Photosystem II electron flow as a measure for phytoplankton gross primary production = [Fotosysteem II elektronentransport als een maat voor de bruto primaire produktie van fytoplankton]

Saturating pulse fluorescence measurements, well known from studies of higher plants for determination of photosystem II (PS II) characteristics, were applied to cultures of the green alga Dunaliella teitiolecta (Chapter 2). The actual efficiency of PS II (φ _{PS II} ), the maximal efficiency of PS II (F _v /F _m ), and both photochemical and non- photochemical fluorescence quenching were determined for cultures of D. tertiolecta growing under varying light intensities. The rate of PS II electron flow (J _E ) estimated as the product of φ _{PS II} , and the photon flux density (PFD), appeared to correlate well with growth rates determined for the D. tertiolecta cultures . The results indicated that the saturating pulse fluorescence method may be successfully used to determine photosynthetic characteristics of phytoplankton. However, an increase of sensitivity by a factor 1000 was found to be needed for the application of this technique to in situ measurements. Conditions were outlined which have led to the development of the Xe-PAM fluorometer with a manyfold higher sensitivity.

The relation between photosynthetic oxygen evolution (J ₀ , expressed as oxygen production per chlorophyll a ) and J _E was investigated for the marine algae Phaeodactylum tricornutum, D. tertiolecta, Tetraselmis sp., Isochrysis sp. and Rhodomonas sp , by varying the ambient PFD (Chapter 3). At limiting light a linear relation was found in all species. At PFD's approaching light saturation linearity was lost. The observed non-linearity at high M's is most probably not caused by photorespiration but by a Mehler-type of oxygen reduction. The relationship could be modelled by including a redox-state dependent oxygen uptake. The linear range between J _E and J ₀ extends to a PFD which is 2 to 10 times higher than the PFD at which the species were grown. The ratio of J _E and J ₀ in the light-limited range is species dependent and related to differences in absorption cross-section of PS II (σ _{PS II} ). The ratio of J _E and J ₀ in the light-limited range is not dependent on temperature. F _v /F _m was found to be temperature dependent with an optimum near 10 °C in the diatom P. tricornutum .

The photosynthetic electron flux in a phytoplankton sample (PEF) was shown to depend on the product of J _E (= φ _{PS II} · PFD), σ _{PS II} and the number of PS II (n _{PS II} ) in the sample (Chapter 4). A mathematical expression was derived which relates the minimal fluorescence (F ₀ ) to n _{PS II} and σ _{PS II} under the condition that the spectral distribution of the ambient light and the measuring light are identical. This condition can be approximated measuring F ₀ with the Xe-PAM fluorometer. The experimental conditions under which the relationship between PEF, φ _{PS II} ,  and F ₀ is valid, were examined. The maximal value of φ _{PS II} (F _v /F _m ) was shown to be independent on the wavelength under the measuring conditions. The apparent F _v /F _m   depends on the intensity of the measuring light and the duration and intensity of the saturating light pulse. It is shown that, under certain conditions, the minimal fluorescence can be used as a measure of PS II excitation in the light. F ₀ , obtained with the broad band excitation light of a filtered Xenon flash lamp, thus was used as a measure for the product of  n _{PS II} and σ _{PS II} . The relationship between PEF calculated with this expression and net oxygen evolution (phytoplankton oxygen flux, POF, expressed as oxygen production per sample volume) was found to be similar in the diatom P tricornutum and the green alga D. tertiolecta. Therefore we conclude that the use of PEF as a measure for POF yields better results than the use of J _E for J ₀ . The Xe-PAM fluorometer was found to be sensitive enough for coastal applications.

The relation between PEF and carbon fixation (phytoplankton carbon flux, PCF, expressed as the carbon dioxide fixed per sample volume) was examined in cultures of Isochrysis sp. , Phaeocystis sp. macroflagellates and Skeletonema costatum (Chapter 5). The F ₀ used to calculate PEF was measured at the start of the experiments. Both the PFD and the duration of the incubation were varied. As found before for the relation between J _E and J ₀ , the relation between PEF and PCF was also approximately linear at limiting light and deviated from linearity at saturating light. The linear range between PCF and PEF also extends to a PFD which is 2 to 10 times higher than the PFD at which the species were grown. The length of the incubation did not affect PEF and PCF except for the highest PFD (1530 μmol m ^-2s ^-1). The decline of F _v /F _m of the samples irradiated at the highest PFD showed a fast component within 30 minutes incubation and a minor slow component, indicating that photoinhibition was induced in the first 30 minutes.

PEF, PCF, oxygen production and their relationships were furthermore examined during phytoplankton development in a mesocosm at the field station (Chapter 6). PEF was calculated from F ₀ , φ _{PS II}  and the PFD which were continuously monitored for three weeks in the upper water layer of the mesocosm. In addition PEF and PCF were estimated from laboratory measurements on samples taken from the mesocosms. Daily primary production in the mesocosm, measured as either PSII electron flow (PPE) or carbon fixation (PPC), was calculated using a photosynthesis model. Daily photosynthetic oxygen evolution (PPO) was calculated from changes in the oxygen concentration over the day. In the period between the 2 blooms the ration of PPO and PPE was higher than during the peak of the blooms. The ratio of PPC and PPE was much more constant. In general PPE gave a reasonable measure of both PPC and PPO.

The effects of chlororespiration and state transitions on F ₀ were determined in the diatom P. tricornutum (Chapter 7). Inhibition of chlororespiration by antimycin A or anaerobiosis did not affect F _v /F _m . The observed F ₀ was insignificantly (8%) increased upon addition of antimycin A and slightly decreased upon illumination with farred light (6 μmol m ^-2s ^-1). These effects might be attributed to chlororespiratory activity, but could as well be caused by reduction of Q _A by the weak fluorescence measuring light. Addition of the uncouplers CCCP or nigericin did not increase the maximal fluorescence (F _M ). The data show that under our conditions reduction of Q _A and energy dependent quenching in the dark by chlororespiration do not occur in P. tricornutum . Light-induced increases in F _M , therefore, are suggested to be caused by state transitions. Use of the F ₀ to estimate photon capture in the light might lead to an underestimation of the PEF The error is estimated to be not more than about 10 % as calculated from the increase in maximal fluorescence in the light.

The present work illustrates that the fluorescence pulse method is a reliable technique to get insight into the photosynthetic performance and gross primary production of the population of algal cells in a marine ecosystem.