Assessing the photosynthetic impact of inorganic carbon in symbiotic dinoflagellates in culture and in coral host complexes

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Increased atmospheric carbon dioxide concentrations are causing warming of the Earth's atmosphere and having significant effects on the chemistry of the oceans. Disruption to seawater biochemistry is causing associated pH change known as ocean acidification and increased concentrations of dissolved inorganic carbon. Coral reefs are already at risk from increased seawater temperatures; however the impact of ocean acidification will be in addition to that of temperature change alone. The major objectives of this thesis were to provide insight into the effects of temperature, dissolved inorganic carbon concentration and seawater pH on the photosynthesis of the symbiotic algae Symbiodinium sp. both in vitro and in hospite. While it has been speculated that increased carbon dioxide may stimulate primary production, results presented here for short-term incubations show that Symbiodinium sp. photosynthesis is saturated at present concentrations (2 mM). However, photosynthesis in hospite can become significantly limited if DIC falls below 2 mM. This highlights the significant role of host inorganic carbon transport mechanisms for the maintenance in a healthy symbiosis. Symbiodinium sp. in culture however, show significant photosynthetic plasticity over a range of inorganic carbon concentrations (0.1 - 10 mM), and exhibit no photosynthetic inhibition at inorganic carbon concentrations well below ambient levels (0.4 - 2 mM). This suggests that Symbiodinium sp. have well developed carbon concentrating mechanisms and may be able to adapt to changes in inorganic carbon availability, whereas the same algae in hospite requires a higher DIC supply. Exposure to elevated temperature is known to cause photosynthetic inhibition in the coral symbiont Symbiodinium sp. Models of photophysiologcial thermal damage in corals can be generally divided into two broad groups; those that take place at the site of the light reactions and those that occur in the Calvin cycle. Results presented here have identified species- and cladal-specific heterogeneity in thermal inhibition of the dark reactions in the whole corals Stylophora pistillata and Pocillopora damicornis, and amongst Symbiodinium clades A, Band Cl. Furthermore, these patterns are not consistent when observed in vivo and in vitro. pH-drift experiments of Symbiodinium sp. grown in culture have identified significant cladal-specific pH ranges of clades A, B and subclade C1. Results show that Symbiodinium sp. are well adapted for living in rapidly changing pH and dissolved inorganic carbon environments. However, when measured in hospite in the C !- harbouring corals S. pistillata and P. damicornis, photosynthesis became significantly inhibited at pH 7.2 and 7.8 over short-term incubations. Furthermore, the thermal history of corals measured in the field had a significant effect on pH-susceptibility between summer and winter seasons. Results revealed an synergistic affect of elevated temperature and low pH on photosynthesis. The disparity between results observed in vivo and in vitro suggest that the cnidarian host may be considered the more vulnerable partner to changes in pH leading to photosynthetic inhibition and symbiotic dysfunction.
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