Light Sensing in Open Ocean Eukaryotic Plankton

Most eukaryotic organisms coordinate key biological events to coincide with the day/night cycle using a molecular circadian clock that generates an internal estimate of time. The diel oscillations in biological processes are entrained through the activity of light-sensitive photoreceptors. In the oceans, the plankton community must contend with dramatic changes in both the quantity and quality of light over depth. Photoreceptors allow organisms to respond rapidly to changes in light exposure (from phototaxis to optimizing light harvesting), while avoiding excessive UV radiation that causes DNA damage and oxidative stress. We analyzed publicly available marine eukaryotic genomic and transcriptomic data. We found that eukaryotes across the phylogenetic tree of life utilize photoreceptors of at least one of the investigated types, e.g. the cryptochrome/photolyases, phytochromes, rhodopsins and the LOV-domain (Light, Oxygen, Voltage) proteins, phototropins, aureochromes and helmchromes. We generated a high resolution open-ocean eukaryotic metatranscriptomic dataset from near Station ALOHA and found that the expression of the majority of photoreceptors peak at, or just before, dawn. We also found that species belonging to the Haptophyta and Stramenopiles express, in a controlled diel manner, novel light-responsive elements, combining the sensory LOV domain with different types of transcription factor domains, highlighting new regulatory avenues in these understudied organisms.