In the last decade, ocean acidification (OA) has become a major focus of scientific research. A significant fraction of anthropogenic carbon dioxide (C02) emissions is absorbed by the world's oceans, causing a decrease in pH and shifts in the carbonate chemistry of seawater towards increased CO2(aq) Many reef-building coral species obtain their algal symbionts from environmental populations of free-living Symbiodinium via a mechanism known as horizontal transmission. This thesis examined the effect ofOA upon the productivity and growth of four different phylotypes of Symbiodinium (AI, A2, A13 and BI) and also characterised the carbon concentrating mechanisms (CCMs) ofphylotypes A2 and A13, in culture (i.e. 'free-living'). The response to a doubling of pC02 to ~800 ppmv varied between phylotypes; A I and B I were relatively insensitive to OA whilst the productivity of A2 and the growth rate of A13 increased by 40% and 60%, respectively. This phylotype-specific response to OA is likely to affect population dynamics in free-living Symbiodinium, with further implications for those corals that obtain their symbionts via horizontal transmission. Furthermore, the actual mode of iC- acquisition was shown to differ between phylotypes, with A2 capable of indirect bicarbonate (HC03-) uptake, via the catalytic action of external carbonic anhydrase (eCA), whilst A13 was solely dependent upon the CO2(aq) fraction of the inorganic carbon pool in seawater. These differences in iC-acquisition provide the basis for the phylotype-specific effects of OA on these two phylotypes. Future studies will be able to build on these findings to examine the effect of OA upon a wider representation of the Symbiodinium genus and contrast their ability to regulate the CCM in response to changes in seawater pC02 and, ultimately, contribute towards a better understanding of the effects OA will have upon the form and function of coral reef ecosystems.