Species-specific isotope tracking of mercury uptake and transformations by pico-nanoplankton in an eutrophic lake

The present study aims to explore the bioaccumulation and biotic transformations of inorganic (iHg) and monomethyl mercury (MMHg) by natural pico-nanoplankton community from eutrophic lake Soppen, Switzerland. Pico-nanoplankton encompass mainly bacterioplankton, mycoplankton and phytoplankton groups with size between 0.2 and 20 μm. Species-specific enriched isotope mixture of ¹⁹⁹iHg and ²⁰¹MMHg was used to explore the accumulation, the subcellular distribution and transformations occurring in natural pico-nanoplankton sampled at 2 different depths (6.6 m and 8.3 m). Cyanobacteria, diatoms, cryptophyta, green algae and heterotrophic microorganisms were identified as the major groups of pico-nanoplankton with diatoms prevailing at deeper samples. Results showed that pico-nanoplankton accumulated both iHg and MMHg preferentially in the cell membrane/organelles, despite observed losses. The ratios between the iHg and MMHg concentrations measured in the membrane/organelles and cytosol were comparable for iHg and MMHg. Pico-nanoplankton demethylate added ²⁰¹MMHg (~4 and 12% per day depending on cellular compartment), although the involved pathways are to further explore. Comparison of the concentrations of ²⁰¹iHg formed from ²⁰¹MMHg demethylation in whole system, medium and whole cells showed that 82% of the demethylation was biologically mediated by pico-nanoplankton. No significant methylation of iHg by pico-nanoplankton was observed. The accumulation of iHg and MMHg and the percentage of demethylated MMHg correlated positively with the relative abundance of diatoms and heterotrophic microorganisms in the pico-nanoplankton, the concentrations of TN, Mg²⁺, NO₃^-, NO₂^-, NH₄⁺ and negatively with the concentrations of DOC, K⁺, Na⁺, Ca²⁺, SO₄^2-. Taken together the results of the present field study confirm the role of pico-nanoplankton in Hg bioaccumulation and demethylation, however further research is needed to better understand the underlying mechanisms and interconnection between heterotrophic and autotrophic microorganisms.

In vivo study of the effect of lactoferrin on iron metabolism and bioavailability from different iron chemical species for formula milk fortification

Iron fortification in infant formulas is a common practice for providing iron to newborns in order to avoid its deficiency (anemia). Depending on the physicochemical species used, its bioavailability might be insufficient to meet iron requirements. In this vein, the influence of Lactoferrin (Lf) presence on iron bioavailability in 2-week-old wistar rats fed with formula milk fortified with ⁵⁷ Fe(III)₂ -Lf or ⁵⁷ Fe(II)SO₄ (in presence of Lf) using quantitative speciation (by HPLC-ICP-MS) and Isotope Pattern Deconvolution (IPD) is studied here. Results obtained were compared among fortifiers and also with the maternal group. In RBCs, iron was mainly bound to hemoglobin in all the assayed groups in the same extent. Regarding serum samples, several iron-proteins were observed (such as transferrin and albumin). In both samples, iron content in the fractions studied was similar in all groups compared and exogenous ⁵⁷ Fe incorporation of intaked iron was always above 50%, showing no significative differences between physicochemical forms but related to the dose administered. Regarding iron stores (liver) the group fed with formula milk fortified with the higher dose of ⁵⁷ FeSO₄ in presence of Lf presented the highest values of total iron even superior than those found in the maternal group, and also the highest exogenous (⁵⁷ Fe) incorporation. In conclusion, it was proved that iron fortification is required to ensure proper iron levels in all body compartments. No significative differences were observed between different physicochemical species when iron is administered at low doses. However, higher iron doses lead to a greater incorporation in all the iron-proteins studied.