Symbiont diversity and coral bleaching: An antioxidant view on thermal stress

The functioning of coral reef systems, as biodiversity hotspots, is largely dependent on the symbiotic association between dinoflagellate symbionts (Symbiodinium spp.) and scleractinian coral hosts. The breakdown of this symbiosis (coral bleaching), as a result of global warming and other stressors, therefore has profound implications for the tropical marine environment. Corals associate with a variety of Symbiodinium genotypes, and it is this mosaic nature that contributes to the variable stress thresholds of corals. Research over the past 25 years has established that the generation and scavenging of reactive oxygen species (ROS) in both partners, under light and thermal stress, is a fundamental element of the bleaching response. However, while the existence of more thermally susceptible and tolerant symbiont types has been recognized, the differences in the antioxidant systems that may accompany these properties have received less attention. The purpose of this thesis was to explore the role of the antioxidant network in explaining the different thermal susceptibilities of various symbiont types and how the activity of key antioxidants in both partners under thermal stress relates to bleaching patterns in different corals. Thus, the specific objectives were to: (1) assess the antioxidant network response in different Symbiodinium types; (2) investigate the activity and structural diversity of key enzymatic antioxidants in different Symbiodinium types; (3) examine the regulation of these antioxidants at the transcriptomic and proteomic levels; and (4) contrast the symbiont’s and host’s antioxidant responses under bleaching conditions. Symbiodinium types in culture were found to differ significantly with regards to the concentration and activity of specific antioxidants, exhibiting magnitude scale differences in some of them. However, the response of the main removal pathway, involving superoxide dismutase (SOD) and ascorbate peroxidase (APX), under lethal thermal stress was fairly similar. Instead, the typespecific differences were found to lie in more downstream systems, and particularly in those associated with the maintenance of the glutathione redox state. A declining glutathione redox state was the common feature of the three thermally susceptible Symbiodinium types: B1, C1, and E. Indeed, in comparison to the most sensitive type (B1), the tolerant type F1 exhibited stronger antioxidant up-regulation and the successful preservation of the highly reduced glutathione pool. Comparing antioxidant gene orthologues from members of different Symbiodinium clades (A-E) revealed a higher degree of sequence variation at the amino acid level for peroxidases, which reflected the genetic radiation of the genus. In contrast, primary defences in the form of SOD isoforms were highly conserved. Sequence variations between Symbiodinium types involved residues that constitute binding sites of substrates and co-factors, and therefore likely affect the catalytic properties of these enzymes. While expression of antioxidant genes was successfully measured in Symbiodinium B1, it was not possible to assess the link between transcriptomic expression and proteomic activity due to high variability in expression between replicates, and little response in their enzymatic activity over three days. In contrast to previous findings, up-regulation of antioxidant defences was not evident in Symbiodinium cells inside the host (i.e. in hospite). In fact, oxidative stress in the thermally sensitive corals Acropora millepora and Pocillopora damicornis was only apparent from increased host catalase activity, which interestingly preceded photosynthetic dysfunction of their symbionts. Baseline antioxidant activities of thermally tolerant and susceptible host species showed no differences, though the scavenging activities of the hosts were considerably higher than those of the symbionts. Baseline activities for the symbionts were different, however, with Symbiodinium C15 from the thermally tolerant coral Montipora digitata exhibiting the lowest activities for SOD and catalase peroxidase. This thesis provides significant findings with respect to the variability in antioxidant activity, structure, and network response in different Symbiodinium types in culture, and how these relate to thermal tolerance. What effect these differences have on the response in the intact symbiosis remains unclear, however, as the findings contradict the classic bleaching model of photoinhibition and symbiont-derived ROS. I argue, using previously published data, that heating rates might profoundly affect the way we perceive the antioxidant response of both partners to thermal stress, and that host antioxidant defences might not be as easily overwhelmed by symbiont ROS as suggested previously. This thesis reports important findings on the antioxidant system in different Symbiodinium types, but also raises new questions about the antioxidant response of the intact coral.