Population Recovery and Differential Heat Shock Protein Expression for the Corals Agaricia Agaricites and A. Tenuifolia in Belize
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MARINE ECOLOGY PROGRESS SERIES Vol. 283: 151–160, 2004 Published November 30 Mar Ecol Prog Ser Population recovery and differential heat shock protein expression for the corals Agaricia agaricites and A. tenuifolia in Belize Martha L. Robbart1, 3, Paulette Peckol1,*, Stylianos P. Scordilis1, H. Allen Curran2, Jocelyn Brown-Saracino1 1Department of Biological Sciences, and 2Department of Geology, Smith College, Northampton, Massachusetts 01063, USA 3PBS&J Environmental Services, 2001 Northwest 107th Avenue, Miami, Florida 33172, USA ABSTRACT: Over recent decades, coral reefs worldwide have experienced severe sea-surface temperature (SST) anomalies. Associated with an El Niño-Southern Oscillation (ENSO) event of 1997–1998, nearly 100% mortality of the space-dominant coral Agaricia tenuifolia was reported at several shelf lagoonal sites of the Belize barrier reef system; a less abundant congener, A. agaricites, had lower mortality rates. We assessed A. agaricites and A. tenuifolia populations at coral reef ridges in the south-central sector of the Belize shelf lagoon and forereef sites to document recovery follow- ing the 1998 ENSO event and subsequent passage of Hurricane Mitch. To investigate the difference in heat stress tolerance between the 2 species, heat shock protein (HSP) expression was examined in the laboratory under ambient (28°C) and elevated (+6°C) temperatures. Populations of A. agaricites and A. tenuifolia surveyed at forereef sites in 1999 showed after effects from the 2 disturbances (par- tial colony mortality was ~23 and 30% for A. agaricites and A. tenuifolia,respectively), but partial mortality declined by 2001. At reef ridge sites, A. tenuifolia exhibited 75 to 95% partial colony mor- tality in 1999 compared to 18% in the less abundant A. agaricites. We measured a significant increase in percentage live cover at ridge sites for both Agaricia species from 1999 to 2001, except at Tunicate Ridge; at this site, which has restricted water flow, live A. tenuifolia cover remained low (~10%) 3.5 yr after the 1998 warming event, due in part to high sponge cover (>75%). Immunoblotting results indi- cated that A. agaricites had twice as much HSC 70 (16.9 µg cm–2) as A. tenuifolia (8.7 µg cm–2) at ambient temperatures and 6× as much under the +6°C treatment. In addition to the inducible response by A. agaricites, this species expressed HSP 90, whereas A. tenuifolia did not. The distinc- tive patterns of population recovery and HSP expression suggest that A. tenuifolia has a lesser abil- ity to produce HSPs for protection against environmental stress than A. agaricites. Such differences in resilience to large-scale environmental disturbances such as intermittent ENSO episodes may drive a dramatic change in coral species abundance patterns. KEY WORDS: Agaricia · Coral reefs · ENSO episodes · Heat shock proteins · Population dynamics Resale or republication not permitted without written consent of the publisher INTRODUCTION experiments (Hayes & King 1995). Long-term studies of coral reefs affected by thermal anomalies with resul- Most reef-building scleractinian corals live near their tant coral mortality have revealed that recovery may upper thermal limits and may experience heat stress take several decades (Glynn 1993, Eakin 1996). with an increase of only a few degrees centigrade Environmental stresses, such as increase or decrease (Jokiel & Coles 1977, Fitt et al. 2001). Widespread coral in temperature or salinity, or exposure to heavy metals bleaching has been associated with thermal anomalies may deleteriously affect normal metabolic functioning that occur with elevated sea-surface temperatures at the cellular level. Proteins that are critical to the (SST) accompanying El Niño-Southern Oscillation maintenance of cell function may become denatured (ENSO) events (Brown 1997, Wilkinson 2000). Bleach- as a result of prolonged or severe environmental stress. ing also has been induced in laboratory heat shock For example, heat stress may cause a breakdown of *Corresponding author. Email: [email protected] © Inter-Research 2004 · www.int-res.com 152 Mar Ecol Prog Ser 283: 151–160, 2004 enzymatic pathways with consequent biochemical and While bleaching events have affected the wider metabolic dysfunction (Cossins & Bowler 1987); effects Caribbean since the early 1980s (Lasker et al. 1984, from such a stressor may be exacerbated by influences Williams et al. 1987), there were no records of mass from other environmental factors (i.e. irradiance, salin- bleaching of corals on the Belize (Mesoamerican) bar- ity, circulation) (Fitt et al. 2001). rier reef system before the El Niño-Southern Oscilla- Often, such environmental challenges elicit a cellu- tion (ENSO) episode of 1995 (Burke et al. 1996, lar response to protect an organism from permanent McField 1999). Associated with this event, McField damage that may result from exposure to these ex- (1999) measured over 50% coral bleaching on the for- treme conditions (Craig 1985, Welch 1992). In response ereef region off Belize. Although most coral colonies to thermal anomalies or UV radiation, Lesser et al. recovered within 6 mo, ~10% experienced at least par- (1990) demonstrated increased activities of enzymes tial colony mortality. Again, in 1998, the Belize barrier responsible for detoxifying active forms of oxygen in reef was subjected to large-scale disturbances by a symbionts of the zoanthid Palythoa caribaeorum. Simi- severe ENSO event (early August to October 1998) and larly, heat shock proteins (HSPs) serve a protective or Hurricane Mitch (October 25 to 31). Aronson et al. reparative function and directly reduce the accumula- (2002a) reported positive (>1°C) SST anomalies during tion of denatured proteins caused by significant envi- autumn 1998 for shelf lagoon and forereef regions off ronmental change, including heat stress, heavy metals south-central Belize. SSTs exceeded the HotSpot and other metabolic poisons (Craig 1985, Welch 1992, threshold for over 2 wk and reached +4°C in late Sep- Sharp et al. 1997). Once exposed to non-lethal stress tember. Hurricane Mitch, a category-5 storm on the that elicits a response, namely, increased synthesis of Saffir/Simpson Hurricane Scale, passed directly over HSPs, organisms often show a greater resistance to the Bay Islands of Honduras, south of Belize, and intro- future, similar environmental disturbances. HSPs are duced enough fresh water to create a freshwater lens 3 thus vital to organisms in variable environments and m deep at Carrie Bow Caye in southern Belize (Aron- are responsible for acclimation at the cellular level son et al. 2002a). Following these major disturbances, (Lindquist & Craig 1988, Laszlo 1992, Welch 1992). Kramer & Kramer (2000) reported significant remnant The cosmopolitan distribution of HSPs across widely bleaching at forereef sites off Belize 8 mo after the ini- disparate taxa led researchers to focus on the role of tial bleaching response. In contrast, Mumby (1999) these proteins in the physiological responses of scler- found that although ~80% of coral colonies on the actinian corals to thermal stress. Several HSPs have forereef region (8 to 10 m) of Glovers Atoll (Belize) been documented in coral species, including constitu- appeared to be either fully or partially bleached, these tive (cognate) heat shock protein 70 (HSC 70), and colonies regained their usual coloration in subsequent inducible HSP 70, HSP 60 and HSP 25, under heat and months. UV radiation stress (Hayes & King 1995, Fang et al. The 1998 ENSO episode had a particularly strong 1997, Sharp et al. 1997, Branton et al. 1999, Downs et impact on the shallow shelf lagoonal region off south- al. 2000). HSP 70 is the best-documented heat shock central Belize. Mass bleaching was first observed in protein, largely because it is highly inducible. For this area in early September 1998 (Nemecek 1999). In example, field transplant experiments with the Pacific the Pelican Cayes, nearly 100% mortality was docu- coral Goniopora djiboutiensis demonstrated elevated mented for the spacedominant coral Agaricia tenuifo- levels of HSC/HSP 70 following heat shock (Sharp et lia at lagoonal reef sites to a depth of 15 m (Aronson et al. 1997). al. 2000, 2002a, Peckol et al. 2003) following the mas- However, many studies identifying a 70 kDa stress sive and prolonged bleaching event. Patch reefs also protein did not differentiate between the constitutive experienced severe and lingering effects from bleach- (HSC 70) and inducible (HSP 70) forms (Welch 1992, ing, particularly for Montastraea annularis, a major Thompson et al. 2001). Differentiation between the reef-building coral (Peckol et al. 2003). A congeneric of constitutive and inducible forms is important because A. tenuifolia, A. agaricites, showed lower percentage increased synthesis of HSC 70 may act to stabilize colony bleaching (Peckol et al. 2001). proteins at the onset of environmental stress prior to Heat stress associated with the 1998 ENSO episode HSP 70 induction. Thompson & Scordilis (2001) docu- was hypothesized to be responsible for this mass mor- mented elevated levels of HSC 70 in mice 15 min after tality event of Agaricia tenuifolia in the lagoonal reefs exercise stress, suggesting this so-called ‘constitutive’ of the Pelican Cayes, Belize (Aronson et al. 2000, protein was acting as a ‘first defender’ following the 2002a). Fitt & Warner (1995) found that congener stress. Therefore, distinguishing between HSC 70 and A. lamarcki was also sensitive to temperature stress. HSP 70