| dc.description.abstract | This thesis puts emphasis on an evaluation of the Pulse Amplitude Modulated (PAM) fluorescence (that is, the in vivo variable Chl a fluorescence) as non-invasive means to estimate the oxygen evolution rate. The estimated rate depends on the light absorption rate of microalgae, which in turn is both species-specific and affected by the photo-acclimation status. To estimate oxygenic photosynthesis from PAM fluorescence, the amount of photons absorbed by Photosystem II (PSII) has to be quantified. I have employed three different biooptical approaches to estimate the fraction of light absorbed by PSII to calculate the O2- production rate from quantum yield of charge separation in PSII. These approaches were validated against simultaneous measurements of O2-production using microelectrodes. I concluded that spectrally weighted PSII absorption normalized to Chl a ( * aPSII ) provides the best measure of light absorbed by PSII. In combination with PAM fluorescence measurements, it provides a non-invasive and fast method for determination of gross oxygen production rate in microalgae.
Short-term temperature effects on photosynthesis were investigated to evaluate if the in vivo variable Chl a fluorescence is temperature dependent. Three different methods were used in monocultures of three marine phytoplankton species: measurements of O2-production using microelectrodes, calculated O2-production from PAM fluorescence, or 14C-incorporation rates. Photosynthesis vs. irradiance curves were measured at seven temperatures (from 0 to 30°C). The three techniques for measuring photosynthetic rates showed the same relative response to a short-term temperature change. In conclusion, it appears that the PAM technique can be used to study temperature responses of photosynthesis in microalgae.
To evaluate responses in photochemical efficiency the dinoflagellate species, Alexandrium tamarense was grown under 5 different nitrogen: phosphate (N:P) supply ratios. The photochemical efficiency, measured as operational quantum yield of charge separations in PSII, was lowest under high N:P supply ratio although the cells contained higher amounts of pigments. A. tamarense is known to produce saxitoxin, which induces Paralytic Shellfish Poison. The results suggested that this N-rich toxin (and possibly other toxins) may represent part of the storage N pool in A. tamarense grown under high N:P supply ratios. In contrast, under low N:P supply ratios, the toxins might be degraded, to release N that can be used for de novo synthesis of pigments and eventually to sustain photosynthesis and cell division under N-deplete conditions.
Lastly, to assess if the protocol established for calculating O2-production from PAM fluorescence could be used to estimate gross O2-production rates in situ, diurnal times-series of photosynthetic responses vs. irradiance in three different assemblages of Antarctic marine microalgae are described. My findings agree with physiological characteristics of low-light acclimated algae: low Ek, low amount of photoprotective carotenoids, and correspondingly high amounts of light harvesting pigments. The results also indicate that there might be a difference in photosynthetic efficiency in psychrophilic algae when measured at ambient temperatures close to –2ºC and laboratory experiments of the same algae at +2ºC. | nb_NO |
