Search

SUMMARYLithium (Li) has a wide range of uses in science, medicine and industry but its isotopy is underexplored, except in nuclear science and in geoscience. 6Li and 7Li isotopic ratio exhibits the second largest variation on Earth’s surface and constitutes a widely used tool for reconstructing past oceans and climates. As large variations have been measured in mammalian organs, plants or marine species, and as 6Li elicits stronger effects than natural Li (~95% 7Li) a central issue is the identification and quantification of biological influence of Li isotopes distribution. We show here that membrane ion channels and Na+-Li+/H+ exchangers (NHEs), strongly fractionate Li isotopes. This systematic 6Li enrichment is driven by membrane potential for channels, and by intracellular pH for NHEs, where it displays cooperativity, a hallmark of dimeric transport. Evidencing that transport proteins discriminate between isotopes differing by one neutron, opens new avenues for transport mechanisms, Li physiology, and paleoenvironments.

In the Western Tropical South Pacific, a hotspot of N2-fixing organisms has recently been identified. The survival of these species depends on the availability of dissolved iron (dFe). dFe was measured along a transect from 175 °E to 166 °W near 19-21 °S. The distribution of dFe showed high spatial variability: low concentrations (~0.2 nmol kg-1) in the South Pacific gyre and high concentrations (up to 50 nmol kg-1) west of the Tonga arc, indicating that this arc is a clear boundary between iron-poor and iron-rich waters. An optimal multiparameter analysis was used to distinguish the relative importance of physical transport relative to non-conservative processes on the observed dFe distribution. This analysis demonstrated that distant sources of iron play a minor role in its distribution along the transect. The high concentrations observed were therefore attributed to shallow hydrothermal sources massively present along the Tonga-Kermadec arc. Nevertheless, in contrast to what has been observed for deep hydrothermal plumes, our results highlighted the rapid decrease in dFe concentrations near shallow hydrothermal sources. This is likely due to a shorter residence time of surface water masses combined with several biogeochemical processes at play (e.g., precipitation, photoreduction, scavenging, biological uptake). This study clearly highlights the role of shallow hydrothermal sources on the dFe cycle within the Tonga-Kermadec arc where a strong link to biological activity in surface waters can be assessed. It also emphasizes the need to consider the impact of these shallow hydrothermal sources for a better understanding of the global iron cycle.

International audience; Lithium production has dramatically increased over the past decade, and the first cases of environmental Li pollution have been recently reported in urban and mining regions. While elevated Li concentrations may be toxic for living organisms, tools to monitor Li in the environment have not yet been developed. Consequently, its impact on key biota and human health is still poorly known. The present laboratory-based study shows that the soft tissues of blue mussels (Mytilus edulis) can be used to quantify Li contamination in coastal waters. Stable Li isotope ratios (7 Li/ 6 Li) measured in these soft tissues correlate positively with seawater Li concentrations and show precisely the threshold above which mussels shift their depuration mechanism. Combined with other data from the natural environment, the experimental results have profound implications for the fate of coastal ecosystems and shellfish consumption living under a high Li environmental level. We also highlight the need to develop innovative tools to extract Li from wastewaters before its release into rivers and, ultimately, the ocean.

International audience

This study reports Li concentrations and Li isotope compositions of soft tissues and bones from six international reference materials of biological origin, i.e., lobster hepatopancreas (TORT-2 and TORT-3), dogfish muscle (DORM-2 and DORM-4), mussel tissue (ERM-CE278k), and bone ash (SRM-1400). An in-house standard of plankton material (PLK-VLFR) has also been characterized. The seven materials display Li concentrations ranging from 0.05 μg g−1 DW (for DORM-2) to 10.5 μg g−1 DW (for PLK-VLFR). The Li purification technique used for Li isotope analyses is classical, with AG-50-X12 resin. Using the MC-ICP-MS Neptune Plus for Li isotope ratio measurements, the average δ7Li values, based on 3 to 9 replicates for each material, are 30.98 ± 0.47‰, 19.28 ± 4.90‰, 22.64 ± 2.81‰, 29.17 ± 0.24‰, 24.17 ± 0.32‰, 14.69 ± 0.35‰, and −1.72 ± 0.48‰ for PLK-VLFR, DORM-2, TORT-2, DORM-4, TORT-3, ERM-CE278k, and SRM-1400 respectively. The long-term analytical reproducibility (∼1 year) is estimated at ±0.18‰ (2SD) based on 20 replicate analyses of the pure reference solution Li7-N above 2 ng mL−1. It decreases at ±2.5‰ when the solutions are measured below 0.5 ng mL−1. For the reference materials of biological origin studied here, the Li isotope ratio uncertainties always remain below ±0.5‰ (2SD) when the Li amount loaded on the column is above 2 ng. However, it increases exponentially below this limit (up to ±2.4‰ for 1 ng Li). For the material with the lowest Li concentrations, DORM-2, a standard addition method was employed to precisely measure Li isotope ratios and to better assess the corresponding accuracy and reproducibility. This method leads to a much better precision (2SD = 1.1‰ vs. 4.9‰). Overall, this study characterizes the Li isotope composition of materials of biological origin and advises to consider a lower precision when discussing δ7Li values of low Li materials measured without standard addition.