Consequences of light spectra for pigment composition and gene expression in the cryptophyteRhodomonas salina

Summary: Algae with a more diverse suite of pigments can, in principle, exploit a broader swath of the light spectrum through chromatic acclimation, the ability to maximize light capture via plasticity of pigment composition. We grew Rhodomonas salina in wide-spectrum, red, green, and blue environments and measured how pigment composition differed. We also measured expression of key light-capture and photosynthesis-related genes and performed a transcriptome- wide expression analysis. We observed the highest concentration of phycoerythrin in green light, consistent with chromatic acclimation. Other pigments showed trends inconsistent with chromatic acclimation, possibly due to feedback loops among pigments or high-energy light acclimation. Expression of some photosynthesis-related genes was sensitive to spectrum, although expression of most was not. The phycoerythrin α-subunit was expressed two-orders of magnitude greater than the β-subunit even though the peptides are needed in an equimolar ratio. Expression of genes related to chlorophyll-binding and phycoerythrin concentration were correlated, indicating a potential synthesis relationship. Pigment concentrations and expression of related genes were generally uncorrelated, implying post-transcriptional regulation of pigments. Overall, most differentially expressed genes were not related to photosynthesis; thus, examining associations between light spectrum and other organismal functions, including sexual reproduction and glycolysis, may be important. Originality-Significance Statement Most work on light and algal photophysiology focuses on light intensity rather than light spectrum. Given the large spectral variation of light in aquatic systems, explaining how such algae respond to spectral variation will provide a better foundation for understanding the base of aquatic food webs. Much of the light spectrum is poorly absorbed by chlorophyll, which creates an opportunity for photosynthetic species with other pigments. We quantified physiological and genetic responses to light spectrum in replicate experimental populations of Rhodomonas salina , an alga with a phycoerythrin in addition to chlorophylls. We predicted photophysiology and gene expression would change to maximize R. salina’s capacity to capture available light, in accordance with the theory of chromatic acclimation. Our results show that responses to light spectra are more complex than predicted. Some aspects of photophysiology did support the theory’s predictions, but gene expression was generally unrelated to variation of light spectrum or photophysiology. This not only suggests that chromatic acclimation is potentially regulated post-transcriptionally, but also that physiological processes – notably glycolysis and the transition to sexual reproduction – that may be regulated by light spectrum. Our work adds to the generally limited work on light spectrum and physiology by investigating a eukaryote from a phylum with a great diversity of photosynthetic pigments.