Date of Award

Summer 2023

Document Type

Open Access Dissertation


Biological Sciences

First Advisor

Tammi Richardson


Cryptophytes are a group of unicellular eukaryotic algae that can be found in a wide range of underwater habitats. Part of their ecological success can be attributed to their diverse array of cryptophyte phycobiliproteins (Cr-PBPs), a pigment class that captures wavelengths of light that are poorly absorbed by chlorophylls. Cryptophytes gained photosynthesis via secondary endosymbiosis in which their ancestor engulfed a red algal endosymbiont. Following endosymbiosis, they deconstructed the red algal photosynthetic machinery to form the Cr-PBP. Since then, the Cr-PBPs have diversified into at least 9 spectrally distinct forms. I investigated the diversity of Cr-PBP light absorption across 76 cryptophyte strains and found that there were many overlooked differences within the commonly accepted Cr-PBP “types.” I also found that adding criteria beyond the commonly used wavelength of maximum absorption can help distinguish between similar Cr-PBPs.

While cryptophytes are generally found in low-light intensity environments that are spectrally limited, little work has been done to determine whether they are low-light adapted and unable to live in environments with high light intensities. I constructed photosynthesis versus irradiance curves for 3 cryptophyte species in white, green, and red light using a 14C radiolabel. I found that all three species had low (less than 100 μmol photons m-2 s-1 ) compensation irradiance, but there was only photoinhibition in white light.

Finally, I used experimental evolution to determine whether 2 cryptophyte species in the genus Hemiselmis (Hemiselmis rufescens and Hemiselmis tepida) could adapt their Cr-PBP absorption spectra when grown in white, blue, green, or red light for hundreds of generations. Not only did H. tepida evolve in green light to use a Cr-PBP that resembles that of H. rufescens—it also evolved plasticity that allowed it to quickly switch between its ancestral Cr-PBP and the evolved Cr-PBP depending on the light environment. This finding may explain the diversity in Cr-PBP found in the genus Hemiselmis.


© 2023, Kristiaän Merritt

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