, 20‒200 μm). The radioisotope 197Hg was used to trace the Hg2+ incorporation in microbiota. The specific Hg2+ uptake was assessed considering the morphology and size of the different species. Hg uptake was analyzed as a function of cell surface (SCF: surface concentration factor) and cell volume (VCF: volume concentration factor), as proxies of adsorption and internalization, respectively. These organisms incorporate substantial amounts of dissolved Hg2+, displaying different Hg internalization and therefore, varying potential for Hg transfer. The SCF and VCF ranged between 38 and 1660 pL μm-2, and from 2 to 324 pL μm-3, respectively. In both cases, the values were highest for the dinoflagellate, followed by O. naumanii in SCF and by Synechococcus sp. in VCF. The high bioconcentration of Gymnodinium may be due to the adsorption on its external mucus sheath. In addition to surface area and quality, surface:volume ratio may likely control Hg2+ uptake in microplanktonic organisms. Hg bioaccumulated by pelagic protists can be transferred to higher trophic levels, regenerated to the dissolved phase by excretion, and/or transferred to the bottom sediments by particle sinking. Overall, in ultraoligotrophic Andean Patagonian lakes, picoplankton and planktonic protists are key components of lake food webs, linking the pelagic and benthic Hg pathways, and thereby playing a central role in Hg trophodynamics.