Modular device for simultaneous analysis of phototaxis and NPQ in photosynthetic organisms

While light is essential for the fixation of carbon dioxide by photosynthetic organisms, exposure to overly intense light can be detrimental. To mitigate the adverse effects of high light intensity, photosynthetic organisms have evolved two primary short-term protective strategies. The first one is the ability to optimize light exposure through movement (phototaxis) while the second one consists in dissipating the excess energy as heat within the light-harvesting antennae by processes referred to as non-photochemical quenching (NPQ). If these two strategies are already described independently in the literature for various organisms, how these mechanisms jointly contribute to optimize light exposure is not yet comprehensively studied. We present a device that enables such joint measurements by simultaneously conducting motility analysis (using computer vision) and chlorophyll fluorescence measurement. The device is designed with modularity at its core, equipped with LEDs that can be positioned perpendicularly to Petri dishes or multiwell plates to produce an adjustable light gradient in both intensity or spectrum. Fluorescence analysis leverages components from the SpeedZen fluorescence camera (JBeamBio), a device well-established for its reliability in fluorescence measurements, which we have adapted with a custom power supply and image analysis software for our specific requirements. Additionally, the inclusion of a motorized rotating filter system enables easy switching between timelapse imaging and fluorescence modes or to enhance image contrast for clearer observation. We are currently in the process of validating this setup with a variety of mutant strains of the unicellular alga Chlamydomonas reinhardtii defective in motility, light perception or NPQ, alongside the use of specific inhibitors that target photosynthesis and NPQ processes. The device will be open-source, allowing researchers to modify or replicate the system as needed to investigate photosynthetic responses and light management strategies across a range of organisms. In this respect, we aim to comprehensively investigate phototaxis and photoprotection mechanisms of the marine flatworm Symsagittifera roscoffensis and its photosynthetic symbiont Tetraselmis convolutae. We have already confirmed that the flatworm loses its phototactic ability at high light intensities, suggesting that light saturation of its photoreceptors may hinder its ability to locate the light source. Concurrently, we hypothesize that in this condition, the symbiotic algae develop NPQ, indicating a prominent mechanism to mitigate excessive light stress within the symbiosis. Moving forward, our research will focus on comparing the photosynthetic and phototactic behaviors of the worm with those of the free-living alga. This comparative analysis will provide valuable insights into the adaptation strategies of both organisms to varying light conditions and their symbiotic relationship.