The pigment composition of Phaeocystis antarctica was monitored under various conditions of light, temperature, salinity, and iron. 19'-Hexanoyloxyfucoxanthin (Hex-fuco) always constituted the major light-harvesting pigment, with remarkably stable ratios of Hex-fuco-to-chl a under the various environmental conditions. Increased pigment-to-chl a ratios at low irradiance confirmed the light-harvesting function of Fucoxanthin (Fuco), 19'-Hexanoyloxy-4-ketofucoxanthin (Hex-kfuco), 19'-butanoyloxyfucoxanthin (But-fuco), and chl c2 and c3. Increased pigment-to-chl a ratios at high irradiance, low iron concentrations, and to a lesser extent at high salinity confirmed the photoprotective function of diadinoxanthin, diatoxanthin, and b,b-carotene. Pigment ratios were not always according to expectations. The consistent increase in But-fuco/chl at high temperature, high salinity, and low iron suggests a role in photoprotection rather than in light harvesting. Low Hex-kfuco/chl ratios at high salinity were consistent with a role as light harvester, but the high ratios at high temperature were not, leaving the function of Hex-kfuco enigmatic. Dedicated experiments were performed to test whether or not the light-harvesting pigment Fuco could be converted into its structural relative Hexfuco, and vice versa, in response to exposure to light shifts. Rapid conversions could not be confirmed, but long-term conversions cannot be excluded. New pigment ratios are proposed for chemotaxonomic applications. The ratios will improve pigment-based diagnosis of algal species in waters dominated by P. antarctica.