The xanthophyll cycle is a photo-protection mechanism in phytoplankton which protects the photosynthestic machinery from damage by high light intensities. Sea ice algae in Antarctica live on the bottom of sea ice often at low irradiances. During sea ice break up, they become exposed to high levels of irradiance. This cycle was investigated in Antarctic sea ice to determine, a) how does the ... xanthophyll cycle respond to changes in irradiance, b) What is the relationship between the xanthophyll cycle and UV radiation, c) What is the relationship between xanthophyll cycle and the water column light profile? Sea ice algae was collected from Cape Royds and used for experiments in the laboratory. PAR sensors were placed under the ice to measure ambient PAR and UV radiation, algae samples were placed in beakers and covered with plastics of different UV transmissions and others were placed in a series of containers with different neutral density filters. All treatments were samples at various time intervals and the xanthophyll cycle analysed. Samples were also collected for laboratory experiments investigating a) the response of the xanthophyll pigments to changing irradiances, b) the relationship between the xanthophyll cycle and UV radiation and c) experiments with the vertical mixing incubator. The xanthophyll cycle in water column algae was simultaneously measured with measurements of kinetic coefficients for an adaptation to a change in irradiance, estimates of the vertical velocity of marine algael cells within the water column and water column characteristics were measured (CTD, PAR, spectroradiometer, multichannel radiometer and TUVR radiometer).