Physiological effects and UVB sensitivity in the marine phytoplankton Emiliania huxleyi in response to elevated carbon dioxide concentration and high photon flux.

Anthropogenic activities are having a range of important impacts on the global climate, including increased carbon dioxide (C02) in the atmosphere and an increase in ultraviolet B (UVB) radiation reaching the earth's surface. The marine coccolithophore, Emiliania huxleyi, is of global biogeochemical importance, due to its capacity to form very dense blooms, its ability to produce calcium carbonate scales (coccoliths) and its consequently major role in the global carbon cycle. Hence this study investigated the impact of various climate change factors on the growth, photosynthesis and nutrient uptake in E. huxleyi. Semi-continuous cultures of two strains of E. huxleyi (a non-calcifying and a calcifying strain) were grown at CO2 concentration corresponding to either the present day (380 ppm) or those predicted for 2100 (1000 ppm), and at either low light (80 ~mol photons m-2 S-I) or high light (250 ~mol photons m-2 S-I). Growth rates, rapid light curves (RLCs), phosphorus and nitrogen uptake rates, and activities of the enzymes nitrogen reductase (NR) and glutamine synthetase (GS) were measured. Cellular carbon, nitrogen and phosphorus contents and C:N:P ratios were measured in the non-calcifying strain. The sensitivity of these parameters to exposure to an acute dose of UVB radiation was also assessed. Elevated C02 concentration negatively affected growth rates in both strains of E. huxleyi investigated, although in the non-calcifying strain high PAR (photosynthetically active radiation) partially reversed the negative effects of CO2 concentration. The specific impacts of C02, whether direct or indirect, on E. huxle)'i physiology are still unclear from this study. However, low pH due to elevated CO2 concentration has been suggested to impact on cellular function and hence may impact on growth. Several physiological parameters were sensitive to high light. These included photosynthetic parameters (rETRmax and NPQ) and the activity of the main rate-limiting enzyme involved in nitrogen assimilation, glutamine synthetase. Whether high light directly damages the enzyme or if damage to photosynthesis indirectly affects nitrogen assimilation through a decrease in energy availability is unclear from this study. In both strains of E. huxleyi investigated, neither elevated CO2 nor light levels led to an increased susceptibility of the physiological parameters measured to short -term UYB exposure. Each environmental factor studied impacted negatively on E. huxleyi. Interestingly however, neither elevated CO2 nor high light intensity increased the susceptibility of cells to damage by acute UYB exposure. This suggests that under future predicted climate scenarios, while the two strains of E. huxle.vi investigated may be less competitive, they will not be further disadvantaged in high UYB conditions close to the ocean surface. Given the global importance of E. fluxle.-vi as a species, the negative impacts under predicted future climate scenarios are of global significance.