Impact of light quality and space weather in Alexandrium catenella (Dinophyceae) cultures

The chain-forming dinoflagellate Alexandrium catenella was grown with LED or fluorescent light. With LED light, the pool of mycosporine-like amino acids (MAAs) with strong antioxidant properties, such as mycosporine-glycine and palythene, was reduced in comparison to fluorescent light. The conjugated MAAs M-320 and M-335/360 presented similar stability to light and oxidation than their respective more sensitive moiety: mycosporine-glycine and palythene, respectively. A. catenella was exposed to extracellular hydrogen peroxide under LED or fluorescent light. It triggered an increase in chain formation at a final concentration of 98 and 196 µM, typical of an inverted U-shaped hormetic response, and reduced cell survival above 294 µM. LED light, minimizing UV-stress photoprotection, was chosen to render cells more susceptible to space weather. Cultures were exposed to 490 µM H₂O₂ multiple times between November 2017 and February 2018, close to the minimum of solar cycle 24. Cell survival was dependent on temperature, geomagnetic activity, solar X-rays and neutron flux. Geomagnetic activity originated an inverted U-shaped survival curve and X-ray flux a J-shaped survival curve, this second type of hormetic response being more rarely found. The percentage of cells in chains increased moderately with temperature, and more significantly with solar X-ray flux. Chain formation reduced along culture growth, but these observations were dependent on the occurring X-ray flux.