SGER: Carbon Isotope Fractionation by Fungal Microorganisms

Johns Hopkins University, Baltimore MD

Investigators

Abstract

The carbon stable isotope (d13Corg) value of fossilized soil organic matter (SOM) is a crucial input during the reconstruction of key paleoenvironmental characteristics, such as pCO2, d13CO2, changes in plant community, and disruption of the carbon cycle. Fossil d13Corg values must be understood in terms of microbial decomposition, which transforms terrestrial organic materials both chemically and isotopically during the fossilization process. Although less celebrated than bacteria, it is the unicellular fungi that are responsible for the terrestrial cycling of most of the carbon produced by plants and animals. For these reasons, the direction and possible magnitude of carbon isotope fractionation imparted by unicellular fungi during the fossilization of SOM will form a critical new piece of information for the interpretation of fossil SOM d13Corg values, and by extension, for paleoenvironmental reconstructions. The investigator proposes to comprehensively characterize the carbon isotope fractionation resulting from the growth of fungal microorganisms on a carbon-rich medium. These results will quantify isotopic fractionation across three fundamental aspects of fungal microorganism biology: 1.) carbon acquisition pathway; 2.) community growth-phase; 3.) environmental temperature. This proposal will also 4.) investigate the biochemical mechanisms of observed carbon-isotope fractionation and 5.) establish the potential for fungal specific compounds (i.e., chitin and individual lipids, especially PLFA 18:2-6,9) to be used as isotopic indicators of fungal paleometabolism. Preliminary data based on the fungal microorganisms C. albicans and S. cerevisiae suggest that each of the above biological aspects exert an influence on carbon isotope fractionation during growth, and yet net isotopic enrichment of the organism relative to the carbon substrate consistently falls between 1 - 2 per cent. By performing an expanded set of growth experiments on a phylogenetically diverse group of organisms, this study will present a generalized quantification of carbon isotope fractionation during the growth of fungal microorganisms, tested across fundamental aspects of microbial biology. The cultures studies will be monitored and contextualized in terms of our observations of biochemical cell synthesis. The project will also evaluate the specific compounds extracted from fungal microorganism cell walls as potential d13C paleoindicators in their own respect. This proposal will establish the fundamental carbon isotope fractionations associated with fungal microorganisms and explore the extension of these ideas to the geological setting. The results of this project will pave the way for a larger integrative proposals intended to A.) examine fungal microorganism metabolism of glucose complexed with simple mineral materials (e.g., silica), and B.) examine the effects of strategic mutation in S. cerevisiae upon net glucose metabolic pathway (letter of support included). Intellectual Merit: the proposed work will constitute an exhaustive characterization of carbon stable isotope fractionation of fungal microorganisms. These values will introduce a key framework for the general interpretation of d13Corg in terrestrial organic matter for paleoenvironmental reconstruction. In particular, it will quantify the isotopic enrichment in soil organic matter that stems from the growth and addition of fungal microorganisms, which perform most of the carbon cycling in terrestrial ecosystems; it will also evaluate the potential for specific compounds to reflect this metabolism in the fossil record. Broader Impacts: The investigator will continue to share her work each semester with Paul Laurence Dunbar High School and ConneXions Community Leadership Academy, two inner-city disadvantaged Baltimore Public Schools.