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Articles

Effect of N Source on the Steady State Growth and N Assimilation of P-limited Anabaena flos-aquae¹

Biology Department, Queen's University, Kingston, Ontario, Canada K7L 3N6

Phosphate-limited chemostat cultures were used to study cell growth and N assimilation in Anabaena flos-aquae under various N sources to determine the relative energetic costs associated with the assimilation of NH₃, NO₃^–, or N₂. Expressed as a function of relative growth rate, steady state cellular P contents and PO₄ assimilation rates did not vary with N-source. However, N-source did alter the maximal PO₄-limited growth rate achieved by the cultures: the NO₃^– and N₂ cultures attained only 97 and 80%, respectively, of the maximal growth rate of the NH₃ grown cells. Cellular biomass and C contents did not vary with growth rate, but changed with N source. The NO₃^–-grown cells were the smallest (627 ± 34 micromoles C · 10^–9 cells), while NH₃-grown cells were largest (900 ± 44 micromoles C · 10^–9 cells) and N₂-fixing cells were intermediate (726 ± 48 micromoles C · 10^–9 cells) in size. In the NO₃^–-and N₂-grown cultures, N content per cell was only 57 and 63%, respectively, of that in the NH₃-grown cells. Heterocysts were absent in NH₃-grown cultures but were present in both the N₂ and NO₃^– cultures. In the NO₃^–-grown cultures C₂H₂ reduction was detected only at high growth rates, where it was estimated to account for a maximum of 6% of the N assimilated. In the N₂-fixing cultures the acetylene:N₂ ratio varied from 3.4:1 at lower growth rates to 3.0:1 at growth rates approaching maximal.

Compared with NH₃, the assimilation of NO₃^– and N₂ resulted either in a decrease in cellular C (NO₃^– and N₂ cultures) or in a lower maximal growth rate (N₂ culture only). The observed changes in cell C content were used to calculate the net cost (in electron pair equivalents) associated with growth on NO₃^– or N₂ compared with NH₃.