Diatoms exhibit dynamic chloroplast calcium signals in response to high light and oxidative stress

Diatoms are a group of silicified algae that play a major role in marine and freshwater ecosystems. Diatom chloroplasts were acquired by secondary endosymbiosis and exhibit important structural and functional differences from the primary plastids of land plants and green algae. Many functions of primary plastids, including photoacclimation and inorganic carbon acquisition, are regulated by calcium-dependent signalling processes. Calcium signalling has also been implicated in the photoprotective responses of diatoms, although the nature of calcium elevations in diatom chloroplasts and their wider role in cell signalling remains unknown. Using genetically encoded calcium indicators, we find that the diatom Phaeodactylum tricornutum exhibits dynamic chloroplast calcium elevations that are distinct from those found in land plants. Chloroplast calcium ([Ca2+]chl) acts independently from the cytosol and is not elevated by stimuli that induce large cytosolic calcium ([Ca2+]cyt) elevations. In contrast, high light and exogenous hydrogen peroxide (H2O2) induce large, sustained calcium elevations in the chloroplast stroma that are not replicated in the cytosol. Measurements using the fluorescent H2O2 sensor roGFP2-Orp1 indicate that [Ca2+]chl elevations induced by these stimuli correspond to the accumulation of H2O2 in the chloroplast. [Ca2+]chl elevations were also induced by the addition of methyl viologen, which acts to generate superoxide within the chloroplast, and by treatments that disrupt non-photochemical quenching (NPQ). The findings indicate that diatoms generate specific calcium elevations in the chloroplast in response to high light and oxidative stress that likely modulate the activity of calcium-sensitive components in photoprotection and other regulatory pathways.