This thesis was conducted to apply new techniques for measuring photosynthesis in Antarctic sea ice diatoms. A systematic approach of investigations was applied to obtain precise measurements of photosynthesis under natural conditions in the field from which questions were derived for further analysis in the laboratory. In situ measurements with the tracer 14C through the entire thickness of a young sea ice floe revealed that algae are able to actively assimilate dissolved inorganic carbon under extreme conditions (e.g. -7°C, < 10 µmol photons m-2) after inclusion into newly formed sea ice. These measurements were conducted with ice slices suspended in a new incubator. Unfortunately such bulk measurements did not provide encompass photosynthesis prevailing in the network of brine channels, pockets or bubbles. New sensors (oxygen micro-optodes) were therefore introduced into sea ice research which withstand freezing without damage, an important prerequisite to measuring oxygen dynamics directly within brine channels or brine pockets without disturbing the ice texture or brine chemistry. A new laboratory sea ice microcosm was developed to simulate natural conditions of sea ice and to cultivating a sea-ice diatom (Fragilariopsis cylindrus) directly within brine channels. Hence, changes of environmental conditions (e.g. melting, freezing, nutrient and light limitation) can now be simulated in the laboratory and effects on diatom photosynthesis can be investigated with the optodes without disturbing the habitat. However, how changes of environmental conditions influence photosynthesis and how these diatoms are generally adapted to their habitat still remained unresolved. Consequently, the first molecular data base of an obligate psychrophilic diatom (Fragilariopis cylindrus) was conducted using an EST (expressed sequence tag) approach under freezing conditions. This preliminary EST-library consists of 189 unique sequences. More than half (59%) of these sequences could not be identified by GenBank comparison, indicating the existence of many unknown genes. A subset of identified genes and additional genes responsible for photosynthesis, respiration and cold adaptation were cloned and arranged on a macro-array to investigate gene expression under freezing conditions. These molecular measurements were paralleled by biophysical and biochemical investigations. Experimental results revealed that acclimation of diatom photosynthesis under freezing temperatures of sea water and different resource limitations (light, nutrients) requires chaperons and repair mechanisms in order to sustain chloroplast membranes and proteins which are responsible for energy generation and carbon dioxide fixation.