During the ANTARES 3 cruise in the Indian sector of the Southern Ocean in October-November 1995, the surface waters of Kerguelen Islands plume, and the surface and deeper waters (30-60 m) along a transect on 62 degrees E from 48 degrees 36'8 to the ice edge (58 degrees 50'S), were sampled. The phytoplankton community was size-fractionated (2 mu m) and cell numbers, chlorophyll biomass and carbon assimilation, through Rubisco and beta-carboxylase activities, were characterized. The highest contribution of <2 mu m cells to total biomass and total Rubisco activity was reported in the waters of the Permanent Open Ocean Zone (POOZ) located between 52 degrees S and 55 degrees S along 62 degrees E. In this zone, the picophytoplankton contributed from 26 to 50% of the total chlorophyll (a + b + c) with an average of 0.09 +/- 0.02 mu g Chl l(-1) for <2 mu m cells. Picophytoplankton also contributed 36 to 64% of the total Rubisco activity, with an average of 0.80 +/- 0.30 mg C mg Chl a(-1) h(-1) for <2 mu m cells. The picophytoplankton cells had a higher P-carboxylase activity than larger cells >2 mu m. The mixotrophic capacity of these small cells is proposed. From sampling stations of the Kerguelen plume, a relationship was observed between the Rubisco activity per picophytoplankton cell and apparent cell size, which varied with the sampled water masses. Moreover, a depth-dependent photoperiodicity of Rubisco activity per cell for <2 mu m phytoplankton was observed during the day/night cycle in the POOZ. In the near ice zone, a physiological change in picophytoplankton cells favouring phosphoenolpyruvate carboxykinase (PEPCK) activity was reported. A species succession, or an adaptation to unfavourable environmental conditions such as low temperature and/or available irradiance levels, may have provoked this change. The high contribution of picophytoplankton to the total biomass, and its high CO2 fixation capacity via autotrophy and mixotrophy, emphasize the strong regeneration of organic materials in the euphotic layer in the Southern Ocean.