which are deposited formed when colonies of cyanobacteria bind with calcium carbonate. a) stromatolites b) pruteen c) superbug d) symbiotic
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Abstract
Microbial carbonate mineralization is widespread in nature and among microorganisms, and of vast ecological and geological importance. However, our understanding of the mechanisms that trigger and control processes such as calcification, i.e., mineralization of CO2 to calcium carbonate (CaCO3), is limited and literature on cyanobacterial calcification is oftentimes bewildering and occasionally controversial. In cyanobacteria, calcification may be intimately associated with the carbon dioxide-(CO2) concentrating mechanism (CCM), a biochemical system that allows the cells to raise the concentration of CO2 at the site of the carboxylating enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) up to 1000-fold over that in the surrounding medium. A comprehensive understanding of biologically induced carbonate mineralization is important for our ability to assess its role in past, present, and future carbon cycling, interpret paleontological data, and for evaluating the process as a means for biological carbon capture and storage (CCS). In this review we summarize and discuss the metabolic, physiological and structural features of cyanobacteria that may be involved in the reactions leading to mineral formation and precipitation, present a conceptual model of cyanobacterial calcification, and, finally, suggest practical applications for cyanobacterial carbonate mineralization.
Keywords: calcification; calcium carbonate; carbon sequestration; cyanobacteria
1. Introduction
In addition to reducing CO2 to organic compounds via photosynthesis, many cyanobacteria mineralize CO2 to recalcitrant carbonates, such as CaCO3. Thus cyanobacteria present two different modes of CO2 capture, one via photosynthesis and the Calvin-Benson-Bassham (CBB) cycle where CO2 is captured and converted to organic compounds, and another via carbonate mineralization, e.g., calcification, where CO2 is converted to carbonate minerals (Figure 1). The ratio of organic to inorganic carbon production (ROI), differs between species of cyanobacteria and environmental conditions. Although the phenomenon of microbial calcification has long been recognized, its physiological function is unknown. Calcification is a prominent feature of many cyanobacterial species [1,2,3,4,5,6] and cyanobacterial calcification is of vast biogeochemical and ecological significance [2,4,5,7,8,9,10,11,12,13]; magnificent illustrations of cyanobacterial calcification are stromatolites [14,15,16] and whiting events, very fast, large-scale precipitations of fine-grained CaCO3 together with organic compounds that can turn entire water bodies such as Lake Michigan into a milky state [17,18,19,20]. A comprehensive understanding of carbonate mineralization is necessary for us to fully appreciate and employ this process in efforts to model and predict carbon cycling and budgets, to elucidate paleoenvironments, and apply it in biological CCS. In this review, we discuss carbonate mineralization in cyanobacteria, with focus on calcification and, in so doing, provide some details about methodology currently applied in the field, and consider the significance of the process in a wider perspective.
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