Calcite Precipitation by a Nitrogen-Fixing Cyanobacterium

Microbiologically induced calcium carbonate precipitation (MICP) is the process through which the metabolic activity of microorganisms causes the precipitation of calcium carbonate, which can result in solidification of sediment. In cyanobacteria, MICP is thought to occur primarily because cells sequester bicarbonate for the photosynthetic process, thereby lowering the pH of the surrounding media. However, these mechanisms are still poorly understood. Here we show direct evidence of MICP caused by the filamentous cyanobacterium Anabaena. Anabaena differentiates into photosynthetic vegetative cells and nitrogen-fixing heterocysts. Using quantitative microscopy, we show that MICP occurs due to two distinct mechanisms: Firstly, mechanical stress on vegetative cells can cause leakage and/or lysis, releasing sequestered bicarbonate into the environment, resulting in formation of new crystals. Secondly, contact between a heterocyst and a calcite crystal seed appears to cause rapid crystal growth. Our results suggest an evolutionary benefit of contact-mediated precipitation to anchor cyanobacteria growing in tidal regions. By providing greater insight into MICP caused by Anabaena, these results could be used to optimize bio-cement production, thereby enabling a green construction material that could assist with carbon sequestration and reducing the impact of climate change.