(505f) Controllable Phycobilin Modification in Proteins: An Alternative Photoacclimation Response in Cryptophyte Algae

Cryptophyte algae are well known for their ability to survive under low light conditions through the use of their auxiliary light harvesting antennas, phycobiliproteins. Mainly acting to absorb light where chlorophyll cannot (500-650 nm), phycobiliproteins also play an instrumental role in helping cryptophyte algae respond to changes in light intensity through the process of photoacclimation. Until recently, photoacclimation in cryptophyte algae was only observed as a change in the cellular phycobiliprotein concentration; however, an additional photoacclimation response was recently discovered that causes shifts in the phycobiliprotein absorbance peaks following growth under light which is restricted in spectral quality, i.e., red, blue, or green light. Here, I show this newly identified photoacclimation response can be reproduced in two other species of cryptophyte algae, P. sulcata and H. pacifica, and elucidate the origin of the response on the protein level. The photoacclimation response was confirmed to originate on the protein level by isolating phycobiliproteins from algae and measuring their absorbance spectra using UV-vis, fluorescence, and transient absorption spectroscopy. Surprisingly, there was no change in amino acid sequence nor tertiary structure, as confirmed using X-ray crystallography to determine structures before and after photoacclimation. Because the spectroscopic changes persist while the protein structure remains unchanged, we conclude that cryptophyte algae are capable of modifying the individual chromophores of their phycobiliprotein β subunits as an additional photoacclimation response to changes in spectral quality of incident light. This talk motivates future work on rationally modifying chromophores in photosynthetic proteins to selectively tune their absorbance spectra and demonstrates the possibility of engineering light harvesting in organisms using directed photoacclimation.