Final Activity Report Summary - IMPULSE (InforMation Processing in PULSEd P Environments: Comparison of (...) between bloom forming diatoms and the coccolithophore Emiliania huxleyi.) Phytoplankton are the base of many marine and freshwater food webs, providing the organic matter that sustains higher trophic levels leading to fish, mammals including man and birds. A small proportion of the phytoplankton species have been designated as harmful because they contain toxins, or as nuisance when they bloom to excessively high abundances. However, most phytoplankton are beneficial, in the sense that they play key ecological roles and provide key ecological services. Better management of marine and freshwater systems requires, among other things, that we develop better methods to scientifically assess the factors that limit phytoplankton growth as well as better models for describing the effects of limiting factors on aquatic ecosystems. In order to advance scientific knowledge towards meeting these objectives new research was undertaken on the kinetics, i.e. rate, of phosphorus uptake by marine phytoplankton. Up until now, research on adaptation of photosynthetic micro-organisms to external stimuli such as nutrients and light was mainly focussed on full adaptation. However, full adaptation is unlikely to ever be achieved in nature. The aim of this EIF study was to understand the largely unknown constraints that determined the transition from one adaptive state to another for marine algae. This was not a time invariant phenomenon, but proceeded in a historical context since every adaptation was determined by previous adaptation of an organism. P-limitation was described for both freshwater and marine environments, even if this phenomenon was more common in lakes than in the sea. Outgrow experiments, particularly in the ocean, were often used to estimate the aspect of limited phytoplankton growth and provided a lot of valuable results on algal physiology. Most of these experiments, however, were designed with only one objective, namely the optimisation of growth conditions and high growth rates. However, this required larger additions of phosphorus during experiments than those which were ever likely to be observed in nature. Thus, more insight would be gained if we could assess the effects of nutrient limitation on more natural scales, as was done during this EIF project. Specifically, the [32-P] net incorporation experiments that were undertaken on three marine algae, namely emiliania huxleyi, thalassiosira weissflogii and rhodomonas reticulata were not necessarily linked to growth (Droop) but reflected the activation of the P-uptake system of phytoplankton cells. The phosphorus uptake kinetics were evaluated using three models, emphasising on different backgrounds which were most relevant to understand the time-dependent pulse-to-pulse response of microorganisms. The selected models were the conceptual model of a flow-force relationship by Falkner, an empirical model of first order kinetics and a modified Michaelis-Menten model. The kinetic response to phosphorus acquisition to a sequence of up to 10 pulses in the nano-molar range showed that microorganisms were capable to adjust successively to the pulse pattern. After passing an initial phase of adaptive phosphorus uptake of about one hour, further pulsing of low nano-molar concentrations induced an adjustment of accelerated uptake velocity. This phenomenon of optimised phosphorus uptake in response to environmental stimulation confirmed other studies field observations. These showed that photosynthetic microorganisms were able to adjust to low concentration pulsing by enhanced uptake efficiency in environments of micro-zooplankton excretion in comparison to those of larger pulsing of macro-zooplankton. The extent of transient quenching of photosynthesis, e.g. the decrease of maximum fluorescence measured under light, Fm, increased with time for six minutes until half of the concentration of phosphorus pulse was taken up and supplied around one micro mol. The results of P-uptake experiments with marine cultures were confirmed by a field study on phosphorus pulse addition bioassays for spring phytoplankton in the Red Sea.