Description: Mixed cultures of 4 polar diatoms regularly found in Antarctic pack ice were grown over 20 d in closed bottles at high light (200 to 250 umol photons m-2 s-1) and at 0*C in order to investigate growth physiology and biomass production under conditions simulating the sea ice habitat during summer. Species tested were: Chaetoceros cf. neogracile, Fragilariopsiscylindrus, Thalassiosiraantarctica and Porosira pseudodenticulata. Initially, all species grew exponentially, but exponential growth ceased for P. pseudodenticulata and T.antarctica after 6 d, for F.cylindrus after 8 d, and for C. cf. neogracile after 10 d. Slight increases in cell number were observed for all species 2 d later. Peak biomass amounted to 140 ug chl a (850 umol particulate organic carbon, POC) l-1. At the same time, concentrations of dissolved inorganic carbon (DIC) were reduced by 1000 uM, oxygen concentrations increased to 1400 uM, and pH increased to 10.5. At this stage, a substantial decline in plasma-containing cells was recorded for F.cylindrus. C. cf. neogracile accounted for 80%, and C. cf. neogracile and F.cylindrus accounted for 〉95% of total carbon biomass. The carbon isotope composition of POC (expressed as delta13C) increased from -24 to -9 during the experiment. Model calculations showed that diffusive uptake of dissolved CO2 satisfied cellular carbon demand for all species except P. pseudodenticulata at CO2(aq) concentrations 〉0.5 uM, whereas direct HCO3- utilization was observed for C. cf. neogracile below this concentration. Our data confirm that intense photosynthetic carbon assimilation may lead to profound chemical changes in isolated interstitial brine solutions, with significant consequences for sea ice biota. We propose that the capacity to efficiently utilize ambient DIC, possibly mediated by virtue of favorable surface to volume ratios as well as active pathways of inorganic carbon acquisition, favors growth of small diatoms, and may be an important factor driving ice algal species succession during summer blooms. Since only 2 species continued to grow in fresh medium following experimental incubation (C. cf. neogracile and P. pseudodenticulata), differential tolerance to chemical variations may influence the seeding potential of ice algae following release into the open water.