The desmid
Staurastrum luetkemuellerii Donat et Ruttner and the cyanobacterium
Microcystis aeruginosa Kütz. were grown in mixed cultures with various phosphate (P
i) additions. One pulse of P
i each day (semi-continuous cultures)
favored M. aeruginosa whereas
S. luetkemuellerii was favored when the same quantity of P
i was supplied continuously (chemostats). Both species coexisted under P limitation provided that the nutrient was supplied in an appropriate mode. The ability of each species to compete for P depended on their P
i uptake characteristics and their capability to retain the accumulated P
i. High affinity in uptake at low P
i concentrations contributed considerably to the growth eficiency of
S. luetkemuellerii under continuous supply of P
iM. aeruginosa was, however, consistently superior to
S. luetkemuellerii in accuniulatiug the newly added P, but had a high rate of P
i release. In both -types of cultures, a net high of P went from
M. aeruginosa to S. luetkemuellerii. The kinetic characteristics of the two species were used to simulate the outcome of competition experiments. Simulations agreed with the experimental data f both uptake and P
i release were considered in the model. The zlariable P*(the concentration of P
i at which the net uptake is equal to μ·Q
P is a function of uptake and release of P
i but could not explain the chemostat results.
S. luetkemuellerii was the winner in many experiments even if its P*was higher thou that of M. aeruginosa. Thus, in the present case P
c (the concentration at which the net uptake is zero) was a better predictor of the ability to compete for P
i under steady state as well as transient conditions in the P
i supply.
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