Unlocking the inner cell : linking cell biochemistry to the physiology and ecology of coccolithophores

Coccolithophores — marine calcifying unicellular algae — make a key contribution to phytoplankton community diversity and productivity and have important roles in regulating ocean biogeochemistry. Despite their ecological success, the current understanding of the clade relies on the knowledge of the model species, Emiliana huxleyi, whereas very little is known of their diverse physiological ecology. This thesis presents a detailed analysis of the physiology of a diverse range of coccolithophores, including a meta-analysis of their cell size as well as biochemical data of cellular elemental (i.e., carbon, C; nitrogen, N; phosphorus, P) and macromolecular (i.e., protein, lipid, and carbohydrate) content in nutrient-replete cultures. Coccolithophore differ from other key phytoplankton in that their cell size spectrum is restricted with most extant species smaller than 10 μm in diameter, likely giving them advantages in low nutrient and light environments when competing with other phytoplankton. In addition, coccolithophores are less C-rich than other phytoplankton, providing a coccolithophore-specific relationship between cell organic C content and biovolume. The examination of coccolithophore elemental composition shows that organic C to N ratios are similar to other phytoplankton, implying little additional N cost for calcification and efficient retention and recycling of cell N. On the other hand, C to P ratios imply a greater P demand in coccolithophores, which hints at efficient metabolic strategies for the use of this nutrient by the cells. The macromolecular composition of coccolithophores of this study shows higher lipid and lower protein content than reported previously for haptophytes. Coccolithophore C to N ratios and high lipid relative to proteins have implications for N cycling, as well as C fixation, and export relative to blooms of non-biomineralized phytoplankton. Finally, outputs of a distinct DNA-barcoding field-study of small eukaryote plankton communities (< 200 !m) revealed differences in the species composition between the contrasting depth layers of the water column in the sub-tropical oligotrophic gyre. Nutrient enrichment studies demonstrated the role of P, largely understudied in comparison with N, in constraining eukaryotic marine biodiversity, which has implications for ocean productivity

Allometry of carbon and nitrogen content and growth rate in a diverse range of coccolithophores

As both photoautotrophs and calcifiers, coccolithophores play important roles in ecosystems and biogeochemical cycles. Though some species form blooms in high-latitude waters, low-latitude communities exhibit high diversity and niche diversification. Despite such diversity, our understanding of the clade relies on knowledge of Emiliana huxleyi. To address this, we examine carbon (C) and nitrogen (N) content of strains (n = 9) from the main families of the calcifying Haptophyceae, as well as allometry and cell size frequency across extant species. Coccolithophore cell size is constrained, with ~71% of 159 species smaller than 10 μm in diameter. Growth rates scale with cell biovolume (μ = 1.83 × cell volume(−0.19)), with an exponent close to metabolic theory. Organic carbon (C) per cell is lower than for other phytoplankton, providing a coccolithophore-specific relationship between cell organic C content and biovolume (pg C cell(−1) = 0.30 × cell volume(0.70)). Organic C to N ratios (~8.3 mol:mol) are similar to other phytoplankton, implying little additional N cost for calcification and efficient retention and recycling of cell N. Our results support observations that coccolithophores are efficient competitors in low-nutrient conditions, able to photosynthesize, calcify and run the routine metabolic machinery necessary without any additional need for N relative to noncalcifying algae