The effect of ocean acidification upon free-living Symbiodinium spp

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 of OA upon the productivity and growth of four different phylotypes of Symbiodinium (AI, A2, A13 and BI) and also characterised the carbon concentrating mechanisms (CCMs) of phylotypes 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.

The effect of ocean acidification on the ecology and physiology of marine macroalgae

Ocean acidification (OA) is the decrease in ocean pH due to increasing atmospheric pC02• It is predicted that by the year 2100, the pC02 will rise from ca 385 uatm today to 750 uatrn, with a corresponding decrease in surface ocean pH from 8.1 to 7.8. pH was monitored at five stations around a coastal CO2 vent site in Ischia, Italy and the utility of such areas discussed. An ecological survey of benthic macroalgae revealed a substantial community shift at lower pH toward reduced species richness and diversity. The chlorophytes became more dominant at lower pH, with cover increasing from 45-55% at pH 8.16-7.84, to 67-90% at pH 7.48-7.11. Heavily calcified species disappeared at pH 7.48-7.11, but their total cover did not change significantly between pH 8.16-7.80, suggesting some resilience over this century. At lower pH, the DMSP content in macroalgae from Ischia increased in the chlorophytes while in rhodophytes and phaeophytes it decreased. The dark-adapted algal photophysiology suggested a significant benefit when pH was 7.84-7.80, which was lost at pH 7.48-7.11. Two species of common chlorophyte macroalgae VIva lactuca and VIva clathrata, were incubated under pC02 conditions ranging from 432 to 1514 uatrn, In both species, the results indicated that by the year 2100 there could be a large decrease by 50-58% in DMS production, a reduction in chlororespiration, and increased reproductive output in these species. I conclude that increasing pC02 does not directly fertilise photosynthesis or somatic growth in the Ulvales but, reduces chlororespiration, possibly due to carbon-concentrating mechanism down-regulation. This may be the cause of the large reduction in DMS production seen and may lead to a reallocation of resources towards reproductive output. This may increase the prevalence of chlorophyte macroalgae in the future with major repercussions for coastal ecosystems.