Barium and strontium isotope fractionation by cyanobacteria forming intracellular carbonates

Abstract

While barium (Ba) and strontium (Sr) stable isotopes are increasingly used as tracers of biogeochemical processes and paleo-proxies, the role of biotic processes on Ba and Sr isotope fractionation is poorly understood. Here, Ba and Sr stable isotope fractionations were studied in the laboratory using Gloeomargarita lithophora, a cyanobacterium that selectively hyperaccumulates Ba and Sr within intracellular amorphous carbonate biominerals. Our results show that lighter Ba and Sr isotopes are enriched in G. lithophora cells compared to the initial solution by –0.24‰ to –0.03‰ (δ137Ba) and –0.33‰ to –0.01‰ (δ88Sr) depending on the stage of the experiment. The fractionation of Ba and Sr isotopes is distinct in magnitude from that occurring during abiogenic and other known biogenic carbonate formation cases. Additionally, using a Rayleigh fractionation model, the fractionation factors of Ba and Sr isotopes between G. lithophora cells and the fluid (the growth medium), i.e., Δ137Ba(bac-sol) and Δ88Sr(bac-sol), were equal to –0.25‰ and between –0.46‰ and –0.38‰, respectively. Interestingly, δ137Basol and δ88Srsol decreased at the end of Ba and Sr uptake stages back towards their initial values, which caused departures from the Rayleigh fractionation model. This suggests the existence of a back reaction resulting in a Ba and Sr outflux from cells to the solution. Possible hypotheses for this back-reaction include dissolution of amorphous carbonate inclusions in response to cellular stress, or a first-order rate dependence of amorphous carbonate dissolution on the amount of Ba and Sr inside the cell. Our findings suggest that bacteria forming intracellular amorphous carbonates could introduce Ba and Sr isotope variability in environmental records, especially in environments where they thrive. Moreover, the enrichment of lighter isotopes of Ba and Sr during amorphous carbonate formation is consistent with that occurring during the formation of other biogenic carbonates but slightly differs in magnitude, opening a discussion about the possibility to use Δ137Ba(bac-sol) and Δ88Sr(bac-sol) as an indicator of intracellular amorphous carbonate biomineralization in the fossil rock record. Overall, this work highlights the complexity of the biological uptake of alkali-earth metals and stresses the overlooked role of bacteria forming intracellular amorphous carbonates in Ba and Sr biogeochemical cycles.