Investigating Potential Sugar-Related Uncertainties in a Proxy for Ancient CO2

Author

Liam Cooper, CUNY Bernard M Baruch CollegeFollow

Date of Award

Spring 5-20-2025

Document Type

Thesis

Degree Name

B.A.

Honors Designation

yes

Program of Study

Biology

Language

English

First Advisor

Ana Gonzalez-Nayeck

Second Advisor

Chester Zarnoch

Third Advisor

David Gruber

Abstract

The ε_p proxy for ancient pCO₂ can potentially estimate ancient pCO₂ as from the evolution of rubisco-based photosynthesis, using the difference in stable carbon isotope ratios (δ¹³C) between fixed and inorganic carbon, assuming this difference is proportional to pCO₂. This study aims to address uncertainty with the use of photosynthetic microbes in proxies for ancient pCO₂. While previous studies address uncertainties related to the δ¹³C of initial fixed photosynthate (e.g., growth rate), we aim to address the effects of downstream carbon allocation. Specifically, if a significant fraction of fixed carbon is allocated towards excreted sugars, this could potentially impact ε_p. Literature has previously assumed that cyanobacterial extracellular sugar (glucose) has the same δ¹³C as intracellular sugar. If incorrect, sugar excretion must be accounted for in future use of the proxy. Here, we seek to answer: Is there a carbon isotope fractionation associated with the sugar excreted by cyanobacteria, and does this in turn impact the ɛ_p proxy for ancient pCO₂? We address this question by growing Synechococcus PCC 7002 under two light conditions to promote variable sugar excretion, and we report δ¹³C of DIC, biomass, and relative amounts of sugar excretion to determine the effects of sugar excretion on the calculated ε_p. Overall, our findings indicate several key implications for the proxy. While we indicate glucose excretion does not significantly impact the proxy, we show that atmospheric CO₂ alone is not sufficient in accurately predicting ε_p. Considering our current pCO₂ levels (~420 μatm) have significantly surpassed the maximum levels (~300 μatm) recorded in ice cores, which only record up to ~800,000 years ago, proxies that look further back in the paleoclimate record are of the utmost importance to better understand our current climate crisis. Our findings give assurance that researchers who have relied on the proxy do not have to adjust to account for sugar excretion. However, more research should be done to promote varying EPS excretion in cyanobacteria to confirm these results.

Recommended Citation

Cooper, Liam, "Investigating Potential Sugar-Related Uncertainties in a Proxy for Ancient CO2" (2025). CUNY Academic Works.
https://academicworks.cuny.edu/bb_etds/205