Cryptophytes are a group of freshwater algae that have acquired photosynthesis through secondary endosymbiosis with a red alga and an unknown eukaryote heterotroph. Cryptophytes possess photosynthetic pigment-protein structures called phycobiliproteins (PBPs). Cryptophyte phycobiliproteins are composed of α and β-protein subunits and four chromophores (bilins). There are nine classes of cryptophyte PBPs that absorb different wavelengths of light based on the type of bilins covalently bound to the protein subunits. Three cryptophyte PBPs are phycoerythrins that give the algae a red appearance and six are phycocyanins that give the algae a blue to green appearance. Hemiselmis cryptochromatica (CCMP 1181) is a strain of cryptophyte algae that has demonstrated the ability to shift its PBP maximum absorption peak under different light conditions suggesting its PBP absorption is a plastic phenotype. This unique characteristic is what brought our attention to this strain. The research questions addressed here are, (1) Do changes in gene expression of photosynthetic genes stabilize after the acclimation period (four weeks) in green- or red-light environments? And (2) Do the β-subunit, CPES, PebA, and PebB genes play a role in H. cryptochromatica’s (CCMP 1181) ability to shift its PBP maximum peak wavelength absorption under different light environments? To address these questions, gene expression was examined using RNA-seq data and RT-qPCR. Cultures of H. cryptochromatica (CCMP 1181) were grown under green-light and red-light environments for –six to eight weeks to determine how gene expression of photosynthetic related genes is altered in these environments. For question one, we found that gene expression does not change after the four weeks acclimation period in the red-light environment up to six weeks, but in the green-light environment, we found that significant changes in gene expression occur at eight weeks for three of the four genes. For question two, we found that the β-subunit and PebB do not play a role in this phenotypic plasticity response. However, CPES and PebA may be partially responsible for the shift of its maximum wavelength absorption peak to 625nm under red-light. Both of these genes were upregulated in the red-light environment. PebA is the oxidoreductase responsible for producing the bilin DBV and CPES is an S/U type lyase that can only bind DBV and PEB bilins to the β-subunit. This suggests that an increase in DBV production and attachment to the β-subunit is likely needed for the shift in absorption under red-light. Furthermore, this leads us to think that another bilin is responsible for the dominant peak at 569nm under green-light that is currently unknown. These data indicate that the oxidoreductase that produces this unknown bilin needs to be determined, as well as the lyase that binds the bilin. Therefore, an in-depth RNA-seq experiment and characterization of the bilins of H. cryptochromatica (CCMP 1181) in different light environments will be necessary to fully understand phenotypic plasticity in cryptophytes.
Date of publication
Dr. Matthew Greenwold, Dr. Joe Glavy, Dr. Joshua Banta
Masters in Biology
Aguilar, Chastity D., "DIFFERENTIAL EXPRESSION OF PHOTOSYNTHETIC GENES IN CRYPTOPHYTE ALGAE (HEMISELMIS CRYPTOCHROMATICA CCMP 1181)" (2022). Biology Theses. Paper 72.