Many cyanobacteria are known for their ability to sequester carbon dioxide through the precipitation of calcium carbonate but not many have been known to precipitate biominerals containing barium or strontium. What sets desmid green algae apart is its ability to sequester strontium (Sr) and barium (Ba). Sr and Ba are chemically very similar to calcium and when the elemental molecules are in the presence of one another, calcium often "outcompetes" the other two in forming a precipitant. However, researchers have found that when there is a large amount of dissolved barium in vacuoles where calcium and strontium are concentrated, the bacteria precipitates (Ba, Sr)SO4. They atrribute this finding to (Ba,Sr)SO4's low solubility compared to other options for biomineral precipitants. 90Sr is a harmful radioactive element that has been linked to cancer as it can substitute for calcium in bone. In addition, during environmental disasters, cleanup teams struggle with removing 90Sr in the presence of calcium because of competition between the two for uptake.Edit Summary
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"The desmid green alga Closterium moniliferum belongs to a small number of organisms that form barite (BaSO4) or celestite (SrSO4) biominerals. The ability to sequester Sr in the presence of an excess of Ca is of considerable interest for the remediation of 90Sr from the environment and nuclear waste. While most cells dynamically regulate the concentration of the second messenger Ca2+ in the cytosol and various organelles, transport proteins rarely discriminate strongly between Ca, Sr, and Ba. Herein, we investigate how these ions are trafficked in C. moniliferum and how precipitation of (Ba,Sr)SO4 crystals occurs in the terminal vacuoles. Towards this goal, we simultaneously visualize intracellular dynamics of multiple elements using X-ray fluorescence microscopy (XFM) of cryo-?xed/freeze-dried samples. We correlate the resulting elemental maps with ultrastructural information gleaned from freeze-fracture cryo-SEM of frozen-hydrated cells and use micro X-ray absorption near edge structure (micro-XANES) to determine sulfur speciation. We find that the kinetics of Sr uptake and efflux depend on external Ca concentrations, and Sr, Ba, and Ca show similar intracellular localization. A highly ion-selective cross-membrane transport step is not evident. Based on elevated levels of sulfate detected in the terminal vacuoles, we propose a 'sulfate trap' model, where the presence of dissolved barium leads to preferential precipitation of (Ba,Sr)SO4 due to its low solubility relative to SrSO4 and CaSO4. Engineering the sulfate concentration in the vacuole may thus be the most direct way to increase the Sr sequestered per cell, an important consideration in using desmids for phytoremediation of 90Sr" (Krejci et al. 2011:176).
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