Light Climate and Status of the Photosynthetic Machinery in Macroalgae in the Polar Night

Macroalgae found in the Arctic during the polar night was shown to possess functioning photosynthetic machinery, and was able to endure and recover from long (10 minutes) - of high light irradiance (HL, 727 µmol photons m-2 s-1) exposure, measured as recovery of maximum quantum yield of Chl a fluorescence in PSII (ΦPSIImax). Measurements of ΦPSIImax ranged from 0.5 (Alaria esculenta) to 0.8 (Laminaria digitata), indicating that all species investigated contained a large fraction of functioning reaction centres. In vivo spectral reflectance spectra and pigment analysis (HPLC) showed that functional photosynthetic light harvesting pigments were present, even though in situ light levels were far below threshold irradiance for photosynthesis for 2-3 months. Time-series of ambient irradiance in air, measured over a week in January 2014, indicated a maximum irradiance of 1.7*10m-4 µmol photons m-2 s-1under clear sky conditions. Seasonality in the polar oceans is not like in temperate regions, with four distinct seasons in a year. Polar regions are defined by two distinct light seasons; polar night and polar day, characterized by �complete� darkness or 24 hours of light, with a more gradual change between seasons with decreasing latitude. While some deciduous plants break down and store nutrients from the photosynthetic machinery during winter, we propose that it is cost efficient for macroalgae living in the polar regions to maintain their photosynthetic machinery throughout the polar night, much like evergreen terrestrial plants. This ensures an instantaneous start in primary production when light levels become sufficient for photosynthesis.