Living on the Edge: Protective Mechanisms Underlying Thermal Tolerance in High Latitude Symbiodinium spp.

The association between symbiotic dinoflagellates (Symbiodinium spp.) and corals extends to subtropical and temperate regions, where sea surface temperatures (SSTs) are generally lower than in the tropics and can vary substantially over the course of the year due to seasonal changes. These high latitude coral-dinoflagellate symbioses might be better able to withstand thermal variability and might be particularly well equipped to cope with lower SSTs compared to their tropical relatives. The aim of this thesis was to analyze the cellular mechanisms that underlie heat and/or cold tolerance in a range of reef-building corals (Acropora yongei, Acropora solitariensis, Isopora palifera, Pocillopora damicornis, Porites heronensis and Stylophora sp.), as well as the symbiotic sea anemone Entacmaea quadricolor. In particular, the study focussed on protective mechanisms in their dinoflagellate symbionts as a potential determinant of thermal sensitivity (i.e. bleaching) or resistance of the intact symbiosis. High latitude reef-building corals were analyzed at the world’s southernmost coral reef at Lord Howe Island, while E. quadricolor was sampled at the subtropical coral community at North Solitary Island; both sites are located in New South Wales, Australia. The specific objectives were to assess the roles of: (1) xanthophyll deepoxidation; (2) thylakoid fatty acid composition; (3) Symbiodinium superoxide dismutase (SOD) and ascorbate peroxidase (APX) activity; and (4) D1 repair on the photophysiology, bleaching susceptibility and survivorship of a range of high-latitude coral-Symbiodinium associations from Lord Howe Island when exposed to elevated or decreased temperature. Furthermore, I aimed to: (5) characterise Symbiodinium diversity in the anemone E. quadricolor on the west coast of Australia; and (6) measure the dynamics of Symbiodinium ITS2 populations and SOD activity in two E. quadricolor phenotypes (green and pink colour phenotypes) in response to elevated temperature. I showed that thermal responses in high latitude corals and their dinoflagellate symbionts are highly variable, depending on host species (or phenotype) and Symbiodinium genotype, and that the activation of protective mechanisms in Symbiodinium was not necessarily correlated with sub-lethal bleaching susceptibility or survivorship of their coral hosts. More specifically: (1) In response to short-term heat stress and cold stress, xanthophyll de-epoxidation increased in some but not all bleaching susceptible (e.g. P. damicornis) and bleaching tolerant (P. heronensis) corals; (2) overall unsaturated thylakoid fatty acids increased in symbionts of a bleaching tolerant coral association, yet was not correlated with PSII photochemical efficiency; and (3) SOD and APX activity remained unchanged in the majority of Symbiodinium types regardless of bleaching susceptibility of the coral host, but decreased in bleaching susceptible Pocillopora damicornis when exposed to short-term heat stress. Elevated temperatures resulted in enhanced D1 turnover in two warm-water bleaching susceptible Symbiodinium-host combinations; however a direct link between increased dependence on D1 turnover and bleaching susceptibility was not demonstrated. From the results obtained it seems unlikely that the specific cellular adaptations in Symbiodinium alone determine the tolerance of Lord Howe corals to thermal variations. In contrast, the results highlight the significance of the particular host-symbiont combination and it appears that the host is important in determining, at least in part, the thermal response of the coral. Additionally, this study revealed a high diversity of Symbiodinium ITS2 (internal transcribed spacer 2) types in E. quadricolor from five locations on the west coast of Australia. E. quadricolor predominantly associated with six types of clade C (four of which were novel) and most anemones harboured multiple types simultaneously. At North Solitary Island, anemones simultaneously harboured Symbiodinium C25 and C3.25 (a novel variant of C3). Experimentally, I showed that anemones shuffled the relative proportions of C25 and C3.25 in response to elevated temperature, but not in both anemone colour phenotypes analyzed. Furthermore, baseline photobiological characteristics were distinct in the two different anemone colour morphs but were not correlated with the ratio of Symbiodinium C25 to C3.25, suggesting that host mechanisms such as pigmentation were involved in regulating light utilization by the symbionts. My hypothesis that symbiont shuffling was related to SOD activity, as such that those symbionts with enhanced SOD activity and increased capability to scavenge superoxide anion would increase in relative abundance in response to short-term heat stress, could not be proved. In summary, this thesis provides detailed information on some key cellular mechanisms that could underpin thermal sensitivity and resistance in high latitude Symbiodinium, and most importantly highlights the significance of the host-symbiont combination in determining the response to thermal stress. The various mechanistic findings described here further our understanding of the coral bleaching process in general and particularly give insight into physiological and cellular responses to coldwater stress in reef-building corals at high-latitude sites. The results of this thesis indicate that in light of ongoing climate change, as episodes of cold-water and warm-water anomalies will become more frequent, branching corals such as Acropora yongei or Pocillopora damicornis and their symbionts will experience physiological stress more frequently than massive species such as Porites heronensis. This might have profound impacts on the long-term stability and species composition of high latitude coral reefs

Living on the Edge: Protective Mechanisms Underlying Thermal Tolerance in High Latitude Symbiodinium spp.

The association between symbiotic dinoflagellates (Symbiodinium spp.) and corals extends to subtropical and temperate regions, where sea surface temperatures (SSTs) are generally lower than in the tropics and can vary substantially over the course of the year due to seasonal changes. These high latitude coral-dinoflagellate symbioses might be better able to withstand thermal variability and might be particularly well equipped to cope with lower SSTs compared to their tropical relatives. The aim of this thesis was to analyze the cellular mechanisms that underlie heat and/or cold tolerance in a range of reef-building corals (Acropora yongei, Acropora solitariensis, Isopora palifera, Pocillopora damicornis, Porites heronensis and Stylophora sp.), as well as the symbiotic sea anemone Entacmaea quadricolor. In particular, the study focussed on protective mechanisms in their dinoflagellate symbionts as a potential determinant of thermal sensitivity (i.e. bleaching) or resistance of the intact symbiosis. High latitude reef-building corals were analyzed at the world’s southernmost coral reef at Lord Howe Island, while E. quadricolor was sampled at the subtropical coral community at North Solitary Island; both sites are located in New South Wales, Australia. The specific objectives were to assess the roles of: (1) xanthophyll deepoxidation; (2) thylakoid fatty acid composition; (3) Symbiodinium superoxide dismutase (SOD) and ascorbate peroxidase (APX) activity; and (4) D1 repair on the photophysiology, bleaching susceptibility and survivorship of a range of high-latitude coral-Symbiodinium associations from Lord Howe Island when exposed to elevated or decreased temperature. Furthermore, I aimed to: (5) characterise Symbiodinium diversity in the anemone E. quadricolor on the west coast of Australia; and (6) measure the dynamics of Symbiodinium ITS2 populations and SOD activity in two E. quadricolor phenotypes (green and pink colour phenotypes) in response to elevated temperature. I showed that thermal responses in high latitude corals and their dinoflagellate symbionts are highly variable, depending on host species (or phenotype) and Symbiodinium genotype, and that the activation of protective mechanisms in Symbiodinium was not necessarily correlated with sub-lethal bleaching susceptibility or survivorship of their coral hosts. More specifically: (1) In response to short-term heat stress and cold stress, xanthophyll de-epoxidation increased in some but not all bleaching susceptible (e.g. P. damicornis) and bleaching tolerant (P. heronensis) corals; (2) overall unsaturated thylakoid fatty acids increased in symbionts of a bleaching tolerant coral association, yet was not correlated with PSII photochemical efficiency; and (3) SOD and APX activity remained unchanged in the majority of Symbiodinium types regardless of bleaching susceptibility of the coral host, but decreased in bleaching susceptible Pocillopora damicornis when exposed to short-term heat stress. Elevated temperatures resulted in enhanced D1 turnover in two warm-water bleaching susceptible Symbiodinium-host combinations; however a direct link between increased dependence on D1 turnover and bleaching susceptibility was not demonstrated. From the results obtained it seems unlikely that the specific cellular adaptations in Symbiodinium alone determine the tolerance of Lord Howe corals to thermal variations. In contrast, the results highlight the significance of the particular host-symbiont combination and it appears that the host is important in determining, at least in part, the thermal response of the coral. Additionally, this study revealed a high diversity of Symbiodinium ITS2 (internal transcribed spacer 2) types in E. quadricolor from five locations on the west coast of Australia. E. quadricolor predominantly associated with six types of clade C (four of which were novel) and most anemones harboured multiple types simultaneously. At North Solitary Island, anemones simultaneously harboured Symbiodinium C25 and C3.25 (a novel variant of C3). Experimentally, I showed that anemones shuffled the relative proportions of C25 and C3.25 in response to elevated temperature, but not in both anemone colour phenotypes analyzed. Furthermore, baseline photobiological characteristics were distinct in the two different anemone colour morphs but were not correlated with the ratio of Symbiodinium C25 to C3.25, suggesting that host mechanisms such as pigmentation were involved in regulating light utilization by the symbionts. My hypothesis that symbiont shuffling was related to SOD activity, as such that those symbionts with enhanced SOD activity and increased capability to scavenge superoxide anion would increase in relative abundance in response to short-term heat stress, could not be proved. In summary, this thesis provides detailed information on some key cellular mechanisms that could underpin thermal sensitivity and resistance in high latitude Symbiodinium, and most importantly highlights the significance of the host-symbiont combination in determining the response to thermal stress. The various mechanistic findings described here further our understanding of the coral bleaching process in general and particularly give insight into physiological and cellular responses to coldwater stress in reef-building corals at high-latitude sites. The results of this thesis indicate that in light of ongoing climate change, as episodes of cold-water and warm-water anomalies will become more frequent, branching corals such as Acropora yongei or Pocillopora damicornis and their symbionts will experience physiological stress more frequently than massive species such as Porites heronensis. This might have profound impacts on the long-term stability and species composition of high latitude coral reefs.</p

Living on the Edge: Protective Mechanisms Underlying Thermal Tolerance in High Latitude Symbiodinium spp.

The association between symbiotic dinoflagellates (Symbiodinium spp.) and corals extends to subtropical and temperate regions, where sea surface temperatures (SSTs) are generally lower than in the tropics and can vary substantially over the course of the year due to seasonal changes. These high latitude coral-dinoflagellate symbioses might be better able to withstand thermal variability and might be particularly well equipped to cope with lower SSTs compared to their tropical relatives. The aim of this thesis was to analyze the cellular mechanisms that underlie heat and/or cold tolerance in a range of reef-building corals (Acropora yongei, Acropora solitariensis, Isopora palifera, Pocillopora damicornis, Porites heronensis and Stylophora sp.), as well as the symbiotic sea anemone Entacmaea quadricolor. In particular, the study focussed on protective mechanisms in their dinoflagellate symbionts as a potential determinant of thermal sensitivity (i.e. bleaching) or resistance of the intact symbiosis. High latitude reef-building corals were analyzed at the world’s southernmost coral reef at Lord Howe Island, while E. quadricolor was sampled at the subtropical coral community at North Solitary Island; both sites are located in New South Wales, Australia. The specific objectives were to assess the roles of: (1) xanthophyll deepoxidation; (2) thylakoid fatty acid composition; (3) Symbiodinium superoxide dismutase (SOD) and ascorbate peroxidase (APX) activity; and (4) D1 repair on the photophysiology, bleaching susceptibility and survivorship of a range of high-latitude coral-Symbiodinium associations from Lord Howe Island when exposed to elevated or decreased temperature. Furthermore, I aimed to: (5) characterise Symbiodinium diversity in the anemone E. quadricolor on the west coast of Australia; and (6) measure the dynamics of Symbiodinium ITS2 populations and SOD activity in two E. quadricolor phenotypes (green and pink colour phenotypes) in response to elevated temperature. I showed that thermal responses in high latitude corals and their dinoflagellate symbionts are highly variable, depending on host species (or phenotype) and Symbiodinium genotype, and that the activation of protective mechanisms in Symbiodinium was not necessarily correlated with sub-lethal bleaching susceptibility or survivorship of their coral hosts. More specifically: (1) In response to short-term heat stress and cold stress, xanthophyll de-epoxidation increased in some but not all bleaching susceptible (e.g. P. damicornis) and bleaching tolerant (P. heronensis) corals; (2) overall unsaturated thylakoid fatty acids increased in symbionts of a bleaching tolerant coral association, yet was not correlated with PSII photochemical efficiency; and (3) SOD and APX activity remained unchanged in the majority of Symbiodinium types regardless of bleaching susceptibility of the coral host, but decreased in bleaching susceptible Pocillopora damicornis when exposed to short-term heat stress. Elevated temperatures resulted in enhanced D1 turnover in two warm-water bleaching susceptible Symbiodinium-host combinations; however a direct link between increased dependence on D1 turnover and bleaching susceptibility was not demonstrated. From the results obtained it seems unlikely that the specific cellular adaptations in Symbiodinium alone determine the tolerance of Lord Howe corals to thermal variations. In contrast, the results highlight the significance of the particular host-symbiont combination and it appears that the host is important in determining, at least in part, the thermal response of the coral. Additionally, this study revealed a high diversity of Symbiodinium ITS2 (internal transcribed spacer 2) types in E. quadricolor from five locations on the west coast of Australia. E. quadricolor predominantly associated with six types of clade C (four of which were novel) and most anemones harboured multiple types simultaneously. At North Solitary Island, anemones simultaneously harboured Symbiodinium C25 and C3.25 (a novel variant of C3). Experimentally, I showed that anemones shuffled the relative proportions of C25 and C3.25 in response to elevated temperature, but not in both anemone colour phenotypes analyzed. Furthermore, baseline photobiological characteristics were distinct in the two different anemone colour morphs but were not correlated with the ratio of Symbiodinium C25 to C3.25, suggesting that host mechanisms such as pigmentation were involved in regulating light utilization by the symbionts. My hypothesis that symbiont shuffling was related to SOD activity, as such that those symbionts with enhanced SOD activity and increased capability to scavenge superoxide anion would increase in relative abundance in response to short-term heat stress, could not be proved. In summary, this thesis provides detailed information on some key cellular mechanisms that could underpin thermal sensitivity and resistance in high latitude Symbiodinium, and most importantly highlights the significance of the host-symbiont combination in determining the response to thermal stress. The various mechanistic findings described here further our understanding of the coral bleaching process in general and particularly give insight into physiological and cellular responses to coldwater stress in reef-building corals at high-latitude sites. The results of this thesis indicate that in light of ongoing climate change, as episodes of cold-water and warm-water anomalies will become more frequent, branching corals such as Acropora yongei or Pocillopora damicornis and their symbionts will experience physiological stress more frequently than massive species such as Porites heronensis. This might have profound impacts on the long-term stability and species composition of high latitude coral reefs

The distribution of intra-genomically variable dinoflagellate symbionts at Lord Howe Island, Australia

The symbiotic dinoflagellates of corals and other marine invertebrates (Symbiodinium) are essential to the development of shallow-water coral reefs. This genus contains considerable genetic diversity and a corresponding range of physiological and ecological traits. Most genetic variation arises through the accumulation of somatic mutations that arise during asexual reproduction. Yet growing evidence suggests that occasional sexual reproductive events also occur within, and perhaps between, Symbiodinium lineages, further contributing to the pool of genetic variation available for evolutionary adaptation. Intra-genomic variation can therefore arise from both sexual and asexual reproductive processes, making it difficult to discern its underlying causes and consequences. We used quantitative PCR targeting the ITS2 locus to estimate proportions of genetically homogeneous symbionts and intra-genomically variable Symbiodinium (IGV Symbiodinium) in the reef-building coral Pocillopora damicornis at Lord Howe Island, Australia. We then sampled colonies through time and at a variety of spatial scales to find out whether the distribution of these symbionts followed patterns consistent with niche partitioning. Estimated ratios of homogeneous to IGV Symbiodinium varied between colonies within sites (metres to tens of metres) and between sites separated by hundreds to thousands of metres, but remained stable within colonies through time. Symbiont ratios followed a temperature gradient, with the local thermal maximum emerging as a negative predictor for the estimated proportional abundance of IGV Symbiodinium. While this pattern may result from fine-scale spatial population structure, it is consistent with an increased susceptibility to thermal stress, suggesting that the evolutionary processes that generate IGV (such as inter-lineage recombination and the accumulation of somatic mutations at the ITS2 locus) may have important implications for the fitness of the symbiont and that of the coral host

Photoacclimatory and photoprotective responses to cold versus heat stress in high latitude reef corals

Corals at the world's southernmost coral reef of Lord Howe Island (LHI) experience large temperature and light fluctuations and need to deal with periods of cold temperature

Antioxidant plasticity and thermal sensitivity in four types of Symbiodinium sp.

Warmer than average summer sea surface temperature is one of the main drivers for coral bleaching, which describes the loss of endosymbiotic dinoflagellates (genus: Symbiodinium) in reef-building corals. Past research has established that oxidative stress in the symbiont plays an important part in the bleaching cascade. Corals hosting different genotypes of Symbiodinium may have varying thermal bleaching thresholds, but changes in the symbiont's antioxidant system that may accompany these differences have received less attention. This study shows that constitutive activity and up-regulation of different parts of the antioxidant network under thermal stress differs between four Symbiodinium types in culture and that thermal susceptibility can be linked to glutathione redox homeostasis. In Symbiodinium B1, C1 and E, declining maximum quantum yield of PSII (Fv/Fm) and death at 33°C were generally associated with elevated superoxide dismutase (SOD) activity and a more oxidized glutathione pool. Symbiodinium F1 exhibited no decline in Fv/Fm or growth, but showed proportionally larger increases in ascorbate peroxidase (APX) activity and glutathione content (GSx), while maintaining GSx in a reduced state. Depressed growth in Symbiodinium B1 at a sublethal temperature of 29°C was associated with transiently increased APX activity and glutathione pool size, and an overall increase in glutathione reductase (GR) activity. The collapse of GR activity at 33°C, together with increased SOD, APX and glutathione S-transferase activity, contributed to a strong oxidation of the glutathione pool with subsequent death. Integrating responses of multiple components of the antioxidant network highlights the importance of antioxidant plasticity in explaining type-specific temperature responses in Symbiodinium

Dal successo economico all'arcadia urbanizzata: i nuovi paesaggi del Veneto

Background: The diversity of the symbiotic dinoflagellate Symbiodinium sp., as assessed by genetic markers, is well established. To what extent this diversity is reflected on the amino acid level of functional genes such as enzymatic antioxidants that play an important role in thermal stress tolerance of the coral-Symbiodinium symbiosis is, however, unknown. Here we present a predicted structural analysis and phylogenetic characterization of the enzymatic antioxidant repertoire of the genus Symbiodinium. We also report gene expression and enzymatic activity under short-term thermal stress in Symbiodinium of the B1 genotype.\ud \ud Results: Based on eight different ITS2 types, covering six clades, multiple protein isoforms for three of the four investigated antioxidants (ascorbate peroxidase [APX], catalase peroxidase [KatG], manganese superoxide dismutase [MnSOD]) are present in the genus Symbiodinium. Amino acid sequences of both SOD metalloforms (Fe/Mn), as well as KatG, exhibited a number of prokaryotic characteristics that were also supported by the protein phylogeny. In contrast to the bacterial form, KatG in Symbiodinium is characterized by extended functionally important loops and a shortened C-terminal domain. Intercladal sequence variations were found to be much higher in both peroxidases, compared to SODs. For APX, these variable residues involve binding sites for substrates and cofactors, and might therefore differentially affect the catalytic properties of this enzyme between clades. While expression of antioxidant genes was successfully measured in Symbiodinium B1, it was not possible to assess the link between gene expression and protein activity due to high variability in expression between replicates, and little response in their enzymatic activity over the three-day experimental period.\ud \ud Conclusions: The genus Symbiodinium has a diverse enzymatic antioxidant repertoire that has similarities to prokaryotes, potentially as a result of horizontal gene transfer or events of secondary endosymbiosis. Different degrees of sequence evolution between SODs and peroxidases might be the result of potential selective pressure on the conserved molecular function of SODs as the first line of defence. In contrast, genetic redundancy of hydrogen peroxide scavenging enzymes might permit the observed variations in peroxidase sequences. Our data and successful measurement of antioxidant gene expression in Symbiodinium will serve as basis for further studies of coral health.\u

Differential coral bleaching—contrasting the activity and response of enzymatic antioxidants in symbiotic partners under thermal stress

Mass coral bleaching due to thermal stress represents a major threat to the integrity and functioning of coral reefs. Thermal thresholds vary, however, between corals, partly as a result of the specific type of endosymbiotic dinoflagellate (Symbiodinium sp.) they harbour. The production of reactive oxygen species (ROS) in corals under thermal and light stress has been recognised as one mechanism that can lead to cellular damage and the loss of their symbiont population (Oxidative Theory of Coral Bleaching). Here, we compared the response of symbiont and host enzymatic antioxidants in the coral species Acropora millepora and Montipora digitata at 28 degrees C and 33 degrees C. A. millepora at 33 degrees C showed a decrease in photochemical efficiency of photosystem II (PSII) and increase in maximum midday excitation pressure on PSII, with subsequent bleaching (declining photosynthetic pigment and symbiont density). M. digitata exhibited no bleaching response and photochemical changes in its symbionts were minor. The symbiont antioxidant enzymes superoxide dismutase, ascorbate peroxidase, and catalase peroxidase showed no significant upregulation to elevated temperatures in either coral, while only catalase was significantly elevated in both coral hosts at 33 degrees C Increased host catalase activity in the susceptible coral after 5 days at 33 degrees C was independent of antioxidant responses in the symbiont and preceded significant declines in PSII photochemical efficiencies. This finding suggests a potential decoupling of host redox mechanisms from symbiont photophysiology and raises questions about the importance of symbiont-derived ROS in initiating coral bleaching. (C) 2015 Elsevier Inc. All rights reserved