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JPR Advance Access published online on February 6, 2004
Journal of Plankton Research, doi:10.1093/plankt/fbh025
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Received February 12, 2003;
accepted November 20, 2003

Article

Changes in pigmentation, bio-optical characteristics and photophysiology, during phytoflagellate succession in mesocosms

Matilde S. Chauton 1*, Gavin H. Tilstone 2, Catherine Legrand 3, and Geir Johnsen 1

1 Norwegian University of Science and Technology, Departement of Biology, Trondhjem Biological Station, 7491 Trondheim, Norway
2 Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL1 3DH, UK
3 Univ. Kalmar, Dept. Biol. & Environm. Sci., Marine Sci. Div., S-39182 Kalmar, Sweden

* To whom correspondence should be addressed. E-mail: matilde.chauton{at}bio.ntnu.no.


  Abstract

Pigmentation, bio-optical characteristics and photophysiology, were studied in mesocosms with different N:P ratios. No significant difference in biomass or species composition was seen under different nitrogen to phosphorous ratios (N:P), but a temporal succession of different flagellate groups was observed in all mesocosms. An initial bloom of prymnesiophytes containing chlorophyll (Chl) c and 19'hexanoyloxyfucoxanthin (19'HOF) was followed by prasinophytes containing Chl b. Electron microscope analysis confirmed the presence of genera such as Chrysochromulina (Prymnesiophyceae), Tetraselmis and Pyramimonas (Prasinophyceae). Traces of prasinoxanthin in the pigment samples showed that smaller prasinophytes were also present. Chl b influenced the photophysiology of the prasinophytes resulting in higher Chl a-specific absorption, but a greater difference between absorption and scaled fluorescence excitation spectra indicated that light absorbed by Chl b is associated with photosystem I (PSI). Since a larger fraction of the light was absorbed by chlorophyll in PSI and/or photoprotective carotenoids, the light-saturated Chl a-specific rate of photosynthesis (PmB) and maximum light utilization coefficient ({alpha}B) decreased when [Chl b] increased. The highest PmB values were seen when the ratios of fucoxanthins to Chl a were high, indicating that prymnesiophytes might be more efficient in light harvesting and electron transport through photosystem II (PSII) by fucoxanthins and chl c. Our results therefore indicate different light acclimation strategies in prasinophytes vs. prymnesiophytes, which may be reflected in the successional appearance of these communities in the natural environment. We also suggest that grazing by ciliates and rotifers caused periodic decreases in phytoplankton biomass, which in turn gave rise to the phytoflagellate succession observed in the mesocosms.


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