CAREER: Development of a proxy for past and present plant photorespiration rates based on 13C-D and D-D clumping in wood methoxyl groups

Forests are a visually dominating part of the U.S. landscape, key to our construction industries, and an important area of where future emissions of CO2 are proposed to be sequestered. Thus, having a better understanding of how environmental factors affect the terrestrial biosphere has inherent societal benefit. A fundamental aspect of the study of plants is understanding what controls how well they grow — put another way, what sets the amount of carbon plants can take up from the atmosphere and convert into biomass?

All else being equal, plants are generally found today to grow better the more carbon dioxide (CO2) they have available. The dependence of carbon fixation rates on CO2 is termed the ‘CO2 fertilization effect’. It is based on the idea that increasing CO2 will increase global photosynthetic rates and is of great relevance for future climate.

The focus of this proposal is on the geological past where CO2 concentrations have changed over the past 65 million years by ~1000 ppm from glacial lows of 180 ppm to ~1000 ppm tens of millions of years ago. A question is whether, as seen in modern plants, ancient plants responded to changes in CO2 concentrations over geologic time? Did plants starve during the last glacial episodes due to low CO2 and thrive tens of millions of years ago in CO2 rich atmospheres?

Models predict that this is true, but there are no ‘proxies’ that can reconstruct how plant growth rates responded to changing CO2 levels in the geologic past. Here the investigator proposes to develop such a proxy to test these ideas using the stable isotopic composition of wood based on the abundance of rare stable isotopes of carbon and hydrogen bound together in wood tissues.

He will calibrate the measurement experimentally and on modern plants and then study samples from the geological past. As part of this work, he will additionally create new online teaching resources for 9-12th educators through the Global Climate Change educational resources of the University of California Museum of Paleontology and that will also be integrated into classes taught at UC Berkeley.

Models of earth history predict that changes in CO2 concentrations over the Phanerozoic have altered global terrestrial primary productivity by changing the relative rates of photorespiration vs. photosynthesis in plants. These changes are predicted to have had global-scale consequences, including changing global silicate weathering rates and causing ecosystems to have been in near starvation during glacial pCO2 lows.

There are no available proxies for past photorespiration rates that can be measured on fossil materials to test these hypotheses. This project proposes an integrated research and education plan focused on the development of a first-of-its-kind proxy for photorespiration rates based on the isotopic composition of lignin methoxyl groups with two rare isotopes (termed clumped isotopes; 13CH2D-O-lignin and 12CH2D-O-lignin) that can be measured on modern and fossil material.

The proxy will be developed and calibrated based on theory, experiments, and a survey of modern environmental samples. It will then be applied to test the long-standing hypothesis that terrestrial ecosystems were carbon starved in the last glacial period due to elevated photorespiration rates associated with low atmospheric CO2. Proposed methodologies that will be employed are: (1) Chemical extraction and purification of methoxyl groups from wood lignin and conversion to the analyte CH3Cl. (2) Use of a high-resolution isotope-ratio mass spectrometer to measure intensities of 12CH3+, 12CH2D+, 13CH3+, 13CH2D+, and 12CHD2+ from CH3Cl ion fragments. (3) Growth of plants in growth chambers under controlled conditions. (4) Collection of modern and fossil wood from archives. (5) Theoretical calculations of partition function ratios of isotopologues.

This work will provide the community with a new, calibrated proxy and allow for the immediate testing of a long-standing hypothesis on how changes in pCO2 on geological timescales has or has not affected terrestrial ecosystems. Through an integrated educational and research program, the research will be incorporated into educational materials for the general public and for teachers.

These materials will be hosted and disseminated by the Understanding Global Change web resource developed by the University of California Museum of Paleontology (UCMP). The educational websites hosted by the UCMP receive millions of visitors per month and thus reach a broad, diverse community that includes the general public, students, and educators.

Finally, the research will be integrated into the classes taught by the investigator, including development of a graduate seminar course and creation of new lessons and exercises. The teaching materials developed for these classes will in turn be posted onto the Understanding Global Change website in order to provide examples lesson and exercises for educators using this website to develop their own lesson plans.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.