Coral reef adaptation
From ClimateWiki
From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
Considering the many threats to the health of coral reefs in today’s world of extensive socioeconomic and environmental change, how can these incomparable repositories of underwater biodiversity be expected to escape irreversible bleaching and death? In response to this question, Glynn (1996) pointed out that “numerous reef-building coral species have endured three periods of global warming, from the Pliocene optimum (4.3-3.3 million years ago) through the Eemian interglacial (125 thousand years ago) and the mid-Holocene (6000-5000 years ago), when atmospheric CO2 concentrations and sea temperatures often exceeded those of today.” In fact, Glynn observed that “an increase in sea warming of less than 2°C would result in a greatly increased diversity of corals in certain high latitude locations.”
How does this happen? Living organisms are resilient. Various lifeforms can tolerate temperatures from below freezing to the boiling point of water; others inhabit niches where light intensity varies from complete darkness to full sunlight. One reason for this great versatility is that, given time to adapt, nearly all living organisms can learn to survive in conditions well outside their normal zones of environmental tolerance. As noted by Gates and Edmunds (1999), results of numerous studies indicate that “corals routinely occupy a physically heterogeneous environment,” which “suggests they should possess a high degree of biological flexibility.” And indeed they do, as evidenced by their successful responses to the different threats that cause coral bleaching, which are examined in the following subsections.
Contents |
Response to Solar Radiation Stress
One example of adaptation to stress imposed by high solar irradiance comes from studies of corals that exhibit a “zonation” of their symbiont taxa with depth, where symbiont algae that are less tolerant of intense solar radiation grow on corals at greater depths below the ocean surface (Rowan and Knowlton, 1995; Rowan et al., 1997). It has also been demonstrated that zooxanthellae in corals possess a number of light-quenching mechanisms that can be employed to reduce the negative impacts of excess light (Hoegh-Guldberg and Jones, 1999; Ralph et al., 1999). Both the coral host and its symbionts also have the capacity to produce amino acids that act as natural “sunscreens” (Hoegh-Guldberg, 1999); and they can regulate their enzyme activities to enhance internal scavenging systems that remove noxious oxygen radicals produced in coral tissues as a result of high light intensities (Dykens and Shick, 1984; Lesser et al., 1990; Matta and Trench, 1991; Shick et al., 1996).
Another adaptive mechanism to lessen the stress of solar irradiance is coral tissue retraction, according to Brown et al. (1994), who studied the phenomenon in the scleractinian coral Coeloseris mayeri at coral reefs in Phuket, Thailand by examining the retraction and recovery of coral tissues over a tidal cycle. Results of their analysis showed that extreme tissue retraction was observed approximately 85 minutes after initial sub-aerial coral exposure. Tissue retraction, however, did not involve any reduction in chlorophyll concentration or algae symbiont abundance; the tissues expanded over the coral skeletons to pre-retraction conditions following the return of the tide. The adaptive benefits of tissue retraction, according to the authors, “include increased albedo, leading to a reduction in absorbed solar energy of 10%, … and possible avoidance of photochemical damage or photoinhibition at high solar irradiance.”
Another intriguing idea was proposed by Nakamura and van Woesik (2001), who upon evaluating the bleaching of large and small coral colonies along the western coast of Okinawa, Japan during the summers of 1998 and 2001, argued that small coral colonies should survive thermal and light stress more readily than large coral colonies based on mass transfer theory, which suggests that rates of passive diffusion are more rapid for small colonies than for large colonies. Still another reason why large coral colonies suffer more than small colonies during environmental conditions conducive to bleaching is the fact that small Acropora recruits, according to Bena and van Woesik (2004), “contain high concentrations of fluorescent proteins (Papina et al., 2002), which have photoprotective properties (Salih et al., 2000),” and they note that “a high concentration of photoprotective pigments in early life, when planulae are near the surface and as newly settled recruits, may facilitate survival during this phase as well as during stress events involving both high irradiance and thermal anomalies (van Woesik, 2000).”
In addition to the adaptive phenomena described above, the earth appears to possess a natural “heat vent” over the tropics that suppresses the intensity of solar radiation to which corals are exposed whenever dangerously high water temperatures are approached. According to Hoegh-Guldberg (1999), 29.2°C is the threshold water temperature above which significant bleaching can be expected to occur in many tropical corals. However, as Sud et al. (1999) have demonstrated, deep atmospheric convection is typically initiated whenever sea surface temperatures (SSTs) reach a value of about 28°C, so that an upper SST on the order of 30°C is rarely exceeded. As SSTs reach 28-29°C, the cloud-base airmass is charged with sufficient moist static energy for the clouds to reach the upper troposphere. At this point, the billowing cloud cover reduces the amount of solar radiation received at the surface of the sea, while cool and dry downdrafts produced by the moist convection tend to promote ocean surface cooling by increasing sensible and latent heat fluxes at the air-sea interface that cause temperatures there to decline. This “thermostat-like control,” as Sud et al. describe it, tends “to ventilate the tropical ocean efficiently and help contain the SST between 28-30°C,” which is essentially a fluctuating temperature band of ±1°C centered on the bleaching threshold temperature of 29.2°C identified by Hoegh-Guldberg.
Some other intriguing observations also point to the existence of a natural phenomenon of this nature. Satheesh and Ramanathan (2000), for example, determined that polluted air from south and southeast Asia absorbs enough solar radiation over the northern Indian Ocean during the dry monsoon season to heat the atmosphere there by 1-3°C per day at solar noon, thereby greatly reducing the intensity of solar radiation received at the surface of the sea. Ackerman et al. (2000), however, calculated that this atmospheric heating would decrease cloud-layer relative humidity and reduce boundary-layer mixing, thereby leading to a 25 percent to 50 percent drop in daytime cloud cover relative to that of an aerosol-free atmosphere, which could well negate the surface cooling effect suggested by the findings of Satheesh and Ramanathan. But in a test of this hypothesis based on data obtained from the Extended Edited Cloud Report Archive, Norris (2001) determined that daytime low-level ocean cloud cover (which tends to cool the water surface) not only did not decrease from the 1950s to 1990s, it actually increased … in both the Northern and Southern Hemispheres and at essentially all hours of the day.
Commenting on this finding, Norris remarked that “the observed all-hours increase in low-level cloud cover over the time period when soot aerosol has presumably greatly increased argues against a dominant effect of soot solar absorption contributing to cloud ‘burn-off’.” Hence, he says, “other processes must be compensating,” one of which, we suggest, could be the one described by Sud et al.
Another process is the “adaptive infrared iris” phenomenon described by Lindzen et al. (2001). Working with upper-level cloudiness data obtained from the Japanese Geostationary Meteorological Satellite and SST data obtained from the National Centers for Environmental Prediction, the atmospheric scientists found a strong inverse relationship between upper-level cloud area and the mean SST of cloudy regions, such that the area of cirrus cloud coverage (which tends to warm the planet) normalized by a measure of the area of cumulus coverage (which tends to cool the planet) decreased about 22 percent for each 1°C increase in the SST of the cloudy regions.
“Essentially,” in the words of the scientists, “the cloudy-moist region appears to act as an infrared adaptive iris that opens up and closes down the regions free of upper-level clouds, which more effectively permit infrared cooling, in such a manner as to resist changes in tropical surface temperatures.” So substantial is this phenomenon, Lindzen et al. are confident it could “more than cancel all the positive feedbacks in the more sensitive current climate models,” which are routinely used to predict the climatic consequences of projected increases in atmospheric CO2 concentration.
Is there any real-world evidence the natural thermostat discovered by Sud et al. and Lindzen et al.. has actually been instrumental in preventing coral bleaching? Mumby et al. (2001) examined long-term meteorological records from the vicinity of the Society Islands, which provide what they call “the first empirical evidence that local patterns of cloud cover may influence the susceptibility of reefs to mass bleaching and subsequent coral mortality during periods of anomalously high SST.” With respect to the great El Niño of 1998, Mumby and his colleagues determined that SSTs in the Society Islands sector of French Polynesia were above the 29.2°C bleaching threshold for a longer period of time (two months) than in all prior bleaching years of the historical record. However, mass coral bleaching, which was extensive in certain other areas, was found to be “extremely mild in the Society Islands” and “patchy at a scale of 100s of km.” What provided the coral relief from extreme sun and heat? As Mumby and his associates describe it, “exceptionally high cloud cover significantly reduced the number of sun hours during the summer of 1998,” much as one would have expected earth’s natural thermostat to have done in the face of such anomalously high SSTs. The marine scientists also note that extensive spotty patterns of cloud cover, besides saving most of the coral they studied, “may partly account for spatial patchiness in bleaching intensity and/or bleaching-induced mortality in other areas.”
In conclusion, although the natural thermostat cannot protect all of earth’s corals from life-threatening bleaching during all periods of anomalously high SSTs, it apparently protects enough of them enough of the time to ensure that sufficiently large numbers of corals survive to perpetuate their existence, since living reefs have persisted over the eons in spite of the continuing recurrence of these ever-present environmental threats. And perhaps that is how it has always been, although there are currently a host of unprecedented anthropogenic forces of site-specific origin that could well be weakening the abilities of some species to tolerate the types of thermal and solar stresses they have successfully “weathered” in the past.
Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2 science.org/subject/c/bleachrespsolar.php.
Response to Temperature Stress
As living entities, corals are not only acted upon by the various elements of their environment, they also react or respond to them. And when changes in environmental factors pose a challenge to their continued existence, they sometimes take major defensive or adaptive actions to ensure their survival. A simple but pertinent example of one form of this phenomenon is thermal adaptation, which feature has been observed by several researchers to operate in corals.
Fang et al. (1997), for example, experimented with samples of the coral Acropora grandis that were taken from the hot water outlet of a nuclear power plant near Nanwan Bay, Taiwan. In 1988, the year the power plant began full operation, the coral samples were completely bleached within two days of exposure to a temperature of 33°C. Two years later, however, Fang et al. report that “samples taken from the same area did not even start bleaching until six days after exposure to 33°C temperatures.”
Similar findings have been reported by Middlebrook et al. (2008), who collected multiple upward-growing branch tips of the reef-building coral Acropora aspera from three large colonies at the southern end of Australia’s Great Barrier Reef and placed them on racks immersed in running seawater within four 750-liter tanks that were maintained at the mean local ambient temperature (27°C) and exposed to natural reef-flat summer daily light levels. Then, two weeks prior to a simulated bleaching event—where water temperature was raised to a value of 34°C for a period of six days—they boosted the water temperature in one of the tanks to 31°C for 48 hours, while in another tank they boosted it to 31°C for 48 hours one week before the simulated bleaching event. In the third tank they had no pre-heating treatment, while in the fourth tank they had no pre-heating nor any simulated bleaching event. And at different points throughout the study, they measured photosystem II efficiency, xanthophyll and chlorophyll a concentrations, and Symbiodinium densities.
Results of the study indicated that the symbionts of the corals that were exposed to the 48-hour pre-bleaching thermal stress “were found to have more effective photoprotective mechanisms,” including “changes in non-photochemical quenching and xanthophyll cycling,” and they further determined that “these differences in photoprotection were correlated with decreased loss of symbionts, with those corals that were not pre-stressed performing significantly worse, losing over 40% of their symbionts and having a greater reduction in photosynthetic efficiency,” whereas “pre-stressed coral symbiont densities were unchanged at the end of the bleaching.” In light of these findings, Middlebrook et al. say their study “conclusively demonstrates that thermal stress events two weeks and one week prior to a bleaching event provide significantly increased thermal tolerance to the coral holobiont, suggesting that short time-scale thermal adaptation can have profound effects on coral bleaching.”
Moving out of the laboratory and into the real world of nature, Adjeroud et al. (2005) initiated a monitoring program on 13 islands (eight atolls and five high volcanic islands) in four of the five archipelagoes of French Polynesia, with the goal of documenting the effects of natural perturbations on coral assemblages. For the period covered by their report (1992-2002), these reefs were subjected to three major coral bleaching events (1994, 1998, 2002) and three cyclones (1997), while prior to this period, the sites had experienced an additional seven bleaching events and 15 cyclones, as well as several Acanthaster planci outbreaks.
Results of the monitoring program revealed that the impacts of the bleaching events were variable among the different study locations. In their 10-year survey, for example, they observed three different temporal trends: “(1) ten sites where coral cover decreased in relation to the occurrence of major disturbances; (2) nine sites where coral cover increased, despite the occurrence of disturbances affecting seven of them; and (3) a site where no significant variation in coral cover was found.” In addition, they report that “an interannual survey of reef communities at Tiahura, Moorea, showed that the mortality of coral colonies following a bleaching event was decreasing with successive events, even if the latter have the same intensity (Adjeroud et al., 2002).”
Commenting on their and other researchers’ observations, the seven French scientists say the “spatial and temporal variability of the impacts observed at several scales during the present and previous surveys may reflect an acclimation and/or adaptation of local populations,” such that “coral colonies and/or their endosymbiotic zooxanthellae may be phenotypically (acclimation) and possibly genotypically (adaptation) resistant to bleaching events,” citing the work of Rowan et al. (1997), Hoegh-Guldberg (1999), Kinzie et al. (2001) and Coles and Brown (2003) in support of this conclusion.
Other researchers have also confirmed the phenomenon of thermal adaptation in coral reefs. Guzman and Cortes (2007) studied coral reefs of the eastern Pacific Ocean that “suffered unprecedented mass mortality at a regional scale as a consequence of the anomalous sea warming during the 1982-1983 El Niño.” At Cocos Island (5°32’N, 87°04’W), in particular, they found in a survey of three representative reefs, which they conducted in 1987, that remaining live coral cover was only 3 percent of what it had been prior to the occurrence of the great El Niño four years earlier (Guzman and Cortes, 1992). Based on this finding and the similar observations of other scientists at other reefs, they predicted that “the recovery of the reefs’ framework would take centuries, and recovery of live coral cover, decades.” In 2002, therefore, nearly 20 years after the disastrous coral-killing warming, they returned to see just how prescient they might have been after their initial assessment of the El Niño’s damage, quantifying “the live coral cover and species composition of five reefs, including the three previously assessed in 1987.” The two researchers report that overall mean live coral cover increased nearly five-fold, from 2.99 percent in 1987 to 14.87 percent in 2002, at the three sites studied during both periods, while the mean live coral cover of all five sites studied in 2002 was 22.7 percent. In addition, they found that “most new recruits and adults belonged to the main reef building species from pre-1982 ENSO, Porites lobata, suggesting that a disturbance as outstanding as El Niño was not sufficient to change the role or composition of the dominant species.”
With respect to the subject of thermal tolerance, however, the most interesting aspect of the study was the fact that a second major El Niño occurred between the two assessment periods; Guzman and Cortes state that “the 1997-1998 warming event around Cocos Island was more intense than all previous El Niño events,” noting that temperature anomalies “above 2°C lasted 4 months in 1997-1998 compared to 1 month in 1982-83.” Nevertheless, they report that “the coral communities suffered a lower and more selective mortality in 1997-1998, as was also observed in other areas of the eastern Pacific (Glynn et al., 2001; Cortes and Jimenez, 2003; Zapata and Vargas-Angel, 2003),” which is indicative of some type of thermal adaptation following the 1982-83 El Niño.
One year later in a paper published in Marine Biology, Maynard et al. (2008) described how they analyzed the bleaching severity of three genera of corals (Acropora, Pocillopora and Porites) via underwater video surveys of five sites in the central section of Australia’s Great Barrier Reef in late February and March of 1998 and 2002, while contemporary sea surface temperatures were acquired from satellite-based Advanced Very High Resolution Radiometer data that were calibrated to local ship- and drift buoy-obtained measurements, and surface irradiance data were obtained “using an approach modified from that of Pinker and Laszlo (1991).”
With respect to temperature, the four researchers report that “the amount of accumulated thermal stress (as degree heating days) in 2002 was more than double that in 1998 at four of the five sites,” and that “average surface irradiance during the 2002 thermal anomaly was 15.6-18.9% higher than during the 1998 anomaly.” Nevertheless, they found that “in 2002, bleaching severity was 30-100% lower than predicted from the relationship between severity and thermal stress in 1998, despite higher solar irradiances during the 2002 thermal event.” In addition, they found that the “coral genera most susceptible to thermal stress (Pocillopora and Acropora) showed the greatest increase in tolerance.”
In discussing their findings, Maynard et al. write that they are “consistent with previous studies documenting an increase in thermal tolerance between bleaching events (1982-1983 vs. 1997-1998) in the Galapagos Islands (Podesta and Glynn, 2001), the Gulf of Chiriqi, the Gulf of Panama (Glynn et al., 2001), and on Costa Rican reefs (Jimenez et al., 2001),” and they say that “Dunne and Brown (2001) found similar results to [theirs] in the Andaman Sea, in that bleaching severity was far reduced in 1998 compared to 1995 despite sea-temperature and light conditions being more conducive to widespread bleaching in 1998.”
As for the significance of these and other observations, the Australian scientists say that “the range in bleaching tolerances among corals inhabiting different thermal realms suggests that at least some coral symbioses have the ability to adapt to much higher temperatures than they currently experience in the central Great Barrier Reef,” citing the work of Coles and Brown (2003) and Riegl (1999, 2002). In addition, they note that “even within reefs there is a significant variability in bleaching susceptibility for many species (Edmunds, 1994; Marshall and Baird, 2000), suggesting some potential for a shift in thermal tolerance based on selective mortality (Glynn et al., 2001; Jimenez et al., 2001) and local population growth alone.” Above and beyond that, however, they say their results additionally suggest “a capacity for acclimatization or adaptation.”
In concluding their paper, Maynard et al. say “there is emerging evidence of high genetic structure within coral species (Ayre and Hughes, 2004),” which suggests, in their words, that “the capacity for adaptation could be greater than is currently recognized.” Indeed, as stated by Skelly et al. (2007), “on the basis of the present knowledge of genetic variation in performance traits and species’ capacity for evolutionary response, it can be concluded that evolutionary change will often occur concomitantly with changes in climate as well as other environmental changes.”
One adaptive mechanism that corals have developed to survive the thermal stress of high water temperature is to replace the zooxanthellae expelled by the coral host during a stress-induced bleaching episode by one or more varieties of zooxanthellae that are more heat tolerant, a phenomenon we describe in greater detail in the next section of our report. Another mechanism is to produce heat shock proteins that help repair heat-damaged constituents of their bodies (Black et al., 1995; Hayes and King, 1995; Fang et al., 1997). Sharp et al. (1997), for example, demonstrated that sub-tidal specimens of Goniopora djiboutiensis typically have much lower constitutive levels of a 70-kD heat shock protein than do their intertidal con-specifics; and they have shown that corals transplanted from sub-tidal to intertidal locations (where temperature extremes are greater and more common) typically increase their expression of this heat shock protein.
Similar results have been reported by Roberts et al. (1997) in field work with Mytilus californianus. In addition, Gates and Edmunds (1999) have observed an increase in the 70-kD heat shock protein after six hours of exposure of Montastraea franksi to a 2-3°C increase in temperature, which is followed by another heat shock protein increase at the 48-hour point of exposure to elevated water temperature. They state that the first of these protein increases “provides strong evidence that changes in protein turnover during the initial exposure to elevated temperature provides this coral with the biological flexibility to acclimatize to the elevation in sea water temperature,” and that the second increase “indicates another shift in protein turnover perhaps associated with an attempt to acclimatize to the more chronic level of temperature stress.”
How resilient are corals in this regard? No one knows for sure, but they’ve been around a very long time, during which climatic conditions have changed dramatically, from cold to warm and back again, over multiple glacial and interglacial cycles. Thermal adaptation by coral is a biological response that is overlooked or ignored by the IPCC.
Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/c/bleachresptemp.php.
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