Chlorophyll fluorescence has been used to predict and monitor coral bleaching over short timescales (hours to days), but long-term changes during recovery remain largely unknown. To evaluate changes in fluorescence during long-term bleaching and recovery, Porites compressa and Montipora capitata corals were experimentally bleached in tanks at 30°C for 1 month, while control fragments were maintained at 27°C. A pulse amplitude modulated fluorometer measured the quantum yield of photosystem II fluorescence (Fv/Fm) of the zooxanthellae each week during bleaching, and after 0, 1.5, 4 and 8 months recovery. M. capitata appeared bleached 6 days sooner than P. compressa, yet their fluorescence patterns during bleaching did not significantly differ. Changes in minimum (Fo), maximum(Fm) and variable (Fv) fluorescence throughout bleaching and recovery indicated periods of initial photoprotection followed by photodamage in both species, with P. compressa requiring less time for photosystem II (PS II) repair than M. capitata. Fv/Fm fully recovered 6.5 months earlier in P. compressa than M. capitata, suggesting that the zooxanthellae of P. compressa were more resilient to bleaching stress.

Corals can obtain up to 100% of their daily carbon requirements from photosynthesis of their endosymbiotic zooxanthellae (e.g. Grottoli et al., 2006; Muscatine et al., 1981). Under normal conditions, light is absorbed by antenna pigments of the photosynthetic apparatus in zooxanthellae chloroplasts. Excitation energy is transferred to the reaction centers of photosystem II (PS II) and down the photosynthetic electron transport chain where the primary photochemical reactions of the cell produce reducing power and adenosine triphosphate (ATP)(Krause and Weis, 1991). Photoinhibition occurs when photosynthetic electron transport decreases and absorption of excitation energy increases(Osmond, 1994; Smith et al., 2005). In zooxanthellae, excess excitation energy can produce reactive oxygen species(ROS), ultimately affecting the quantum yield of PS II fluorescence(Fv/Fm; Fv,variable fluorescence; Fm, maximum fluorescence)(Lesser, 1996; Lesser, 2006). Therefore, Fv/Fm is a key indicator of the physiological status of zooxanthellae, particularly the status of chlorophyll a (Chl a) fluorescence and PS II reaction centers.

Zooxanthellae physiology varies seasonally because of changes in temperature, irradiance or a combination of both, with high Fv/Fm in mid-winter to early spring and low in the mid to late summer (Warner et al., 2002). Fv/Fm also fluctuates diurnally, decreasing from a maximum at night to a minimum at mid-day (Brown et al., 1999; Gorbunov et al., 2001; Hoegh-Guldberg and Jones,1999; Jones and Hoegh-Guldberg, 2001; Lesser and Gorbunov, 2001; Ralph et al., 1999; Torregiani and Lesser, 2007; Warner et al.,2006; Winters et al.,2003). These light-dependent variations serve to protect the photosynthetic apparatus from excess excitation energy involved principally in non-photochemical quenching (Gorbunov et al., 2001). Over these seasonal and diurnal patterns, periods of environmental stress can also influence Fv/Fm. Coral bleaching is one stress response, often caused by elevated seawater temperatures that impacts both the zooxanthellae and host. During stressful conditions, zooxanthellae are expelled, photosynthetic pigments are lost, or some combination of both,resulting in a white coral colony (e.g. Brown, 1997; Rodrigues and Grottoli,2007).

Most investigations of the effects of coral bleaching on fluorescence have been experimentally conducted over very short time periods (hours to days), or under extreme bleaching conditions (>4°C above ambient) to pinpoint locations of molecular damage (Bhagooli and Hidaka, 2003; Bhagooli and Hidaka, 2004; Brown et al.,1999; Brown et al.,2000; Hill et al.,2004; Hoegh-Guldberg and Jones, 1999; Jones et al.,1998; Jones et al.,2000; Lesser and Farrell,2004; Torregiani and Lesser,2007; Warner et al.,1996; Warner et al.,1999). One study over slightly longer periods measured fluorescence in bleached corals every 10 to 14 days over a total period of 7 weeks (Rodolfo-Metalpa et al.,2006), whereas long-term (months to years) studies have focused on natural variability of fluorescence(Warner et al., 2002; Winters et al., 2006). The long time span followed by Warner et al.(Warner et al., 2002)encompassed periods of visible bleaching, but the effect of bleaching on fluorescence was difficult to interpret because of the lack of control or non-bleached corals at the same time periods. No studies to our knowledge have experimentally followed bleaching effects on fluorescence over periods longer than weeks.

Variability in fluorescence yields in different species may help predict long-term coral resilience following bleaching. However, understanding these changes in zooxanthellae physiology during bleaching and recovery requires both a comprehensive and long-term study. The current study was designed to answer the following questions: (1) how long does zooxanthellae physiology take to recover from bleaching, (2) after bleaching, how do changes in zooxanthellae physiology vary between coral species, and (3) after bleaching,how do changes in zooxanthellae physiology coincide with known changes in host physiology of these same corals over the same time period(Grottoli et al., 2006; Rodrigues and Grottoli, 2006; Rodrigues and Grottoli, 2007). Active Chl a fluorescence was compared between experimentally bleached and non-bleached Porites compressa Dana 1846 and Montipora capitata Dana 1846 corals during 1 month of bleaching conditions and throughout 8 months of recovery following bleaching. Throughout bleaching and recovery, levels of photoprotection, photoinhibition and photodamage for each species of coral were assessed.

This design allowed for a quantitative assessment of the following hypotheses. (1) During 1 month of bleaching and throughout 8 months of recovery, Fv/Fm and its component variables [Fo (minimum fluorescence), Fm and Fv] are expected to undergo measurable changes in treated relative to control corals and relative to one another. Specifically, one or more of these outcomes may apply: (A) if Fv/Fm does not change while Fo of the treated group is greater than Fo of the control group and Fm of the treated group is greater than Fm of the control group,photodamage may be occurring (e.g. Jones and Hoegh-Guldberg, 2001); (B) if Fv/Fm of the treated group is greater than Fv/Fm of the control group prior to visible color loss in corals, photoprotection may be occurring (e.g. Brown et al., 2000); and/or (C)if Fv/Fm of the treated group is greater than Fv/Fm of the control group, while Fv of the treated group is greater than Fv of the control group or Fm of the treated group is less than Fm of the control group,photoinhibition may be occurring (e.g. Warner et al., 1999). (2)Measurable differences in zooxanthellae fluorescence of P. compressaand M. capitata are expected to reflect known differences in the bleaching response between species(Rodrigues and Grottoli,2007).

Study site and species

Corals were collected from Kaneohe Bay, Hawaii (21°26.18′N;157°47.56′W). Seawater temperatures from June to October average 27±0.012°C (mean ± s.e.m.) and 24.5±0.015°C from November to May (data from Hawaii Institute of Marine Biology weather station).

Porites compressa is branching and yellow-brown to dark brown in color. Montipora capitata is plating to branching and medium to dark brown in color. All collected M. capitata used in this study were branching.

Experimental design

A detailed description of the experimental design is reported in Rodrigues and Grottoli (Rodrigues and Grottoli,2006). Briefly, in August 2003, eight fragments were collected from each of twelve parent colonies from both species (totaling 192 fragments). All 24 parent colonies (12 per species) were located on the reef slope at the same depth (2 m) along a 100 m horizontal transect of the Point Reef off of Coconut Island. The temperature and irradiance levels were assumed to be constant over the small sampling area. The two species used in this study are the dominant coral species present on the fringing reef in this location and are sympatric along the reef slope. The fragments from each parent colony were randomly placed in one of eight tanks filled with filtered seawater that reduced zooplankton and coral heterotrophy. Beginning on 4 September 2003, the seawater temperature of four tanks was raised to 30.1±0.05°C (treated group), while the seawater of the other tanks remained at the ambient temperature of 26.8±0.04°C (control group)for 4 weeks. Within each temperature group, one fragment from each parent colony was randomly assigned to 0, 1.5, 4 or 8 months recovery on the reef. All tanks contained 24 fragments (i.e. 12 fragments from each species), with each parent colony represented once. Corals were rotated within and among tanks of the same treatment to minimize any positional and tank effects. The experiment mimicked the timing, duration and temperature of a 1996 natural bleaching event in Kaneohe Bay (Jokiel and Brown, 2004). All fragments were exposed to the same conditions except temperature during the 4 weeks in the tanks to accurately assess the physiological consequences of bleaching and recovery on each species.

Only dark-acclimated fluorescence was measured to assess the physiological status of the zooxanthellae, using a diving pulse amplitude modulated (PAM)fluorometer (Walz Inc., Effeltrich, Germany). The fluorometer first exposed the corals to a weak pulsed red light (<1 μmol quanta m–2 s–1) to determine Fo(minimal Chl a fluorescence yield), followed by a saturating pulse of light (3000 μmol quanta m–2 s–1) to determine Fm (maximal Chl a fluorescence yield)(Schreiber et al., 1986). Variable Chl a fluorescence yield(Fv=FmFo)and the quantum yield of PS II fluorescence(Fv/Fm) were then calculated and reported. Corals were measured at least 2 h after sunset to allow for dark acclimation. The diving PAM fluorometer measured Chl a fluorescence at excitation and emission wavelengths of 470 nm and 685 nm, respectively. These do not interfere with the wavelength lifetimes of other known fluorescent proteins in coral species(Gilmore et al., 2003). During the period in the tanks, all fragments were repeatedly analyzed at the end of each week for three consecutive weeks on the evenings of 11, 18 and 25 September 2003. On 2 October 2003 corals were returned to the reef for recovery. Then, only those treated and control corals pre-assigned to 0 months recovery were analyzed that same evening. At 1.5 months (16 November 2003), 4 months (2 February 2004) and 8 months (4 June 2004) recovery, the respective pre-assigned treated and control corals were collected, returned to the outdoor, flow-through seawater tanks, and analyzed each evening.

In addition, using published chlorophyll a (Chl a) and zooxanthellae concentration data for these same coral fragments measured at 0,1.5, 4 and 8 months recovery (Rodrigues and Grottoli, 2007), Chl a per zooxanthella was calculated. This calculation provides a direct assessment of zooxanthellae status that can complement the fluorescence measurements from the same time periods.

Statistical analyses

A repeated measures two-way analysis of variance (ANOVA) compared the effects of species and temperature on Fv/Fm, Fo, Fm and Fv during the weeks of bleaching only. Sphericity or the assumption that the repeated samples have similar variances, must be tested and corrected for as it affects the power of a repeated measures ANOVA. Mauchly's test assessed sphericity; if the assumption of sphericity was violated, resulting in a loss of test power, the Huynh–Feldt (H-F) correction factor was used to adjust the P-values of the univariate tests. A posteriori slice tests[e.g. tests of simple effects (Winer,1971)] directly compared the effect of temperature between the treated and control groups at each bleaching week within each species.

During the eight months of recovery, samples were independent of one another (i.e. pre-assigned to a recovery group and sampled only once). Therefore, ANOVA was used to compare the effects of species, genotype,temperature and recovery interval on Fv/Fm, Fo, Fm, Fv and Chl a per zooxanthella. A posteriori slice tests directly compared the effect of temperature between the treated and control groups at each recovery interval and within each species. Since treated and control corals were exposed to identical conditions except temperature during the first 4 weeks of the experiment, observed differences throughout the entire study were independent of season and could be attributed to bleaching alone. The use of replicate genotypes across temperature treatments and recovery times reduced the overall variation between treatments. When treatment values were not statistically different from control values at a single time interval, they are referred to as `fully recovered' throughout the text.

All data were normally distributed according to plots of residuals versus predicted values for each variable. Bonferroni corrections were not used (Quinn and Keough,2002). Statistical analyses were conducted using SAS software,version 9.1.3 of the SAS System for Windows. (Copyright © 2000-2004 SAS Institute Inc. SAS and all other SAS Institute Inc. product or service names are registered trademarks or trademarks of SAS Institute Inc., Cary, NC, USA.) P-values ⩽0.05 were considered significant.

Zooxanthellae status during bleaching

Mauchly's tests revealed that the assumption of sphericity was violated for Fv/Fm during bleaching only, since the variances and covariances between the repeated samples were significantly different from each other (Table 1). As such, P-values adjusted with the H-F correction factor were used for Fv/Fm during bleaching only (Table 2).

For P. compressa, Fv/Fm and Fv were not significantly different in treated and control fragments after the first week of bleaching, whereas Foand Fm were both 113% and 111% higher in treated than control corals, respectively (Fig. 1A–D). Fv/Fm, Fm and Fv of the treated group decreased relative to control values at the end of the second week to 90%, 84%and 77% of control values, respectively, whereas there was no difference in Fo between the treated and control groups(Fig. 1A–D). By the end of the third week, further decreases occurred and Fv/Fm, Fo, Fm and Fv of the treated group were at 84%, 82%, 57% and 49% of control values, respectively(Fig. 1A–D).

M. capitata followed a similar pattern to P. compressafor each week during bleaching. All fluorescence treatment and control variables in M. capitata were not significantly different after the first week of bleaching (Fig. 1E–H). At the end of the second week,Fv/Fm, Fm and Fv of the treated group decreased to 90%, 87% and 83% of control values, respectively, with no difference between treated and control Fo (Fig. 1E–H). By the end of the third week, further decreases occurred and Fv/Fm, Fo, Fm and Fv of the treated group were 87%, 75%, 62% and 58% of the control values,respectively (Fig. 1E–H).

Zooxanthellae status during recovery

One week later, at the end of bleaching and the start of recovery (i.e. 0 months recovery), in P. compressa Fv/Fm, Fo, Fm and Fv of the treated group reached their lowest point at 88%, 42%, 33% and 29% of control values, respectively(Fig. 1A–D). At that same time, Chl a per zooxanthella in treated P. compressa was not significantly different from control values(Fig. 2A). Fv/Fm and Fo of the treated group fully recovered at 1.5 months recovery(Fig. 1A,B), whereas Fo surpassed control values at 4 and 8 months to 137% and 127%,respectively (Fig. 1B). Fm and Fv were 66% and 62% of controls at 1.5 months, respectively, and were fully recovered at 4 and 8 months(Fig. 1C,D). During that time,Chl a per zooxanthella of the treated group dramatically increased compared with control values by more than nine- and eightfold at 1.5 and 4 months, respectively, and was not significantly different from control values at 8 months (Fig. 2A).

For M. capitata, Fv/Fm, Fo, Fm and Fv of the treated group, similar to P. compressa, reached their lowest point at the start of recovery (i.e. 0 months) at 75%, 47%, 31% and 25% of controls, respectively. However, for the remainder of recovery, fluorescence variables and Chl a per zooxanthella in M. capitata followed a different pattern than that of P. compressa. In M. capitata,Fv/Fm of the treated group appeared to recover at 1.5 months, but decreased again to 84% at 4 months, before fully recovering at 8 months (Fig. 1E). Fo was fully recovered by 1.5 months and remained so until the end of the analysis at 8 months(Fig. 1F). Both Fm and Fv of the treated group were 60% and 53% of control values at 1.5 months, respectively, and 46% and 43% of controls at 4 months, respectively, before fully recovering at 8 months(Fig. 1G–H). In contrast to P. compressa, the Chl a per zooxanthella in treated M. capitata was not significantly different from control values at 0,1.5, 4 and 8 months (Fig. 2B).

Interaction effects

Additional patterns were detectable from interaction effects [of species(S), temperature (T), recovery (R) and week (W)] during bleaching and recovery. During bleaching, on average, all fluorescence variables(significant W×T effects) of treated corals decreased more than those of control corals, with similar patterns in both species (non-significant S×T and W×S×T effects; Table 2). All fluorescence variables in treated corals increased more than control corals during recovery(significant T×R effects) and recovery occurred at different months for each species (significant R×S effects; Table 3). Different recovery patterns of Fo and Fm (significant T×S and T×R×S effects) occurred in the two species, but not of Fv/Fm and Fv,where patterns were similar for both species (non-significant T×S and T×R×S effects; Table 3). Chl a per zooxanthella increased in treated compared with control P. compressa, but no change was detected between treated and control M. capitata (significant T×S effect; Table 3).

Zooxanthellae status during bleaching

Most previous studies that measured Fv/Fm of both isolated symbionts and whole corals reported decreased values within hours of temperature stress associated with diminished photochemical efficiency of PS II and dynamic photoinhibition (Bhagooli and Hidaka,2003; Hill et al.,2004; Jones et al.,1998; Jones et al.,1999; Jones et al.,2000; Lesser,1996; Lesser and Farrell,2004; Warner et al.,1996; Warner et al.,1999). However, in the present study, there was no change in Fv/Fm for either species 1 week after the start of bleaching (Fig. 1A,E). Rate of seawater temperature increase occurred rapidly over several hours prior to fluorescence measurements in previous studies(Bhagooli and Hidaka, 2003; Bhagooli and Hidaka, 2004; Dove et al., 2006; Hill et al., 2004; Jones et al., 1998; Jones et al., 2000; Rodolfo-Metalpa et al., 2006; Warner et al., 1996; Warner et al., 1999) compared to a slower increase over several days in the present study, possibly contributing to the different initial outcomes. Although quantum yield remained the same in P. compressa, its component variables,Fo and Fm, both significantly increased(Fig. 1B,C) with a similar, but non-significant pattern in M. capitata(Fig. 1F,G). Increased Fo in higher plants and algae(Krause, 1988) and corals(Jones and Hoegh-Guldberg,2001) has been associated with photodamage, but in all of those cases Fm either remained the same or decreased. There are no reports of Fo and Fm both increasing, as observed here. The initial fluorescence response in P. compressa may simply be associated with zooxanthellae death and/or expulsion occurring during the first week of bleaching; however, further experimentation is required to determine the onset of zooxanthellae loss in this species. Alternatively, measurement of excess excitation energy dissipated as heat (i.e. nonphotochemical quenching) should also be conducted to establish whether the overall upward shift in initial and maximal fluorescence is a form of photoprotection(Gorbunov et al., 2001).

Fv/Fm did not decrease until P. compressa and M. capitata had experienced increased temperatures for 2 weeks – which is similar to the first indication of decreased Fv/Fm in the favid coral, Cladocora caespitosa after 10 days(Rodolfo-Metalpa et al.,2006). For P. compressa this first recorded decrease in Fv/Fm preceded the first visual indication of bleaching (i.e. decreased coloration), whereas some M. capitata colonies had visibly begun to bleach a few days before the first recorded decrease of Fv/Fm. Photoprotection precedes loss of coloration and is typically reversible (e.g. Bhagooli and Hidaka, 2003; Brown et al., 2000; Hill et al., 2004; Jones et al., 2000; Lesser and Gorbunov, 2001). This type of photoprotection probably occurred at week 2 in P. compressa. In M. capitata, photoprotection may have occurred earlier in the experiment or may not normally occur in this species,suggesting an alternative strategy for dealing with the onset of elevated seawater temperatures. In either case, the decrease in quantum yield resulted from decreased Fm and no change in Fo,so that Fv also decreased in both species. Decreased Fv often occurs when heat dissipation from reaction centers increase (Jones and Hoegh-Guldberg,2001). This probably occurred in both species in this study and may have been responsible for the decreased Fv that continued throughout the remainder of the bleaching period.

Zooxanthellae status during recovery

Zooxanthellae fluorescence recovery varied between coral species by several months with the return of normal quantum yield of PS II occurring 6.5months earlier in P. compressa than M. capitata(Fig. 1A,E). This indicates considerable differences in the photochemical efficiency of zooxanthellae from sympatric coral species following the same bleaching event. Furthermore, when experimental conditions are comparable to a natural bleaching event, as they are here, recovery of normal quantum yield seems to take much longer than previous studies have suggested. The length of time corals were exposed to temperature stress in this study (one month) was much longer than the hours to days of previous studies (Bhagooli and Hidaka, 2003; Bhagooli and Hidaka, 2004; Hill et al.,2004; Jones et al.,1998; Jones et al.,2000; Warner et al.,1996; Warner et al.,1999), possibly contributing to the differences in recovery. In addition, the long time period required for recovery probably increases the susceptibility of corals to consecutive bleaching events. Both species incurred disruptions to photochemical efficiency during recovery (as indicated from statistically similar Fv/Fm), yet the underlying physiological causes were different for each species (as indicated from statistically different Fo and Fm; Table 3).

Although the quantum yield of treated P. compressa colonies was fully recovered by 1.5 to 8 months, both Fm and Fv remained low at 1.5 months. This may reflect the period of time required for complete zooxanthellae recovery [at 4 months(Rodrigues and Grottoli,2007)]. This also suggests chronic photoinhibition, since both Fm and Fv recovered at 4 months. Fo increased and was over-compensating at 4 and 8 months,suggesting that photodamage had affected the structures and functions of PS II(Jones and Hoegh-Guldberg,2001). The extensive period of photodamage further suggests that the D1 protein was most likely affected (e.g. Warner et al., 1999). At the same time, these small changes in fluorescence were marked by an eight- to tenfold increase in Chl a per zooxanthella in the treated group compared with that of control corals (Fig. 2A). Likewise, in several groups of marine phytoplankton,including dinoflagellates, significant increases in Chl aconcentration did not result in significant and corresponding changes in fluorescence (Rochelle-Newall and Fisher,2002), suggesting the importance of non-fluorescent pigments. For P. compressa, elevated Chl a per zooxanthella values may be due to a greater increase in the peridinin-chlorophyll-protein complex, which has been shown to have light-shielding properties(Dove et al., 2001). Further research is needed to elucidate the type of non-fluorescent chlorophyll molecule responsible for the dramatic increase in Chl a per zooxanthella concentration in P. compressa.

Unlike P. compressa, there was little evidence of the same type of photodamage in M. capitata, as Fo was fully recovered between 1.5 and 8 months and there were no changes in Chl aper zooxanthella throughout recovery. However, low Fm and Fv values, lasting for at least the first 4 months of recovery, indicated long-term photoinhibition that was probably chronic(Krause and Weis, 1984) and characteristic of photoinhibition on the donor side of PS II in higher plants(Bertamini and Nedunchezhian,2004). Although chlorophyll pigments were lost, the total number of zooxanthellae was retained when M. capitata was stressed with increased ultraviolet radiation(Grottoli-Everett and Kuffner,1995) or elevated seawater temperature(Rodrigues and Grottoli,2007). Therefore, repair of the donor side of PS II took at least 4 months, since expulsion and acquisition of new zooxanthellae did not occur in M. capitata.

Although several parameters of dark-acclimated fluorescence were affected throughout recovery in both species (Fig. 1), mid-day photosynthesis had recovered by 1.5 and 4 months in P. compressa and M. capitata, respectively(Rodrigues and Grottoli,2007). Similarly, Hoogenboom et al.(Hoogenboom et al., 2006)found no measurable difference in coral photosynthesis in high light, although the electron transport rate from PS II declined by more than half. Dynamic photoinhibition during the day protected the photosynthetic apparatus and maintained normal photosynthesis rates(Lesser and Gorbunov, 2001). However, reduced metabolic rates (decreased net photosynthesis and coral plus zooxanthellae respiration) did occur after bleaching in both species(Rodrigues and Grottoli,2007), preceding the period of photodamage in P. compressa and chronic photoinhibition in M. capitata. Together this suggests that reduced coral metabolism may be an indication of severe stress in the remaining zooxanthellae.

Zooxanthellae and host strategies

P. compressa and M. capitata have previously been found to contain zooxanthellae types C15 and C31, respectively(LaJeunesse et al., 2004);zooxanthellae type was not confirmed in the present study. Although P. compressa took 6 days longer (this study) and 36 days longer after a natural event than M. capitata to visibly bleach(Grottoli et al., 2004), PS II fluorescence was not different between species during the bleaching period. The mechanisms underlying the different bleaching rates in both species are difficult to identify, since at the level of PS II both zooxanthellae types were similarly affected. Activity at one or more other levels within the chloroplast (i.e. electron transport, photosystem I, ATP synthase, or carbon fixation) may differ between zooxanthellae types and coral species and account for the visible differences in bleaching (e.g. Smith et al., 2005; Tchernov et al., 2004). Despite their similarities during bleaching, zooxanthellae from each coral species differed in how they recovered. Type C15 in P. compressa was more prone to long-term photodamage (i.e. increased Fo; Fig. 1B), significant increases in Chl a per zooxanthella (Fig. 2A) and recovery of Chl a and zooxanthellae concentrations at 4 and 8 months, respectively(Rodrigues and Grottoli,2007). By contrast, type C31 in M. capitata experienced chronic photoinhibition of the donor side of PS II (i.e. decreased Fm and Fv; Fig. 1G–H) and recovery of Chl a concentration at 8 months(Rodrigues and Grottoli,2007), with no significant change to Chl a per zooxanthella (Fig. 2B) or zooxanthellae concentrations (Rodrigues and Grottoli, 2007). Quantum yield of PS II fluorescence(Fv/Fm) also recovered faster in P. compressa, indicating that its zooxanthellae type may be more resilient than that of M. capitata. Together these differences may account for previously observed differences in Chl a recovery, since P. compressa recovered Chl a at least 4 months sooner than M. capitata (Rodrigues and Grottoli, 2007).

The two coral species used in this study also exhibited contrasting host strategies during recovery from bleaching: P. compressa relies on stored energy reserves and photosynthetically acquired carbon and M. capitata relies on heterotrophically acquired carbon when photosynthesis was not available (Grottoli et al.,2006; Rodrigues and Grottoli,2006; Rodrigues and Grottoli,2007). Changes to zooxanthellae fluorescence and differences in Chl a per zooxanthella further support these two distinct strategies. The more resilient zooxanthellae of P. compressa play an important role in colony recovery, since increases in Chl a per zooxanthella are required before energy reserves(Rodrigues and Grottoli, 2007)and calcification (Rodrigues and Grottoli,2006) can recover. By contrast, the zooxanthellae of M. capitata are probably not as essential to colony recovery, since heterotrophy can be relied upon during the recovery period(Grottoli et al., 2006). Altogether our data highlight two strategies utilized by these two coral species to survive and recover from bleaching events, with P. compressa relying primarily on more resilient zooxanthellae and M. capitata relying on a more resilient host. Ultimately, coral bleaching and recovery appear to involve the combined effects of zooxanthellae and host physiology.

LIST OF ABBREVIATIONS

     
  • Chl a

    chlorophyll a

  •  
  • Fm

    maximal Chl a fluorescence yield measured after dark acclimation

  •  
  • Fo

    minimal Chl a fluorescence yield measured after dark acclimation

  •  
  • Fv

    variable Chl a fluorescence yield measured after dark acclimation(Fv=FmFo)

  •  
  • Fv/Fm

    quantum yield of photosystem II fluorescence measured after dark acclimation

  •  
  • H-F

    Huynh–Feldt correction

  •  
  • PAM

    pulse amplitude modulated

  •  
  • PS II

    photosystem II

We thank the Hawaii Institute of Marine Biology, the University of Pennsylvania, P. Jokiel, C. Smith, P. Petraitis, L. Bloch, O. Gibb, M. Cathey and two anonymous reviewers. Funding was provided by a William Penn Fellowship to L.J.R., the Mellon Foundation and the National Science Foundation programs in Chemical Oceanography (OCE 0610487) and Biological Oceanography (OCE 0542415) to A.G.G. L.J.R. was a PhD graduate student in A.G.G.'s laboratory during the execution of this research. This is contribution number 1316 of the Hawaii Institute of Marine Biology.

Bertamini, M. and Nedunchezhian, N. (
12004
). Photoinhibition and recovery of photosynthesis in leaves of Vistis berlandieri and Vistis rupestris.
J. Plant Physiol.
161
,
203
-210.
Bhagooli, R. and Hidaka, M. (
2003
). Comparison of stress susceptibility of in hospite and isolated zooxanthellae among five coral species.
J. Exp. Mar. Biol. Ecol.
4153
,
1
-17.
Bhagooli, R. and Hidaka, M. (
2004
). Release of zooxanthellae with intact photosynthetic activity by the coral Galaxea fascicularis in response to high temperature stress.
Mar. Biol.
145
,
329
-337.
Brown, B. E. (
1997
). Coral bleaching: Causes and consequences.
Coral Reefs
16
,
S129
-S138.
Brown, B. E., Ambarsari, I., Warner, M. E., Fitt, W. K., Dunne,R. P., Gibb, S. W. and Cummings, D. G. (
1999
). Diurnal changes in photochemical efficiency and xanthophyll concentrations in shallow water reef corals: Evidence for photoinhibition and photoprotection.
Coral Reefs
18
,
99
-105.
Brown, B. E., Dunne, R. P., Warner, M. E., Ambarasari, I., Fitt,W. K., Gibb, S. W. and Cummings, D. G. (
2000
). Damage and recovery of Photosystem II during a manipulative field experiment on solar bleaching in the coral Goniastrea aspera.
Mar. Ecol. Prog. Ser.
195
,
117
-124.
Dove, S. G., Hoegh-Guldberg, O. and Ranganathan, S.(
2001
). Major colour patterns of reef-building corals are due to a family of GFP-like proteins.
Coral Reefs
19
,
197
-204.
Dove, S., Ortiz, J. C., Enriquez, S., Fine, M., Iglesias-Prieto,R., Thornhill, D. and Hoegh-Guldberg, O. (
2006
). Response of holosymbiont pigments from the scleractinian coral Montipora monasteriata.
Limnol. Oceanogr.
51
,
1149
-1158.
Gilmore, A. M., Larkum, A. W. D., Salih, A., Itoh, S., Shibata,Y., Bena, C., Yamasaki, H., Papina, M. and van Woesik, R.(
2003
). Simultaneous time resolution of the emission spectra of fluorescent proteins and zooxanthellae chlorophyll in reef-building corals.
Photochem. Photobiol.
77
,
515
-523.
Gorbunov, M. Y., Kolber, Z. S., Lesser, M. P. and Falkowski, P. G. (
2001
). Photosynthesis and photoprotection in symbiotic corals.
Limnol. Oceanogr.
46
,
75
-85.
Grottoli-Everett, A. G. and Kuffner, I. B.(
1995
). Uneven bleaching within colonies of the Hawaiian coral Montipora verrucosa. In
Ultraviolet Radiation and Coral Reefs
. Vol.
41
(ed. D. Gulko and P. L. Jokiel), pp.
115
-120. Hawaii: HIMB Technical Report.
Grottoli, A. G., Rodrigues, L. J. and Juarez, C.(
2004
). Lipids and stable carbon isotopes in two species of Hawaiian corals, Montipora verrucosa and Porites compressa,following a bleaching event.
Mar. Biol.
145
,
621
-631.
Grottoli, A. G., Rodrigues, L. J. and Palardy, J. E.(
2006
). Heterotrophic plasticity and resilience in bleached corals.
Nature
440
,
1186
-1189.
Hill, R., Larkum, A. W. D., Frankart, C., Kuhl, M. and Ralph, P. J. (
2004
). Loss of functional Photosystem II reaction centres in zooxanthellae of corals exposed to bleaching conditions: using fluorescence rise kinetics.
Photosyn. Res.
82
,
59
-72.
Hoegh-Guldberg, O. and Jones, R. J. (
1999
). Photoinhibition and photoprotection in symbiotic dinoflagellates from reef-building corals.
Mar. Ecol. Prog. Ser.
183
,
73
-86.
Hoogenboom, M. O., Anthony, K. R. N. and Connolly, S. R.(
2006
). Energetic cost of photoinhibition in corals.
Mar. Ecol. Prog. Ser.
313
,
1
-12.
Jokiel, P. L. and Brown, E. K. (
2004
). Global warming, regional trends and inshore environmental condition influence coral bleaching in Hawaii.
Glob. Change Biol.
10
,
1627
-1641.
Jones, R. J. and Hoegh-Guldberg, O. (
2001
). Diurnal changes in the photochemical efficiency of the symbiotic dinoflagellates (Dinophyceae) of corals: photoprotection, photoinactivation and the relationship to coral bleaching.
Plant Cell Environ.
24
,
89
-99.
Jones, R. J., Hoegh-Guldberg, O., Larkum, A. W. D. and Schreiber, U. (
1998
). Temperature-induced bleaching of corals begins with impairment of the CO2 fixation mechanism in zooxanthellae.
Plant Cell Environ.
21
,
1219
-1230.
Jones, R. J., Kildea, T. and Hoegh-Guldberg, O.(
1999
). PAM chlorophyll fluorometry: A new in situtechnique for stress assessment in scleractinian corals, used to examine the effects of cyanide from cyanide fishing.
Mar. Pollut. Bull.
38
,
864
-874.
Jones, R. J., Ward, S., Amri, A. Y. and Hoegh-Guldberg, O.(
2000
). Changes in quantum efficiency of Photosystem II of symbiotic dinoflagellates of corals after heat stress, and of bleached corals sampled after the 1998 Great Barrier Reef mass bleaching event.
Mar. Freshw. Res.
51
,
63
-71.
Krause, G. H. (
1988
). Photoinhibition of photosynthesis. An evaluation of damaging and protective mechanisms.
Physiol. Plant
7
,
566
-574.
Krause, G. H. and Weis, E. (
1984
). Chlorophyll fluorescence as a tool in plant physiology. II. Interpretation of fluorescence signals.
Photosyn. Res.
5
,
139
-157.
Krause, G. H. and Weis, E. (
1991
). Chlorophyll fluorescence and photosynthesis: The basics.
Annu. Rev. Plant Physiol. Plant Mol. Biol.
42
,
313
-349.
LaJeunesse, T. C., Thornhill, D. J., Cox, E. F., Stanton, F. G.,Fitt, W. K. and Schmidt, G. W. (
2004
). High diversity and host specificity observed among symbiotic dinoflagellates in reef coral communities from Hawaii.
Coral Reefs
23
,
596
-603.
Lesser, M. P. (
1996
). Elevated temperatures and ultraviolet radiation cause oxidative stress and inhibit photosynthesis in symbiotic dinoflagellates.
Limnol. Oceanogr.
41
,
271
-283.
Lesser, M. P. (
2006
). Oxidative stress in marine environments: Biochemistry and physiological ecology.
Annu. Rev. Physiol.
68
,
253
-278.
Lesser, M. P. and Farrell, J. H. (
2004
). Exposure to solar radiation increases damage to both host tissues and algal symbionts of corals during thermal stress.
Coral Reefs
23
,
367
-377.
Lesser, M. P. and Gorbunov, M. Y. (
2001
). Diurnal and bathymetric changes in chlorophyll fluorescence yields of reef corals measured in situ with a fast repetition rate fluorometer.
Mar. Ecol. Prog. Ser.
212
,
69
-77.
Muscatine, L., McCloskey, L. R. and Marian, R. E.(
1981
). Estimating the daily contribution of carbon from zooxanthellae to coral animal respiration.
Limnol. Oceanogr.
25
,
601
-611.
Osmond, C. B. (
1994
). What is photoinhibition?Some insights from comparisons of shade and sun plants. In
Photoinhibition of Photosynthesis: From Molecular Mechanisms to the Field
. (ed. N. R. Baker and J. R. Boyer), pp.
1
-24. Oxford: BIOS Scientific Publishers.
Quinn, G. P. and Keough, M. J. (
2002
). Multifactor analysis of variance, factorial designs. In
Experimental Design and Data Analysis for Biologists
,pp.
221
-259. Cambridge: Cambridge University Press.
Ralph, P. J., Gademann, R., Larkum, A. W. D. and Schreiber,U. (
1999
). In situ underwater measurements of photosynthetic activity of coral zooxanthellae and other reef-dwelling dinoflagellate endosymbionts.
Mar. Ecol. Prog. Ser.
180
,
139
-147.
Rochelle-Newall, E. J. and Fisher, T. R.(
2002
). Production of chromophoric dissolved organic matter fluorescence in marine and estuarine environments: an investigation into the role of phytoplankton.
Mar. Chem.
77
,
7
-21.
Rodolfo-Metalpa, R., Allemand, D., Bianchi, C. N., Morri, C. and Ferrier-Pages, C. (
2006
). Response of zooxanthellae in symbiosis with the Mediterranean corals Cladocora caespitosa and Oculina patagonica to elevated temperatures.
Mar. Biol.
150
,
45
-55.
Rodrigues, L. J. and Grottoli, A. G. (
2006
). Calcification rate and the stable carbon, oxygen, and nitrogen isotopes in the skeleton, host tissue, and zooxanthellae of bleached and recovering Hawaiian corals.
Geochim. Cosmochim. Acta
70
,
2781
-2789.
Rodrigues, L. J. and Grottoli, A. G. (
2007
). Energy reserves and metabolism as indicators of coral recovery from bleaching.
Limnol. Oceanogr.
52
,
1874
-1882.
Schreiber, U., Schliwa, U. and Bilger, W.(
1986
). Continuous recording of photochemical and nonphotochemical chlorophyll fluorescence quenching with a new type of modulation fluorometer.
Photosyn. Res.
10
,
51
-62.
Smith, D. J., Suggett, D. J. and Baker, N. R.(
2005
). Is photoinhibition of zooxanthellae photosynthesis the primary cause of thermal bleaching in corals?
Glob. Chang. Biol.
11
,
1
-11.
Tchernov, D., Gorbunov, M. Y., de Vargas, C., Yadav, S. N.,Milligan, A. J., Haggblom, M. and Falkowski, P. G. (
2004
). Membrane lipids of symbiotic algae are diagnostic of senstivity to thermal bleaching in corals.
Proc. Natl. Acad. Sci. USA
101
,
13531
-13535.
Torregiani, J. H. and Lesser, M. P. (
2007
). The effects of short-term exposures to ultraviolet radiation in the Hawaiian coral Montipora verrucosa.
J. Exp. Mar. Biol. Ecol.
340
,
194
-203.
Warner, M. E., Fitt, W. K. and Schmidt, G. W.(
1996
). The effects of elevated temperature on the photosynthetic efficiency of zooxanthellae in hospite from four different species of reef coral: a novel approach.
Plant Cell Environ.
19
,
291
-299.
Warner, M. W., Fitt, W. K. and Schmidt, G. W.(
1999
). Damage to photosystem II in symbiotic dinoflagellates: A determinant of coral bleaching.
Proc. Natl. Acad. Sci. USA
96
,
8007
-8012.
Warner, M. E., Chilcoat, G. C., McFarland, F. K. and Fitt, W. K. (
2002
). Seasonal fluctuations in the photosynthetic capacity of photosystem II in symbiotic dinoflagellates in the Caribbean reef-building coral Montastrea.
Mar. Biol.
141
,
31
-38.
Warner, M. W., LaJeunesse, T. C., Robison, J. D. and Thur, R. M. (
2006
). The ecological distribution and comparative photobiology of symbiotic dinoflagellates from reef corals in Belize:Potential implications for coral bleaching.
Limnol. Oceanogr.
51
,
1887
-1897.
Winer, B. J. (
1971
).
Statistical Principles in Experimental Design. 2nd Edn.
New York:McGraw-Hill.
Winters, G., Loya, Y. and Rottgers, R. (
2003
). Photoinhibition in shallow-water colonies of the coral Stylophora pistillata as measured in situ.
Limnol. Oceanogr.
48
,
1388
-1393.
Winters, G., Loya, Y. and Beer, S. (
2006
). In situ measured seasonal variations in Fv/Fm of two common Red Sea corals.
Coral Reefs
25
,
593
-598.