ABSTRACT. The influence of temperature on NO3(-) and NH4(+) uptake, and the activity of the assimilatory enzyme NO3(-) reductase (NR) was compared to inorganic C uptake (photosynthesis) in natural assemblages of Antarctic sea-ice microalgae. NO3(-) and NH4(+) uptake reached a maximum between 0.5 -2.0 C and 2.0 -3.0 C, respectively, which was close to that for photosynthesis (2.5 -3.0 C). NR showed a distinctly higher temperature maximum (10.0 -12.0 C) and a lower Q-10 value than inorganic N and C transport. Our data imply that, owing to differential temperature characteristics between N transport and N assimilation at in situ temperature (-1.9 C), and the incorporation of extracellular NO3(-) into cellular macromolecules may be limited by transport of NO3(-) into the cell rather than the intracellular reduction of NO3(-) to NH4(+). Despite differences in temperature maxima between N transport and N assimilation, the overall low temperature maxima of inorganic N metabolism characterizes Antarctic sea-ice microalgae as psychrophilic. Our study is the first to examine the temperature dependence of inorganic N uptake and assimilation in sea-ice microbial communities.