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

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Abstract
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 .cntdot. 10-9 cells), while NH3-grown cells were largest (900 .+-. 44 micromoles C .cntdot. 10-9 cells) and N2-fixing cells were intermediate (726 .+-. 48 micromoles C .cntdot. 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.