Sensing the rainbow : genetic and physiological responses to light quality in Ostreococcus, an ecologically important picoeukaryote phytoplankton

The photosynthetic activity of phytoplankton is vital, not only as primary production in the ocean, or for carbon sequestration, but also as a source of almost half the oxygen in our atmosphere. This project is an investigation of the genetic and physiological consequences of light responses in Ostreococcus, a genus of eukaryotic picophytoplankton. The light quality responses of two different ecotypes of Ostreococcus are compared: the coastal and lagoon associated OTH95, and the ocean associated RCC809. It is known that Ostreococcus ecotypes can adapt to high or low intensities of light, however their responses to light of different wavelengths are unknown. It was hypothesised that Ostreococcus ecotypes possess distinct light quality niches, to address this, differential gene expression was determined under monochromatic red, green, and blue light conditions, and functional annotation used to describe the likely physiological effects of the transcriptional responses. These transcriptional light quality responses were ecotype-specific and were tested by measuring physiological photosynthetic parameters using pulse-amplitude modulation fluorometry analysis (PhytoPAM) technology, and relative pigment contents using high performance liquid chromatography (HPLC). These responses represent likely adaptations to the spectral conditions in the ecotypes’ light quality environments. Finally, potential mechanisms controlling the light quality responses were identified by locating candidate regulatory sequence motifs in the promoters of responsive genes and comparing these to transcription factor DNA binding sites. This allowed prospective markers for light quality responses to be identified and enables future investigations of phytoplankton communities in an environmental context. This work provides evidence of distinct light quality responses in Ostreococcus ecotypes and demonstrates the importance of light quality for the adaptation of eukaryotic phytoplankton to ecological niches in the marine environment