Sea-ice algae contribute up to 40% of Antarctic primary production in the ice-covered regions of Antarctica and are an important source of resources for the associated Southern Ocean marine ecosystems. The sea-ice habitat presents them with extreme environmental stressors including temperature, osmotic stresses, UV radiation and light. These various environmental stressors directly influence the rate of many biochemical reactions that affect the photosynthetic and other biochemical abilities of sea-ice algae, while the large-scale cycles of sea-ice formation and decay are also an important influence on their physiology. These communities in turn show important adaptations that enable them and other dependent organisms to survive here. Using the sea-ice ecosystem present along the Victoria Land coast, Antarctica, we studied the influence of light, UV radiation and temperature on aspects of photosynthetic performance, chloroplast ultrastructure and biochemical stress responses of fresh-collected representative sea-ice communities in the field. We also examined the responses of several unialgal cultures of Antarctic and Arctic species of sea-ice algae to similar stress challenges in the laboratory. We found limited flexibility in the ability of the photosynthetic systems of these algae to respond to thermal variation (increase in temperature), supporting the general description of these species as either psychrotolerant or true psychrotrophs, while there was interspecific variation in the abilities of different species examined in culture. Likewise, stress responses (e.g. production of extracellular polysaccharides) were best maintained at low experimental temperatures. While the sea ice is normally well-buffered from exposure to higher temperatures, certain habitats intimately associated with it, such as tidepools, may commonly experience temperatures well above zero for considerable periods and share community members with the marine and sea-ice algal communities, thus the mechanisms by which these species respond to higher temperatures are of important ecophysiological interest.