Speaker – Mike Kipp, Duke University, Division of Earth & Climate Sciences, Nicholas School of the Environment, Levine Science Research Center, Zoom 

Seminar Title – The evolution of plant-microbial nitrogen-fixing symbioses: An isotopic perspective

Bio – Mike is a paleo-bio-geo-chemist, i.e., he studies the co-evolution of life and its environment through Earth history. He has a particular interest in reconstructing oxygen levels and major nutrient (nitrogen & phosphorus) cycling in ancient environments, with the goal of elucidating feedbacks between the geosphere and biosphere that impact planetary habitability. His group primarily uses stable isotope geochemistry to interrogate the ancient rock record, with occasional help from simple biogeochemical models. He obtained a B.S. in Biology from the University of Notre Dame, received his Ph.D. in Earth & Space Sciences at the University of Washington, worked as a postdoctoral fellow at the California Institute of Technology, and as of 2024 is an Assistant Professor in the Nicholas School of the Environment at Duke University.

Abstract – Nitrogen (N) stable isotope ratios are widely used as tracers of N cycling through terrestrial ecosystems. However, until recently, such data had not been successfully applied to the study of fossil plants. We have developed methods for N isotope analysis of carbonaceous compression fossils, and have begun exploring the utility of these data for studying symbiotic associations between plants and N-fixing bacteria. In modern ecosystems, plants with N-fixing symbionts have diagnostic N isotope ratios that resemble the atmosphere, whereas other plants have variable isotopic compositions reflecting soil N sources. We used this distinction to study the evolutionary history of the N-fixing symbiosis between cycads and cyanobacteria. Cycads are gymnosperms that originated over 250 million years ago, with an abundant fossil record, namely in the Mesozoic. All extant cycads harbor N-fixing cyanobacteria, implying an ancestral origin of this trait; however, our fossil N isotope data suggest that this symbiosis emerged much more recently (< 50 Ma), and occurred independently across cycad families. This implies selective pressure in the mid-to-late Cenozoic related to nutrient acquisition. We are now beginning to study the potential drivers of such a trend in cycads, as well as other N-fixing plants such as legumes.