physiological mechanisms of acclimatization in tropical reef corals, The

The Physiological Mechanisms of Acclimatization in Tropical Reef Corals1

SYNOPSIS. The ability of scleractinian corals to survive changes that are predicted in the global environment over the next century will lie in their physiological mechanisms of acclimatization. Corals display rapid modifications in behavior, morphology and physiology enabling them to photoacclimate to changing light conditions, a scenario that demonstrates considerable biological flexibility. Here we argue that the acclimatization mechanisms in corals are fundamentally similar to those exhibited by other invertebrate taxa. We discuss protein metabolism as a mechanism underlying acclimatization responses in reef corals, and explore the relationship between protein turnover, metabolic rate, growth rate, and acclimatization capacity. Our preliminary analyses suggest that corals with low growth rates ((mu)Ca/mgN/h) and high metabolic rates ((mu)O^sub 2^/cm^sup 2^/hr), such as the massive species, acclimatize more effectively than those with high growth rates and low metabolic rates, a feature that is characteristic of branching species. We conclude that studies of protein turnover, combined with temporally relevant investigations into the dynamic aspects of coral dinoflagellate symbioses will provide considerable insight into why corals exhibit such a high level of variation in response to the same environmental challenge. Furthermore, a more detailed understanding of acclimatization mechanisms is essential if we are to predict how a coral assemblage will respond to present and future environmental challenges.

INTRODUCTION

There is concern regarding the ability of scleractinian corals and reef communities to tolerate changes in the global environment predicted to result from CO2 emissions and global warming. In the marine environment, these changes are likely to be manifest as increases in surface sea water temperatures, intensities of ultraviolet radiation and rates of sea-level rise, and a decrease in aragonite saturation state (Gattuso et al., 1999; Pittock, 1999). The intensity and frequency of El Nino/Southern Oscillation events (ENSO) and tropical cyclones may increase, resulting in riverine flooding and regional incidents of lower salinity, elevated turbidity and high nutrient levels (Pittock, 1999).

To gain insight into the responses of coral reef communities to these predicted environmental challenges, it is necessary to assess how such changes might impact the physiology of the organisms within the reef, particularly the hermatypic corals. The organismic response to physical change is affected by both the magnitude of the change and the rate at which the environment shifts. The present predictions forecast major changes in the global environment on a temporal scale of decades to centuries (Pittock, 1999) and, as such, the ability of individual corals to tolerate the changing conditions will lie in their physiological mechanisms of acclimatization. Acclimatization refers to compensatory changes in the metabolism of an organism exposed to multiple natural variations in the environment, such as those resulting from changing seasons (Prosser, 1973). Thus, acclimatization represents the physiological response of an organism to a suite of co-occurring, and temporally variable environmental stimuli which range from gradual to abrupt in onset and from mild to acute in magnitude and/ or duration. Given that the frequency of sexual reproduction in reef corals is greater than the predicted temporal scale of global climate change, adaptation will also play a fundamental role in determining how coral populations will respond to changing environmental conditions, a subject that is discussed in detail elsewhere (Kinzie, 1999; Lasker and Coffroth, 1999).

While the persistence of many coral species through geologic time provides compelling evidence that they can adapt to a changing global environment (Veron, 1995), there are only a few examples that demonstrate corals rapidly acclimatize to changes in their physical environment in the present. The best documented of these is photoacclimation, where modifications in organismic behavior, morphology, physiology and biochemistry allow corals to acclimatize to changes in solar radiation during growth (reviewed by Barnes and Chalker, 1990; Falkowski et al., 1990; Brown, 1997a), or experimental transplantation (Dustan, 1979). On modern reefs, corals exhibit resilience to both seasonal changes in light and temperature regimes (Gates, 1990; Hoegh-Guldberg and Salvat, 1995; Barnes and Lough, 1996; Brown, 1997a, b; Fitt et al., 1998), and acute shifts associated with meteorological events such as ENSO (Lang et al., 1992; Glynn, 1993; Fitt and Warner, 1995; Brown, 1997a, b). Corals also experience, and withstand, widely fluctuating environmental conditions occurring over a diurnal cycle. Oxygen concentrations adjacent to living corals change from hyperoxic during the day, to hypoxic at night (Shashar et al., 1993; Kuhl et al., 1995), with oxygen, pH, and light levels in the tissues exhibiting marked differences from those on the surrounding reef (Kuhl et al., 1995). These data emphasize that corals routinely occupy a physically heterogeneous environment and suggests they should possess a high degree of biological flexibility.