EVALUATION OF COMPETITIVE ABILITY OF STAURASTRUM LUETKEMUELLERII (CHLOROPHYCEAE) AND MICROCYSTIS AERUGINOSA (CYANOPHYCEAE) UNDER P LIMITATION1

Abstract

ABSTRACTThe desmid Staurastrum luetkemuellerii Donat et Ruttner and the cyanobacterium Microcystis aeruginosa Kütz. showed pronounced differences in chemical composition and ability to maintain P fluxes. The cellular P:C ratio (Qp) and the surplus P:C ratio (Qsp) were higher in M. aeruginosa, indicating a lower yield of biomass C per unit of P. The subsistence quota (Qp) was 1.85 μg P·mg C−1in S. luetkemuellerii and 6.09 μg P·mg C−1in M. aeruginosa, whereas the respective Qp of P saturnted organisms (Qs) were 43 and 63 μg P·mg C−1. These stores could support four divisions in S. luetkemuellerii and three divisions in M. aeruginosa, which suggests that the former exhibited highest storage capacity (Qs/Q0). M. aeruginosa showed a tenfold higher activity of alkaline phosphatase than S. luetkemuellerii when P starved. The optimum N:P ratio (by weight) was 5 in S. luetkemuellerii and 7 in M. aeruginosa.The initial uptake of Pi pulses in the organisms was not inhibited by rapid (<1 h) internal feedback mechanisms and the short term uptake rote could be expressed solely as a function of ambient Pi. The maximum cellular C‐based uptake rate (Vm) in P starved M. aeruginosa was up to 50 times higher than that of S. luetkemuellerii. It decreased with increasing growth rate (P status) in the former species and remained fairly constant in the latter. The corresponding cellular P‐based value (Um= Vm/Qp) decreased with growth rate in both species and was about 10 times higher in P started M. aeruginosa than in S. luetkemuellerii. The average half saturation constant for uptake (Km) was equal for both species (22 μg P·L−1) and varied with the P status. S. luetkemuellerii exhibited shifts in the uptake rate of Pi that were characterized by increased affinity (Um/Km) at low Pi, concentrations (<4 μg P·L−1) compared to that at higher concentrations. The species thus was well adapted to uptake at low ambient Pi, but M. aeruginosa was superior in Pi uptake under steady state and transient conditions when the growth rate was lower than 0.75 d−1. Moreover, M. aeruginosa was favored by pulsed addition of Pi. M. aeruginosa relpased Pi at a higher rate than S. luetkemuellerii. Leakage of Pi from the cells caused C‐shaped μ vs. Pi curves. Therefore, no unique Ks for growth could be estimated. The maximum growth rate (μm) (23° C) was 0.94 d−1for S. luetkemuellerii and 0.81 d−1for M. aeruginosa.The steady state concentration of Pi (P*) was lower in M. aeruginosa than in S. luetkemuellerii at medium growth rates. The concentration of Pi at which the uptake and release of Pi was equal (Pc was, however, lower in S. luetkemuellerii.