Abiotic Stress-Induced Chloroplast and Cytosolic Ca Dynamics in the Green Alga Chlamydomonas reinhardtii.

Calcium (Ca)-dependent signalling plays a well-characterised role in the perception and response mechanisms to environmental stimuli in plant cells. In the context of a constantly changing environment, it is fundamental to understand how crop yield and microalgal biomass productivity are affected by external factors. Ca signalling is known to be important in different physiological processes in microalgae but many of these signal transduction pathways still need to be characterised. Here, compartment-specific Ca dynamics were monitored in Chlamydomonas reinhardtii cells in response to environmental stressors, such as nutrient availability, osmotic stress, temperature fluctuations and carbon sensing. An in vivo single-cell imaging approach was adopted to directly visualise changes of Ca2+ concentrations at the level of specific subcellular compartments, using C. reinhardtii lines expressing a genetically encoded ratiometric Ca indicator. Hyper-osmotic shock caused cytosolic and chloroplast Ca elevations, whereas high temperature and inorganic carbon availability primarily induced Ca transients in the chloroplast. In contrast, hypo-osmotic stress only induced Ca elevations in the cytosol. The results herein reported show that in Chlamydomonas cells compartment-specific Ca transients are closely related to specific external environmental stimuli, providing useful guidance for studying signal transduction mechanisms exploited by microalgae to respond to specific natural conditions.