All life on Earth depends upon the fixation of carbon dioxide to organic carbon by organisms that power this process with light energy or high-energy chemicals. There are four known pathways that catalyze carbon fixation. Based on phylogenetic distributions and functional attributes, it has been posited that the Calvin Benson Bassham cycle, found in plants, algae, and some bacteria, dominates in aerobic terrestrial and marine aerobic habitats due to its resilience to oxygen, while other 'oxygen sensitive' carbon fixation pathways are relegated to hypoxic or anoxic environs like vents and hot springs.
Contrary to this supposition, there are symbioses found in hypoxic diffuse flows around hydrothermal vents in which the bacterial partner uses the Calvin Benson Bassham cycle. Tubeworm-bacterial symbioses, in particular Riftia pachyptila ("Riftia"), are the dominant keystone species at hydrothermal vents in the Pacific Ocean. These symbiotic systems fix carbon at mass specific rates comparable to the fastest growing plants. Surprisingly, recent studies suggest that the bacterial symbionts use two carbon fixation pathways, the Calvin Benson Bassham cycle and the reductive tricarboxylic acid cycle. The use of these two pathways by a single organism (the bacterium) to fix carbon is unprecedented, and may be a strategy to cope with the high variability in environmental conditions encountered by hydrothermal vent organisms.
For the work proposed here, Riftia will be incubated in high-pressure aquaria under conditions that mimic the environmental variations found in situ. Biochemical assays on the bacteria and tubeworm host will be used to ascertain the relationship between environmental conditions, metabolic activity and differential use of the two pathways, to understand how they act in concert to sustain carbon fixation in a dynamic environment. These data will considerably further the understanding of the influence of environment on carbon fixation by bacteria as well as plants, and will also be helpful for determining why different organisms have the different carbon fixing pathways.
We are equally committed to the proposed scientific research and our proposed education and outreach. We plan to support three major programs: (1) graduate student development (2) undergraduate mentoring, and (3) the design of high-impact educational curricula using real research data and experiences. This study will enable the support and training of undergraduate and graduate students who will be intimately involved in designing and engineering the experiments, analyzing the data, and formally presenting and documenting the work. The proposed research contains significant field and laboratory components, which affords students and teachers the opportunities to participate in this project at a variety of levels.
