Ph.D. The University of Michigan
The long term objective of my research program is to understand the physiology and ecology of nitrogen fixation in actinorhizal plants. These plants form a symbiosis (root nodules) with the nitrogen fixing bacterium (actinomycete) Frankia. Actinorhizal plants belong to eight different plant families and play important roles in the nitrogen budgets of the ecosystems in which they occur. Frankia differs from rhizobia in that it can fix nitrogen at atmospheric conditions when grown in pure culture and thus is able to provide its own oxygen regulation. In contrast, rhizobia depend on their legume hosts to regulate oxygen. I frequently collaborate with John Tjepkema, professor of plant physiology, and share a laboratory with him.
1. Hemoglobin in Frankia. We have recently discovered that Frankia can produce a hemoglobin. This hemoglobin has kinetic constants for oxygen binding that are consistent with the facilitation of oxygen diffusion over short distances such as those within Frankia cells. We are currently addressing the following questions:
1) Is more hemoglobin produced by Frankia in culture when oxygen is limiting?
2) Does the concentration of hemoglobin in alder nodules increase when the nodules are grown under hypoxic conditions such as those common in wetlands?
3) Is the oxygenation and reduction status of hemoglobin produced in cultures and in nodules consistent with a role in oxygen transport?
2. Establishment of actinorhizal plant seedlings. In order for new individuals to become established, seeds must germinate and the resulting germlings must successfully establish themselves. Conditions that allow seed germination are not always suitable for seedling establishment. We have found the conditions needed for seed germination in three common actinorhizal plants in Maine and are now beginning to examine the conditions needed for establishment of the germlings.
- Tjepkema, J.D., R.E. Cashon, J. Beckwith, and C.R. Schwintzer. 2002. Hemoglobin in Frankia, a nitrogen-fixing actinomycete. Applied and Environmental Microbiology 68: 2629-2631.
- Schwintzer, C.R. and J.D. Tjepkema. 2001. Effect of elevated carbon dioxide in the root atmosphere on nitrogenase activity in three actinorhizal plants. Canadian Journal of Botany 79: 1010-1018.
- Tjepkema, J.D., Schwintzer, C.R., Burris, R.H., Johnson, G.V., and Silvester, W.B. 2000. Natural abundance of 15N in actinorhizal plants and nodules. Plant and Soil 219: 285-289.
- Barkmann, J. and Schwintzer, C.R. 1998. Rapid N2 Fixation in Pines? - Results of a Maine Field Study. Ecology 79: 1453-1457.
- Schwintzer, C.R. and Tjepkema, J.D. 1997. Field nodules of Alnus incana ssp. rugosa and Myrica gale exhibit pronounced acetylene-induced declines in nitrogenase activity. Canadian Journal of Botany 75: 1415-1423.
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