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Title: Effect of photoacclimation on bio-optical properties of marine phytoplankton
Author: Phongphattarawat, Sornsiri
ISNI:       0000 0004 9355 4079
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2020
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Photoacclimation in marine phytoplankton is manifested through changes in pigment composition and has a significant impact on in vivo light absorption. Results obtained from this thesis work clearly show that the variability in the chlorophyll a (Chl-a)-specific absorption coefficient of phytoplankton at 440 nm (a*ph (440)) observed over a broad latitudinal gradient of the Atlantic Basin was influenced significantly by changes in the concentration of non-photosynthetic pigments (NPPs) relative to total pigments (the NPP index), whereas pigment indices of cell size showed a weaker correlation with a*ph (440). Spatial and seasonal changes in the NPP index were the combined result of photoacclimation and the photoadaptive variation in pigment composition of dominant prokaryotic and eukaryotic groups. Variability in the spectral shape (indexed by the spectral slope) of phytoplankton absorption could be largely explained by NPPs, particularly zeaxanthin (a marker for marine picocyanobacteria) which was highly abundant in the (sub)tropical oligotrophic gyres due to the predominance of the marine picocyanobacterium Prochlorococcus. In Prochlorococcus-dominated waters, depth-dependent variation in the NPP index was driven primarily by variation in the intracellular concentration of photosynthetic pigments (PSPs), rather than NPPs, which was the result of both photoacclimation and a shift in dominant ecotypes. Photoacclimation of this globally-significant genus was also clearly reflected in the strong vertical change in the intracellular concentration of the dominant light harvesting pigment, divinyl Chl-a (DVChl-a). The flexibility in the DVChl-a cell−1 in Prochlorococcus leads to a more highly variable carbon-to-DVChl-a ratio (θPro) observed in this genus than in other phytoplankton groups. θPro was shown to be highly correlated with both the NPP index and the spectral slope between 443 and 550 nm, which suggests that both of these indices may serve as useful predictors of θPro. In strongly-stratified, oligotrophic waters where the NPP index may be considered a proxy of ambient light levels, the NPP index could account for a significant fraction of the variability in the parameters describing the photosynthesis-irradiance (PE) response curve. Using estimates of θPro derived from its relationship with the NPP index, the maximum growth rates for surface Prochlorococcus populations (μm, Pro) were calculated from maximum Chl-a-specific photosynthetic rates measured at the Prochlorococcus-dominated stations, yielding an average μm, Pro value of 0.22 d−1 , which is consistent with field and culture studies. Thus, the NPP index can be considered a useful proxy of both the photoacclimatory status and photophysiological characteristics of phytoplankton in subtropical marine ecosystems.
Supervisor: Bouman, Heather ; Zhang, Qiong Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Biological oceanography