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 Q10 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), 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.
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