Final Report Summary - CALCYAN (A living carbonate factory: how do cyanobacteria make rocks? (Calcification in Cyanobacteria))
Cyanobacteria are among the most important bacteria involved in the interactions between the geosphere and the biosphere. Indeed, they had a pivotal role on several global geochemical cycles throughout Earth’s history, including C, N and O. In particular, they have been contributing significantly to the global carbon cycle by assimilating CO2 to organic carbon and by triggering CaCO3 precipitation through photosynthesis. Although the exact age is still debated, it is widely assumed that cyanobacteria appeared more than 2.3 billion years ago and that they contributed to the formation of many carbonate rocks. However, despite the geochemical importance of cyanobacteria-mediated CaCO3 biomineralization, the mechanistic details of this process and their variety in diverse cyanobacteria are yet poorly understood. Overall, this obscures our capacity to assess the role of life in the formation of ancient carbonate deposits and more widely the global cycle of carbon. ERC Calcyan stemmed from the discovery of one species of deep-branching cyanobacteria that forms intracellular Ca-Mg-Sr-Ba carbonate minerals. Deep-branching means that they diverged early in the evolution from other cyanobacterial lineages, and that their comparison with other lineages may offer insight into the characteristics of ancestral cyanobacteria populating the surface of the Earth hundreds of millions years or billions years ago. So far, calcification by cyanobacteria was considered as exclusively extracellular. However, the existence of intracellularly calcifying cyanobacteria questioned that paradigm.
In ERC Calcyan, we achieved major fundamental advances in the knowledge of this biomineralization process. We revealed the existence of a previously unknown broad diversity of cyanobacterial species forming intracellular Ca-carbonates. These cyanobacteria are present worldwide in very different environments, including seawater, lakes, soils or hydrothermal environments. We also discovered that one of these cyanobacterial species is the closest extant relative of plastids, the cellular compartment of algae and plants in which photosynthesis occurs and which possibly appeared more than a billion years ago. This questions whether the capability to form intracellular Ca-carbonates may have been transmitted upon evolution from bacteria to algae at that time. Another breakthrough achieved by ERC Calcyan is the discovery that the formation of intracellular Ca-carbonates by cyanobacteria involves specific mechanisms costing energy to the cells. This energy cost possibly result from the formation of an intracellular envelope enclosing the Ca-carbonate inclusions as well as an active uptake of Ca by the cells from their surrounding environment. Interestingly, this high accumulation of Ca goes in some species with an efficient sequestration of strontium and barium, two pollutants in some environments. This opens exciting perspectives for the development of new remediation strategies using such bacteria.
Overall, we still ignore the precise selective advantages provided by this capability to form intracellular Ca-carbonates to some cyanobacteria, possibly serving as a source of carbon and/or calcium to the cells. However, ERC Calcyan has clearly shown that this process is connected in some cases with the division of the cells and in any case involves specific genes, the function and the evolutionary history of which remain to be determined in the near future.
In ERC Calcyan, we achieved major fundamental advances in the knowledge of this biomineralization process. We revealed the existence of a previously unknown broad diversity of cyanobacterial species forming intracellular Ca-carbonates. These cyanobacteria are present worldwide in very different environments, including seawater, lakes, soils or hydrothermal environments. We also discovered that one of these cyanobacterial species is the closest extant relative of plastids, the cellular compartment of algae and plants in which photosynthesis occurs and which possibly appeared more than a billion years ago. This questions whether the capability to form intracellular Ca-carbonates may have been transmitted upon evolution from bacteria to algae at that time. Another breakthrough achieved by ERC Calcyan is the discovery that the formation of intracellular Ca-carbonates by cyanobacteria involves specific mechanisms costing energy to the cells. This energy cost possibly result from the formation of an intracellular envelope enclosing the Ca-carbonate inclusions as well as an active uptake of Ca by the cells from their surrounding environment. Interestingly, this high accumulation of Ca goes in some species with an efficient sequestration of strontium and barium, two pollutants in some environments. This opens exciting perspectives for the development of new remediation strategies using such bacteria.
Overall, we still ignore the precise selective advantages provided by this capability to form intracellular Ca-carbonates to some cyanobacteria, possibly serving as a source of carbon and/or calcium to the cells. However, ERC Calcyan has clearly shown that this process is connected in some cases with the division of the cells and in any case involves specific genes, the function and the evolutionary history of which remain to be determined in the near future.