[ESAC Seminar] Maddie Paoletti

“Physiological controls on carbon fixation and assimilation in green sulfur bacteria”

The reductive tricarboxylic acid (rTCA) cycle is a carbon fixation pathway thought to be central to early autotrophic life. Today, it is most associated with green sulfur bacteria (GSB), whose pigments appear in the geologic record as biomarkers dating back 1.64 billion years. Traditionally considered strict autotrophs, GSB can also assimilate simple organic molecules, such as acetate, in a mixotrophic metabolism. It has not been directly tested how this carbon metabolic flexibility influences their diagnostic carbon isotopic (δ¹³C) signature, and thus interpretations of the biomarker record. To address this, I cultured Chlorobium limicola under autotrophic and mixotrophic conditions. Preliminary results show that mixotrophic growth produces biomass and carotenoids significantly more depleted in carbon-13 than autotrophic growth, indicating acetate assimilation strongly affects isotopic signals. I expanded these experiments to investigate how their sulfur metabolism varies with light level and acetate availability. These results suggest that acetate supports higher cell densities, and its assimilation coincides with temporal shifts in sulfur oxidation during GSB blooms. Under low light, cells show a higher sulfide-to-acetate uptake ratio, indicating a greater reliance on mixotrophy to persist. Future work will pair isotopic measurements of cellular products with gene expression analyses to directly link these physiological observations to geochemical results. By integrating several lines of experimental data, this research will refine metabolic interpretations of GSB biomarkers and improve reconstructions of ancient euxinic environments.

ESAC Early-Career Seminar Series —

A forum for students and postdocs to share recent research, hone presentation skills, and build community among peers, sponsored by the EAPS Student Advisory Committee. Open to current EAPS graduate and undergraduate students and postdocs. Typically hosted on Thursdays during the semester, including pizza lunch.

Contact: esac.officers@gmail.com