Abstract: Thallus growth, photoynthetic oxygen evolution and rates of carbon fixation were determinedalong the lamina of the endemic Antarctic brown alga Ascoseira mirabilis (Ascoseirales), grown under simulated Antarctic condtions. The meristem is basally located and forms new blade tiddue under spring-conditions. Light saturated net photosynthesis (P_,ax), measures as O? production, was higher in ther intermediate region of the plant (9..8 µmol O? g?¹ fw h?¹). In general, photosynthetic parameters such as dark respiration, gross photosynthesis, photosynthetic efficiency (?) and photosynthetic light compensation (I_c) increased significantly towards the distal region. Carbon-fixation in A. mirabilis also showed thllus-dependent variation. Rates of light and light independent (dark) carbon fixation increased towards the distal regions ranging between 7.6-9.5 and 1.2-2.0 µmol C g?¹ fw h?¹ respectively. The percentage of light independent carbon fixation (in relation to light ¹?C-fixation) also increased from the basal to the distal parts reaching 24% in the distal region of the thallus. he contents of Chl a and Chl c, were close to 0.37 and 0.14 mg g?¹ fw respectively and were notably uniform along the lamina. The results indicate that the formation of the blade by a basal meristem and the increase of light carbon fixation rates from base to the distal regions in A. mirabilis are similar compared with certain Laminariales, especially members of the genus Laminaria. However, light independent carbon fixation is highest in the meristem of Laminaria, opposite to the results obtained here for A. mirabilis

Abstract: Changes in physico-chemical conditions, phytoplankton biomass, biochemical composition and primary productivity were investigated during autumnal sea-ice formation in the southeastern Weddell Sea, Antarctica. During sea-ice growth, brine salinities gradually increased with decreasing temperature. Nutrient concentrations in the brine of sea ice older than 2 weeks were lower than calculated from initial surface seawater values. The concomittant accumulation of phytoplankton biomass could not be explained solely by physical enrichment. We suggest that several microalgal species retained the capacity to assimilate nutrients and continued to grow in newly formed sea ice. However, nutrient depletions were moderate, and biochemical analyses did not indicate nutrient stress of algal metabolism. Relative abundance of smaller diatom species increased during ice growth, suggesting that pore space available for colonization in conjunction with physiological acclimation capacity were major factors determining successional patterns in recently formed sea ice. Even though ice algal assemblages apparently sustained the capacity to acclimate to reduced irradiances brought about by ice growth and increasing snow cover, maximum primary production was considerably lower than values usually reported from spring and summer ice communities. Therefore, autumnal primary production in newly formed sea ice may not add greatly to total annual production, but may provide an important food source for ice-associated grazers during the winter period, when phytoplankton biomass in the water column is extremely low.

Abstract: The physiological responses of a small unicellular Chaetoceros species, isolated from the Weddell Sea, Antarctica, to changes in temperature, salinity and irradiance simulating those that occur during new-ice formation were investigated. The combination of increased salinity, increased quantum irradiance and decreased temperature significantly reduced growth and photosynthetic rates compared to the control, although cellular metabolism was not inhibited. The cells retained the capacity to photoacclimate, which was observed in the variations in cellular chlorophyll a concentrations and carbon allocation patterns. In terms of photosynthesis, a doubling of quantum irradiance apparently compensated for the adverse effects of increased salinity and lowered temperature. It is thus hypothesized that at least some species of the late season phytoplankton population survive incorporation into ice and continue to photosynthesize and grow under the extreme conditions encountered during sea-ice formation.