Cyanobacterium Creates Uncommon Carbonate

A newly identified cyanobacterium species produces within its cell an unusual amorphous carbonate mineral containing magnesium, calcium, strontium, and barium (Science, DOI: 10.1126/science.1216171). Cyanobacteria commonly produce extracellular carbonates, driven by export of photosynthesis by-product CO₃^2–, but intracellular mineral formation is unusual in bacteria and has not been observed previously in cyanobacteria. A team led by Karim Benzerara and Purificación López-García of France’s National Center for Scientific Research (CNRS) discovered the new species, christened Candidatus Gloeomargarita lithophora, on rock samples taken from an alkaline lake in Mexico. The single-celled organisms are shaped like rods and are about 3.9 µm long and 1.1 µm wide. They contain granules about 0.27 µm wide of an amorphous mineral with the formula (SrBa_2.7Mg_1.4Ca_0.9)Ca₆Mg(CO₃)₁₃. The researchers propose that the granules may provide an alternative sink for CO₃^2– and note that the cells may have to actively import Sr²⁺ and Ba²⁺ to make the mineral. The granules occupy about 6% of each cell’s volume and would increase cell density by 12%, perhaps adding weight to help the organisms stay put, so they can live on rocks or sediments, the authors say.