THERMAL ADAPTION

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Coral Reefs (Responses to Temperature Stress) -- Summary
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 insure 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. (2008) 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 fifteen 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 ten-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% 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 horrendous damage, quantifying "the live coral cover and species composition of five reefs, including the three previously assessed in 1987."

So what did they find?

The two researchers report that overall mean live coral cover increased nearly five-fold, from 2.99% in 1987 to 14.87% 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%. 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; and 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 [our italics], 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." Consequently, it can be appreciated that if global warming were to start up again (it has been in abeyance for about the last decade), it need not spell the end for earth's highly adaptable corals.

But how is it done? How do corals adjust to rising temperatures? 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 (see Symbiont Shuffling). 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."

So 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. And in this regard, we see no reason why history cannot be expected to successfully repeat itself, even as the current interglacial experiences its "last hurrah."

source:
CO2 Science

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there was a discussion in reeftank that led to this article. Pretty nice and loooong read really but very good information to better understanding how nature work.