Despite research advances in studies of microbial diversity, life in extreme environments and prebiotic chemistry, the origin of life on Earth remains an enigma. Key unresolved questions include: When did life first appear? What was the nature of the first organisms? and Did early life have a heterotrophic or an autotrophic origin?

    Autotrophy is a requirement for life to exist anywhere, and organisms capable of autotrophic metabolism were an integral part of early ecosystems. They serve an important function by making inorganic carbon available to other organisms, a central component of the global carbon cycle. The balance between the fixation of inorganic carbon and its release through heterotrophic processes is primarily responsible for the carbon dioxide and oxygen concentrations in the atmosphere. The microbial communities on early Earth most likely existed under conditions that are today confined to what one may describe as extreme environments. Organisms that thrive under those conditions primarily utilize carbon fixation pathways other than the Calvin cycle. Yet very little is known about the organisms responsible for primary production in these microbially dominated ecosystems, making this project an important component to further our understanding about the functioning of these systems.

    This project has the goal to better understand the evolutionary history of one of the most ancient carbon fixation pathways, the reductive TCA cycle, and to lay the foundation for an assessment of its environmental significance. Many consider the reductive TCA cycle to be the most ancient autotrophic carbon fixation pathway. This cycle occurs in phylogenetically-diverse prokaryotes whose growth conditions fit well with our view of the primitive Earth. One of the two key enzymes of this pathway is ATP citrate lyase (ACL). Limited gene sequence data are available for this enzyme and we have only limited information about the diversity, distribution and importance of organisms using the reductive TCA cycle in nature. This information is required in order to understand the carbon cycle on our planet today and in the geological past, and the evolution of this ancient carbon fixation pathway. The objective of this project is to examine the occurrence of the ACL gene in organisms where this activity is known to be present, and whether it occurs in other prokaryotic autotrophs where information about CO₂ fixation pathways is lacking. Newly-acquired sequence data is used to explore phylogenetic patterns generated by ACL for insights into the evolution of this ancient pathway on Earth. The data obtained so far support the view of the rTCA as an ancient metabolic pathway from which other pathways could have evolved. Furthermore the data indicate that the rTCA might be more common for carbon fixation than previously thought and that organisms utilizing this pathway are likely to contribute significantly to biomass production at various extreme habitats, like submarine hydrothermal vents, oxic/anoxic interfaces, and the subsurface.