03/23/2010
Expanded Role for Fixation of Carbon Dioxide and Nitrogen in Photosynthetic Bacteria
The Science
Primary productivity on Earth depends on the ability of plants and microbes to convert atmospheric CO2 and N2 into biologically useful forms. The enzymatic circuit known as the Calvin Cycle is responsible for the conversion of CO2 into cellular biomass of plants and photosynthetic microbes. Conversion of N2 to ammonia (i.e. nitrogen fixation) is mediated by nitrogenase, an enzyme possessed by only certain microbial species. A new report now shows that the Calvin Cycle and nitrogenase also play a critical role in maintaining the balance of oxidation and reductions processes during growth of Rhodopseudomanas palustris, a metabolically versatile bacterium that can grow in oxygen free environments using a combination of photosynthesis and consumption of organic acids produced by other fermentative microbes.
The Impact
This finding represents a significant advance in our understanding of central metabolic processes of a class of microbes that are both relevant to bioenergy applications and occupy a critical connective role in the global carbon cycle. The new publication is the inaugural article for Caroline S. Harwood of the University of Washington as a new member of the National Academy of Sciences.
Summary
The Calvin-Benson-Bassham cycle (Calvin cycle) catalyzes virtually all primary productivity on Earth and is the major sink for atmospheric CO2. A less appreciated function of CO2 fixation is as an electron-accepting process. It is known that anoxygenic phototrophic bacteria require the Calvin cycle to accept electrons when growing with light as their sole energy source and organic substrates as their sole carbon source. These results underscore that N2 fixation and CO2 fixation have electron-accepting roles separate from their better-known roles in ammonia production and biomass generation. Some nonphotosynthetic heterotrophic bacteria have Calvin cycle genes, and their potential to use CO2 fixation to recycle reduced cofactors deserves closer scrutiny.
Principal Investigator
Caroline S. Harwood
University of Washington
References
McKinlay, J. B. and C. S. Harwood. 2010. “Carbon Dioxide Fixation as a Central Redox Cofactor Recycling Mechanism in Bacteria,” Proceedings of the National Academy of Science (USA). DOI:10.1073/pnas.1006175107.