STORAGE OF PHOSPHORUS IN NITROGEN-FIXING ANABAENA-FLOS-AQUAE (CYANOPHYCEAE)

P accumulation and metabolic pathway in N2-fixing Anabaena flos-aquae (Lyngb.) Breb were investigated in P-sufficient (20 muM P) and P-limit ed (2 muM P) turbidostats in combined N-free medium. The cyanobacteriu m grew at its maximum rate (mu(max) 1.13 d-1) at the high P concentrat ion and at 65% of mu(max) under P limitation, with total cell P concen trations (Q(P)) at steady states of 12.0 and 5.2 fmol.cell-1, respecti vely. 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-limi ted 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-li mited cells. Pulse chase experiments with P-32 showed that P-rich cell s initially incorporated the labeled P into the acid-soluble PP(i) fra ction within the first few minutes and to a lesser extent into nucleot ide P. Radioactivity in the PP(i) then declined rapidly with concomita nt increases in sugar P and nucleotide P fractions. In contrast, in P- limited cells, no radiolabel was detected in acid-soluble PP(i), and P -32 was initially incorporated into nucleotide P, sugar P, and ortho P fractions. The latter two fractions then subsequently declined. There fore, under N2-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 un der P-sufficient conditions, PP(i) accounted for about 15% of steady-s tate Q(P), but no sugar P was detected. Therefore, the same organism s tored P in different cell P fractions depending on its N sources.