Environmental controls on the physiology of the marine coccolithophore Emiliania huxleyi strain NIWA 1108
Anthropogenic activities have affected the global climate rapidly; different environmental factors important for oceanic productivity have been altered. The concurrent changes in multiple environmental drivers affect the physiology of marine phytoplankton, both individually and interactively, in a complex way. Emiliania huxleyi, the most abundant and widely-distributed coccolithophore in the ocean, is a model organism for understanding the marine carbon cycle. As a calcifying species, E. huxleyi is considered particularly susceptible to the increasing surface seawater CO2 concentration and decreasing pH, termed ocean acidification. The thesis presents a series of step-wise studies on the controls of five environmental drivers (nitrate concentration, phosphate concentration, irradiance, temperature and CO2) on the physiology of a southern hemisphere E. huxleyi strain isolated from the Chatham Rise, New Zealand. The goal of this thesis is to examine the importance of ocean acidification relative to four other environmental drivers both individually and interactively on the physiology of the ecologically important coccolithophore species. First, E. huxleyi strain NIWA 1108 was subjected to a series of semi-continuous incubation experiments by changing the conditions of one environmental driver at a time. The importance of each environmental driver on each measured physiological metric of E. huxleyi was ranked using a semi-quantitative approach by comparing the percentage change caused by each environmental driver on the measured physiological metrics at the projected conditions for the year 2100 relative to the present day conditions in the Chatham Rise, New Zealand. The results reveal that a 33% decrease in nitrate concentration played the most important role in controlling the growth, photosynthetic and calcification rates of E. huxleyi; rising pCO2 decreased the calcification:photosynthesis and cellular particulate inorganic carbon:particulate organic carbon ratios the most; warming was the major driver controlling both cellular particulate organic carbon and particulate inorganic carbon contents; and nutrient concentrations were the most important drivers regulating the cellular particulate nitrogen and cellular particulate phosphorus contents of E. huxleyi. Then a two-way (ocean acidification plus changes in either nitrate concentration, phosphate concentration, irradiance or temperature) and multiple factorial (manipulation of all the five environmental drivers) manipulation experiment was conducted. The results exhibited an interesting connection with the single environmental driver effects: interaction of ocean acidification and a 33% decrease in nitrate concentration had the largest synergistic negative effects on most of the E. huxleyi physiological metrics among all the two-way factorial manipulations. The simultaneous manipulation of all the five environmental drivers to the projected future (2100) conditions had the most prominent negative effects on the growth, photosynthetic and calcification rates of E. huxleyi. Finally, the gene expression study suggests that changing pCO2 probably affects E. huxleyi photosynthesis and calcification through regulating the carbon concentrating mechanism and pH homeostasis at the molecular level. Furthermore, the substantial down-regulation of most of the investigated genes associated with inorganic carbon acquisition and calcification by multi-factorial manipulation of all the five environmental drivers indicates a link between significant suppression of functional genes and the substantially decreased physiological rate processes (growth, photosynthetic and calcification rates) in E. huxleyi. Overall, the thesis reveals that other environmental drivers may play more important roles than ocean acidification in regulating the physiological responses of E. huxleyi, and suggests that the interplay between ocean acidification and other drivers is likely to have antagonistic, additive or synergistic effects on different physiological metrics of E. huxleyi. The thesis contributes to our understanding of how the physiology of E. huxleyi will respond to the concurrent changes of multiple environmental drivers.
Advisor: Hurd, Catriona L.; Roleda, Michael Y.; Boyd, Philip W.; Dickinson, Kath
Degree Name: Doctor of Philosophy
Degree Discipline: Botany
Publisher: University of Otago
Keywords: coccolithophore; Emiliania huxleyi; ocean acidification; multiple stressors; growth; calcification; photosynthesis; elemental composition; gene expression
Research Type: Thesis