Tolerance Of Corals Research Articles

Species–specific interactions between algal endosymbionts and coral hosts define their bleaching response to heat and light stress

The impacts of warming seas on the frequency and severity of bleaching events are well documented, but the potential for different Symbiodinium types to enhance the physiological tolerance of reef corals is not well understood. Here we compare the functionality and physiological properties of juvenile corals when experimentally infected with one of two homologous Symbiodinium types and exposed to combined heat and light stress. A suite of physiological indicators including chlorophyll a fluorescence, oxygen production and respiration, as well as pigment concentration consistently demonstrated lower metabolic costs and enhanced physiological tolerance of Acropora tenuis juveniles when hosting Symbiodinium type C1 compared with type D. In other studies, the same D-type has been shown to confer higher thermal tolerance than both C2 in adults and C1 in juveniles of the closely related species Acropora millepora. Our results challenge speculations that associations with type D are universally most robust to thermal stress. Although the heat tolerance of corals may be contingent on the Symbiodinium strain in hospite, our results highlight the complexity of interactions between symbiotic partners and a potential role for host factors in determining the physiological performance of reef corals.

A community change in the algal endosymbionts of a scleractinian coral following a natural bleaching event: field evidence of acclimatization.

The symbiosis between reef-building corals and their algal endosymbionts (zooxanthellae of the genus Symbiodinium) is highly sensitive to temperature stress, which makes coral reefs vulnerable to climate change. Thermal tolerance in corals is known to be substantially linked to the type of zooxanthellae they harbour and, when multiple types are present, the relative abundance of types can be experimentally manipulated to increase the thermal limits of individual corals. Although the potential exists for this to translate into substantial thermal acclimatization of coral communities, to date there is no evidence to show that this takes place under natural conditions. In this study, we show field evidence of a dramatic change in the symbiont community of Acropora millepora, a common and widespread Indo-Pacific hard coral species, after a natural bleaching event in early 2006 in the Keppel Islands (Great Barrier Reef). Before bleaching, 93.5% (n=460) of the randomly sampled and tagged colonies predominantly harboured the thermally sensitive Symbiodinium type C2, while the remainder harboured a tolerant Symbiodinium type belonging to clade D or mixtures of C2 and D. After bleaching, 71% of the surviving tagged colonies that were initially C2 predominant changed to D or C1 predominance. Colonies that were originally C2 predominant suffered high mortality (37%) compared with D-predominant colonies (8%). We estimate that just over 18% of the original A. millepora population survived unchanged leaving 29% of the population C2 and 71% D or C1 predominant six months after the bleaching event. This change in the symbiont community structure, while it persists, is likely to have substantially increased the thermal tolerance of this coral population. Understanding the processes that underpin the temporal changes in symbiont communities is key to assessing the acclimatization potential of reef corals.

High-sediment tolerance in the reef coral Turbinaria mesenterina from the inner Great Barrier Reef lagoon (Australia)

Sedimentation is an important stressor on coral reefs subjected to run-off, dredging and resuspension events. Reefs with a history of high-sediment loads tend to be dominated by a few tolerant coral species. A key question is whether such species live close to their tolerance thresholds or near their niche optima. Here, we analyse experimentally the sediment tolerance of a spatially dominant coral, Turbinaria mesenterina (Dendrophylliidae), at nearshore reefs in the central Great Barrier Reef lagoon. Testing was conducted in a 5-week tank experiment under manipulated sediment loading and flow conditions. Physiological stress was assessed based on the behaviour of three key response variables: skeletal growth rate, energy reserves (lipid content) and photosynthetic performance. Because sediment effects are likely to vary between flow regimes, sediment and flow responses were tested using a full factorial design. Sediment loads greater than 110 mg cm −2 had no effect on any of the physiological variables, regardless of flow (0.7–24 cm s −1). Turbinaria mesenterina is thus tolerant to sediment loads an order of magnitude higher than most severe sediment conditions in situ. Likely mechanisms for such tolerance are that: (1) colonies covered in sediment (60–120 μm) in low-flow were able to clear themselves rapidly (within 4–5 h); and (2) sediment provides a source of food. These results suggest that intensified sediment regimes on coastal reefs may shift coral communities towards dominance by a few well-adapted species.

Factors Affecting the Evolution of Bleaching Resistance in Corals

We present a mathematical model of coevolutionary interactions between partners in a coral-algae mutualistic symbiosis. Our goal is to better understand factors affecting the potential evolution of bleaching resistance in corals in response to increased average sea temperatures. We explore the evolutionary consequences of four factors: (i) trade-offs among fitness components, (ii) different proximate mechanisms of coral bleaching, (iii) the genetic determination of bleaching resistance, and (iv) the mode of sexual reproduction. We show that traits in mutualistic symbioses, such as thermal tolerance in corals, are potentially subject to novel kinds of evolutionary constraints and that these constraints are mediated by ecological dynamics. We also show that some proximate mechanisms of bleaching yield faster evolutionary responses to temperature stress and that the nature of interspecific control of bleaching resistance and the mode of sexual reproduction interact to strongly influence the rate of spread of resistance alleles. These qualitative theoretical results highlight important future directions for empirical research in order to quantify the potential for coral reefs to evolve resistance to thermal stress.

The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for coral reefs in an era of climate change

The ability of coral reefs to survive the projected increases in temperature due to global warming will depend largely on the ability of corals to adapt or acclimatize to increased temperature extremes over the next few decades. Many coral species are highly sensitive to temperature stress and the number of stress (bleaching) episodes has increased in recent decades. We investigated the acclimatization potential of Acropora millepora, a common and widespread Indo-Pacific hard coral species, through transplantation and experimental manipulation. We show that adult corals, at least in some circumstances, are capable of acquiring increased thermal tolerance and that the increased tolerance is a direct result of a change in the symbiont type dominating their tissues from Symbiodinium type C to D. Our data suggest that the change in symbiont type in our experiment was due to a shuffling of existing types already present in coral tissues, not through exogenous uptake from the environment. The level of increased tolerance gained by the corals changing their dominant symbiont type to D (the most thermally resistant type known) is around 1-1.5 degrees C. This is the first study to show that thermal acclimatization is causally related to symbiont type and provides new insight into the ecological advantage of corals harbouring mixed algal populations. While this increase is of huge ecological significance for many coral species, in the absence of other mechanisms of thermal acclimatization/adaptation, it may not be sufficient to survive climate change under predicted sea surface temperature scenarios over the next 100 years. However, it may be enough to 'buy time' while greenhouse reduction measures are put in place.

Flow reorganizations in the Panama Seaway: A cause for the demise of Miocene corals?

A large number of coral species in the Caribbean Sea disappeared in the early Miocene. Based on climate model simulations, we propose that this regional extinction is related to a flow reorganization in the Panama Seaway. It is shown that large‐scale tectonic events, such as the opening of Drake Passage and the closing of the Tethys Seaway, reorganized the global ocean currents to induce a reversal of the volume transport between Atlantic and Pacific. This flow reversal, which is timed near the Oligocene‐Miocene boundary, allowed cool Pacific waters to enter the Caribbean Sea possibly affecting the habitat of warm water tolerant coral species.

Photoinhibition of photosynthesis is reduced by water flow in the reef-building coral Acropora digitifera

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 301:109-118 (2005) - doi:10.3354/meps301109 Photoinhibition of photosynthesis is reduced by water flow in the reef-building coral Acropora digitifera T. Nakamura1, R. van Woesik2,*, H. Yamasaki1 1Center of Molecular Biosciences, University of the Ryukyus, Senbaru 1, Nishihara, Okinawa 903-0213, Japan2Department of Biological Sciences, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901-6988, USA *Corresponding author. Email: rvw@fit.edu ABSTRACT: While photosynthesis of symbiotic algae is essential for reef-building corals, excess irradiance inhibits photosynthesis through photoinhibition, which can lead to coral bleaching under elevated temperature conditions. Here we show that water flow reduces photoinhibition of in hospite endosymbionts in the coral Acropora digitifera. Diurnal monitoring of chlorophyll fluorescence, under 2 different flow regimes (<3 and 20 cm s–1 flow rates) in an outdoor aquarium, showed reduced photoinhibition, but only under moderate flow conditions (20 cm s–1). Experimental (laboratory) measurements, on time scales ranging from minutes to hours, showed that flow-mediated reductions in photoinhibition occurred not by enhancing recovery of the damaged photosystem, but rather through inducing differential photodamage. Moreover, experiments involving sequential light oscillations (500/20 and 1000/20 µmol photons m–2 s–1) at 3 flow regimes, <3, 10, and 20 cm s–1, on a time scale ranging from hours to days, revealed water-velocity-dependent reductions of dynamic photoinhibition. These results, on time scales ranging from minutes to weeks, confirm that reduced water flow amplifies photodamage of algal photosynthesis under strong irradiance, which in turn affects coral tolerance to strong irradiance and temperature. KEY WORDS: Coral bleaching · Photoinhibition · Water-flow rates · Passive diffusion Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 301. Online publication date: October 11, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Coral bleaching — capacity for acclimatization and adaptation

Coral bleaching, i.e., loss of most of the symbiotic zooxanthellae normally found within coral tissue, has occurred with increasing frequency on coral reefs throughout the world in the last 20 years, mostly during periods of El Nino Southern Oscillation (ENSO). Experiments and observations indicate that coral bleaching results primarily from elevated seawater temperatures under high light conditions, which increases rates of biochemical reactions associated with zooxanthellar photosynthesis, producing toxic forms of oxygen that interfere with cellular processes. Published projections of a baseline of increasing ocean temperature resulting from global warming have suggested that annual temperature maxima within 30 years may be at levels that will cause frequent coral bleaching and widespread mortality leading to decline of corals as dominant organisms on reefs. However, these projections have not considered the high variability in bleaching response that occurs among corals both within and among species. There is information that corals and their symbionts may be capable of acclimatization and selective adaptation to elevated temperatures that have already resulted in bleaching resistant coral populations, both locally and regionally, in various areas of the world. There are possible mechanisms that might provide resistance and protection to increased temperature and light. These include inducible heat shock proteins that act in refolding denatured cellular and structural proteins, production of oxidative enzymes that inactivate harmful oxygen radicals, fluorescent coral pigments that both reflect and dissipate light energy, and phenotypic adaptations of zooxanthellae and adaptive shifts in their populations at higher temperatures. Such mechanisms, when considered in conjunction with experimental and observational evidence for coral recovery in areas that have undergone coral bleaching, suggest an as yet undefined capacity in corals and zooxanthellae to adapt to conditions that have induced coral bleaching. Clearly, there are limits to acclimatory processes that can counter coral bleaching resulting from elevated sea temperatures, but scientific models will not accurately predict the fate of reef corals until we have a better understanding of coral-algal acclimatization/adaptation potential. Research is particularly needed with respect to the molecular and physiological mechanisms that promote thermal tolerance in corals and zooxanthellae and identification of genetic characteristics responsible for the variety of responses that occur in a coral bleaching event. Only then will we have some idea of the nature of likely responses, the timescales involved and the role of 'experience' in modifying bleaching impact.

Reef-Building Corals—Symbiotic Autotrophic Organisms: 2. Pathways and Mechanisms of Adaptation to Light

Ways, mechanisms, and responses of ontogenetic adaptation of reef-building corals to light are discussed on the basis of original and literature data. The possible ways of photoacclimation of corals within the range of tolerance to light are shown: (1) adaptation to bright light (>70% incident photosynthetic active radiation, PARs), (2) adaptation to moderate shade (50–10% PARs), and (3) adaptation to extremely low light (<5% PARs). In each of the ways, general and specific mechanisms and reactions are involved in photoacclimation of corals. Adaptive changes take place in plant (zooxanthellae) and animal (polyps) components of the symbiotic organism. They involve morphology, physiology, and biochemistry of colonial polyps and their zooxanthellae. During changes of the light regime, some adaptive reactions last several months and others occur within a few days. Some physiological and biochemical alterations occur as early as the next day after the light regime changes. The wide range of light tolerance of corals and a great number of mechanisms and reactions of photoacclimation allowing corals to adapt to bright and low light with minimum losses in their metabolic activity give grounds to classify them as a single ecological group of light- and shade-tolerant organisms.

Global climate change and the thermal tolerance of corals

The thermal environment of reef-building corals is critical to their distribution and survival. Recent evidence has revealed that the mean global temperature has increased by 1 degree Celsius over the past century. Examination of tropical sea temperatures reveal that they have also increased by almost 1°C over the past 100 years and are currently increasing at the rate of approximately 1-2°C per century. Japanese waters are no exception. Waters off Okinawa have increased by 1.4°C since 1930 and are currently increasing at the rate of 2.3°C per century (95% confidence interval, 1.4-3.1°C). Coral bleaching occurs when the thermal tolerance of corals and their zooxanthellae are exceeded, probably due to an increased sensitivity of the zooxanthellae to chronic photoinhibition. The consequences of bleaching can be devastating, with corals dying in vast numbers and such aspects as coral reproduction being severely curtailed. In 1998, the world's coral reefs experienced the worst bleaching on record. Japanese reefs were severely effected, with bleaching coinciding with a massive temperature anomaly in July-August 1998. Vast numbers of corals died. The intention of this paper is collect what we know about coral bleaching from biochemical, physiological and ecological perspectives and to discuss to how reefs (especially around Okinawa) might change in the next century. A single important issue surfaces in these deliberations. If temperatures continue to increase, then corals will experience greater incidences of bleaching and mortality unless they can acclimate physiologically or adapt genetically. The conclusions that stem from this issue are of great concern. Firstly, available evidence suggests that corals are not acclimating physiologically to any really extent to the sporadic and seasonal changes in sea temperature. Secondly, the rate of change in sea temperature arguably exceeds the capacity of coral populations to genetically adapt fast enough. The third conclusion is the most worrying. If corals are not changing fast enough, then coral reefs will experience more frequent and more intense bleaching. Given the outcome for many coral reefs across the planet during the 1998 episodes of bleaching, this has to be of great concern to coral reef users and managers everywhere.

Stable isotope geochemistry of corals from Costa Rica as proxy indicator of the EL Niño/southern Oscillation (ENSO)

We analyzed the δ 18O and δ 13C time-series contained in coral skeletons collected from Isla del Caño, Costa Rica, that survived the 1982–1983 El Niño warming event. Coral δ 18O give a record of thermal histories with a precision of ~0.5°C. For this locality, we have determined that the average Δδ w effect in the coral skeletons is equivalent to 33% of the skeletal δ 18O range. Therefore, if δ w effects are not compensated for, the annual skeletal-δ 18O range displays a temperature range of 1 °C lower than actual values. The isotopic record of Porites lobata skeletons shows simultaneous depletions in 18O and 13C at skeletal levels corresponding to 1983, coincident with the El Niño/Southern Oscillation (ENSO) event. Therefore, the El Niño event is not only recorded as negative δ 18O anomalies in the skeleton, suggesting the warming of ambient waters, but also in the δ 13C signal as negative anomalies, indicating coral bleaching. Contrary to the predictions of the “ 13C-insolation model” that the annual carbon isotope variation should be attenuated with depth in proportion to the decrease in light-intensity variation with depth, we found a clear trend where Δδ 13C increases with depth. Coral bioenergetics, which depends on both coral physiology and ecology, may adequately explain the unexpected increase in δ 13C range with depth, without contradicting the seasonal character of δ 13C variability with the solar irradiance cycle. Although some authors have determined the presence of hiatuses in the skeletal record due to severe stress and growth cessation, in this study it has been found that skeletal growth was not seriously diminished during the El Niño year of 1983. We attribute the good agreement of the δ 18O record with the timing and magnitude of the El Niño 1982–1983 warming event to the nature of the warming trend in the area, as well as to some synergistic implications of coral tolerance to thermal stress.

An evaluation of the tolerance of corals to nutrients and related water quality characteristics

The effects of nutrient enrichment on coral reef systems have received little attention compared to the effects of enrichment on freshwater systems. In this present investigation the available data on the general characteristics and effects of nutrient enrichment in coral reef ecosystems have been collated and evaluated. Relationships among coral growth rate, nutrient levels and associated water quality parameters were used to quantitatively evaluate the effects of nutrient enrichment. To illustrate the utility of this approach, tolerances have been calculated for corals from the Great Barrier Reef, Australia. This indicates that phosphate levels, together with BOD and suspended particulate matter concentrations are sensitive indicators of nutrient enrichment.

Water Quality and Management in the Great Barrier Reef Marine Park

Use of the Great Barrier Reef Region for tourism, the economic value of Reef tourism to Queensland, and the value placed by society on natural settings has been increasing rapidly during the 1980's. The Great Barrier Reef Marine Park Authority has the role of providing for reasonable use of this valuable resource. The concern for reefal water quality is discussed in terms of enhanced nutrient levels in the inshore GBR and the low tolerance of corals to nutrients. Findings of a recent Workshop on Nutrients in the Great Barrier Reef Region are described. This paper summarises the Authority's role in ensuring information exchange, appropriate research, and management in the area of water quality management and tourism.

Experimental investigation of UV tolerance in hermatypic corals (Scleractinia)

Effects of artificial radiation on various species of coral from the Great Barrier Reef were studied. The corals were either exposed to artificial white light during and after UV-treatment or were kept In darkness subsequent to UV exposure. When not exposed to white light, the UV tolerance of corals taken from depths of up to 1.5 m was, on average, double that of those collected at 18 to 20 m. Under the ~nfluence of short wavelength visible light (max. effectiveness 450 to 500 nm) , UV tolerance increased on average by a factor of 6. It is postulated that protection from UV light involves at least 2 different physiolog~cal mechanisms: screening by UV-absorbing compounds and recovery due to photoreact~vation. The most conspicuous symptom of serious radiation damage to polyps was extrusion of mesenterial filaments through the mouth opening and entire body surface Slmllar but less pronounced reactions could be induced in non-irradiated polyps by exposing then1 to rad~ation-damaged polyps, In particular when the unexposed polyps were a different species. Such interactions never occurred, however, between polyps from the same colony. It is therefore conjectured that there is a specific protective mechanism that impedes the spread of secondary damage from damaged polyps to others in the same colony

Reef Coral Tolerance of High Temperatures and Salinities

Reef Coral Tolerance of High Temperatures and Salinities