STORAGE OF PHOSPHORUS IN NITROGEN‐FIXING <i>ANABAENA FLOS‐AQUAE</i> (CYANOPHYCEAE)¹

Abstract

ABSTRACT P accumulation and metabolic pathway in N 2 ‐fixing Anabaena flos‐aquae (Lyngb.) Bréb were investigated in P‐sufficient (20 μMP) and P‐limited (2 μMP) turbidostats in combined N‐free medium. The cyanobacterium grew at its maximum rate (μ max , 1.13 d −1 ) at the high P concentration and at 65% of μ max under P limitation, with total cell P concentrations (Q P ) at steady states of 12.0 and 5.2 fmol·cell −1 , respectively. At steady state, polyphosphates (PP i ) accounted for only 3% of Q P (0.4 fmol·cell −1 ) in P‐rich cells. Its concentration in P‐limited cells was 5.8% (0.3 fmol·cell −1 ). On the other hand, sugar P was very high at 22% of Q P in P‐rich cells and was undetectable in P‐limited cells. Pulse chase experiments with 32 P showed that P‐rich cells initially incorporated the labeled P into the acid‐soluble PP i fraction within the first few minutes and to a lesser extent into nucleotide P. Radioactivity in the PP i then declined rapidly with concomitant increases in sugar P and nucleotide P fractions. In contrast, in P‐limited cells, no radiolabel was detected in acid‐soluble PP i , and 32 P was initially incorporated into nucleotide P, sugar P, and ortho P fractions. The latter two fractions then subsequently declined. Therefore, under N 2 ‐fixing conditions the cyanobacteria appeared to store P as sugar P and also utilize P through different pathways under P‐rich and ‐limited conditions. When nitrate was supplied as the N source under P‐sufficient conditions, PP i accounted for about 15% of steady‐state Q P , but no sugar P was detected. Therefore, the same organism stored P in different cell P fractions depending on its N sources.