Why do alpha-cyanobacteria with form 1A RuBisCO dominate aquatic habitats worldwide? (CYANORUB)

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RuBisCO is one of the most abundant enzymes on Earth. Virtually all food webs depend on it to supply fixed carbon. In aerobicenvironments, RuBisCO struggles to distinguish efficiently between CO2 and O2. To compensate, many photosynthetic organismshave developed CO2-concentrating mechanisms (CCMs) to increase the [CO2] around the RuBisCO active site. In cyanobacteria,carboxysomes represent one such CCM, of which two independent forms exist: alpha and beta. This ancient photoautotrophiclineage has succeeded in colonizing habitats worldwide, being primary producers of great ecological importance. Amongst them,cells of the genera Prochlorococcus and Synechococcus, the two most abundant photosynthetic taxa on Earth, dominate oceanicecosystems. These marine picocyanobacteria possess a form IA RuBisCO and alpha-carboxysomes (so-called alpha-cyanobacteria).The remainder of the cyanobacterial radiation was thought to possess beta-carboxysomes and a form IB RuBisCO (beta-cyanobacteria), including freshwater unicellular and filamentous bloom-forming taxa comprising model organisms used inlaboratories worldwide e.g. Synechococcus elongatus and Synechocystis. However, recently I have isolated and sequenced thegenomes of many new unicellular freshwater picocyanobacteria that are phylogenetically much closer to their marine counterpartsand which also possess a form IA RuBisCO and alpha-carboxysomes. Moreover, these organisms have been detected in highabundance in freshwater lakes and reservoirs worldwide. Thus, alpha-cyanobacteria dominate all aquatic systems. CYANORUB seeksto address why this is the case. We hypothesize that alpha-cyanobacteria dominate large, temporally stable water masses,characterized by well-buffered pHs and relatively slow changes in carbonate chemistry. CYANORUB will be crucial for accuratelypredicting the biosphere's response to changing CO2, pH and carbonate chemistry and has biotechnological applications aimed atimproving plant growth.