INTERACTION OF NITROGEN FIXATION AND PHOSPHORUS LIMITATION IN APHANIZOMENON FLOS‐AQUAE (CYANOPHYCEAE)1

Abstract

ABSTRACTThe effect of simultaneous nitrogen fixation and phosphorus limitation on the physiological adaptation and growth performance of Aphanizomenon flos‐aquae (L.) Ralfs PCC 7905 was studied in continuous culture. In the absence of ammonia, N2 fixation occurred and the maximum growth rate (as determined in diluted batch cultures) was lower. However, no distinction could be made between the steady‐state N uptake rates (based on cellular N contents) of N2‐fixing cells and cells grown with ammonia. At the higher dilution rates, the residual P concentration increased with increasing dilution rate, more so under N2‐fixing conditions, compared to the cultures grown in the presence of ammonia. More generally, the yield of biomass per consumed P, as the biomass concentration itself, decreased with increasing dilution rate, and both were lower under N2‐fixing conditions. The restricted biomass production under N2‐fixing conditions suggests that reduction of N loading may benefit lake restoration projects. The influence of N2‐fixation on the severity of P limitation is discussed in terms of metabolic control analysis. From the increase of the residual P concentration on switching from ammonium to N2‐fixing conditions, it is deduced that under N2‐fixing and P‐limited conditions, control of growth is shared by N and P metabolism.