Coccolithophores and light: Photophysiology and ecology amongst different species

Abstract

The coccolithophores are a class of unicellular algae and are considered one of the key phytoplankton functional types. Regionally coccolithophores can contribute > 20 % of primary productivity and are major contributors to pelagic calcite production. Amongst the > 280 extant coccolithophore species there is remarkable diversity in morphology, and evidence that different species inhabit distinct ecological niches. Despite this, the current understanding of the biology and ecology of coccolithophores is largely based on the cosmopolitan and bloom-forming species Emiliania huxleyi, as its reflectance signature is easy to detect in field and it is easier to grow in the laboratory than most other species. Light is a key primary resource for the coccolithophores, and the coccolithophore niche is often considered to be characterised by high irradiance, based largely on observations of E. huxleyi ecology and physiology. In this thesis, I tackle this research bias by investigating the response to irradiance in 10 different species, spanning a broad taxonomic range. A detailed analysis of physiology, including pigment content, light absorption properties, and photophysiological parameters, in response to three growth irradiances (25, 100 and 200 µmol photons m-2 s-1) provide an unprecedented wealth of comparative data between species. The results reveal characteristics that are common amongst the coccolithophores as a group. For example, all species had high accessory pigment content (including fucoxanthin derivatives and chlorophyll cs) relative to chlorophyll a regardless of growth irradiance. The shape of spectral light absorption (with elevated absorption in the 440 - 470 nm region) was also conserved amongst all the coccolithophore species. Notable differences between the coccolithophore species were also revealed in, for example, the contribution of the ‘biomarker pigment’ (19’Hexanoyloxyfucoxanthin) to total pigment varied widely between species, and in the photoacclimation strategy. The size range (cell volumes ranging from 11 µm3 to 2120 µm3) of the species used allowed examination of interspecific size scaling with respect to light in the coccolithophores for the first time. Negative size scaling of maximum growth rate and pigment density (per unit cell volume) was found, consistent with previous findings in other phytoplankton groups. However, large coccolithophore cells (> 100 µm3) did not experience a growth rate penalty under low light, which was an unexpected finding. An assessment of coccolithophore communities in the field found differences in the species composition between the contrasting hydrographic regimes of the stratified central shelf and shelf-break of the Celtic Sea. Patchiness in the community on the stratified shelf environment, and a lack of vertical differentiation of the community, underline the complexity of environmental drivers of coccolithophore distributions in the real world. The evidence presented in this thesis sheds light on the considerable physiological diversity within the coccolithophore group, and advocates continuing to examine a higher diversity of coccolithophore species in experimental work. Physiological studies using a variety of coccolithophore species can help to build on the solid foundation of pre-existing research on E. huxleyi, in order to advance our understanding of the coccolithophores as a group.

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ORCID for Anna Hickman: orcid.org/0000-0002-2774-3934

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