Cyanobacteria Accumulate Radium (226Ra) within Intracellular Amorphous Calcium Carbonate Inclusions

Recently, the cyanobacterium Gleomargarita lithophora was shown to be associated with the capability to strongly accumulate 226Ra (radioactive pollutant) and was suggested as a novel bioremediation strategy for the removal of 226Ra. The bioaccumulation of 226Ra was suggested to be linked with the ability of G. lithophora to form intracellular amorphous calcium carbonate mineral inclusions (iACC). However, this claim relied on indirect evidence, and the fate of (intracellularly) sequestered 226Ra remains unresolved because of the limited spatial resolution of conventional analytical instrumentation. Here, using high-resolution nanoscale secondary-ion mass spectrometry (NanoSIMS), we show that sequestered 226Ra is primarily associated with iACC and to a lesser degree within polyphosphate inclusions, which are also present in G. lithophora. Moreover, we show that G. lithophora accumulates 226Ra efficiently in the presence of competing cations such as barium and strontium (frequently present in Ra-bearing effluents). Our results offer fundamental insights into the interactions between microorganisms and 226Ra, benefit the future development of efficient 226Ra bioremediation strategies, and present a new frontier in the mapping of ultratrace elements in microbial samples using NanoSIMS.