In this thesis a comparison is made between phosphate (P) uptake and growth kinetics of three desmid species originating from lakes that differ in trophic status. Experimental results show that Cosmarium abbreviatum var. planctonicum, a taxon characteristic of oligo-mesotrophic waters, is a better competitor under stringent Pi-limited conditions ([Pi] < 0.02 M) than Staurastrum pingue and S. chaetoceras, both isolated from eutrophic lakes. This could be explained by a higher affinity for Pi uptake (and, consequently, a higher growth rate) in C. abbreviatum under those conditions. Internal P quota, Qp, in the three desmid species, when grown under stringent, continuous Pi limitation, were comparable. When Pi limitation was less severe or not present at all (neither continuously, nor periodically), the Staurastrum species were better competitors due to a higher maximum initial Pi uptake rate (Vi,max) and a higher maximum growth rate (max).

As compared to continuously Pi-limited conditions, at interrupted limitation (Pi provided in distinct pulses) Vi,max increased in both C. abbreviatum and S. chaetoceras, which can be considered an adaptation to a fluctuating Pi regime (due to lack of time, part of the experiments were carried out on only one Staurastrum species: S. chaetoceras). In C. abbreviatum, an increase in the affinity constant for uptake (Km) was observed concurrend with the increase in Vi,max. This resulted in a lower affinity for Pi uptake (Vi,max/Km). Whereas such a lowered affinity in C. abbreviatum was measured both before and after a Pi pulse, in S. chaetoceras an increase of Km was only detected just after a pulse, resulting in a temporarily lowered affinity for Pi uptake.

After a saturating addition of Pi, C. abbreviatum accumulated about 3 times as much P in its cells (Q'max) as S. chaetoceras. Only after Q'max was attained, growth was resumed. During growth, C. abbreviatum did not take up Pi from the medium whereas S. chaetoceras took up Pi at a rate compensating its decrease in Qp caused by growth. Because C. abbreviatum has both a higher storage capacity and a lower max relative to S. chaetoceras, maximum cellular P quota (Q'max) of this species can theoretically sustain max over a much longer period than S. chaetoceras. Both under continuous and pulsed Pi limitation, S. chaetoceras exhibited a stronger feedback of Qp on Pi uptake rates than C. abbreviatum.

C. abbreviatum had a higher maximum alkaline phosphatase activity (APAmax) as well as a higher affinity for the hydrolyzation of the organic P substrate used (methylfluoresceinphosphate) than S. chaetoceras. The outcome of competition between these two desmids under a continuous limitation of organic P (Po) could be explained from these species-specific APA characteristics. Contrary to that, it had to be concluded that under pulsed Po conditions Pi uptake kinetics were more decisive to the outcome of competition than APA characteristics. For, APA in C. abbreviatum was calculated to be higher than its potential Pi uptake rate under the given conditions.

C. abbreviatum differed from both Staurastrum species in P uptake and growth kinetics as well as in the structural presence of a copious mucilaginous extracellular envelope. One of the questions we tried to solve in this thesis is whether such an extracellular mucous sheath in planktonic desmids has a function in capturing scarce nutrients, i.e. P. It was noticed that the mucous envelope gave rise to a higher cellular dry weight and that it increased in size when cells were subjected to pulsed Pi-limited conditions, compared to its size under continuous Pi limitation. No storage of Pi in the mucous envelope after a pulse could be detected, nor was any delay in Pi uptake found. The mucous sheath did sorb some Pi passively, but binding was not proportional to the increase in surface caused by this layer. It is suggested that the mucous envelope could act as an action site for APA as this activity was very high in C. abbreviatum. This hypothesis could not be proved as yet.

The above-described species-specific kinetic characteristics may explain the distribution of the species in the field. The high affinity for P and the high APA make C. abbreviatum well adapted to conditions of prolonged, severe P limitation characteristic of oligotrophic waters. Incidental local enhancement of dissolved P can be fully utilized due to its high storage capacity. S. chaetoceras and S. pingue, on the other hand, have high maximum initial P uptake rates and high maximum growth rates which make them better adapted to frequently occurring P pulses which are thought to be characteristic of a eutrophic environment.

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