Photoinhibition in shallow-water colonies of the coral Stylophora pistillata as measured in situ

Continuous pulse amplitude–modulated (PAM) fluorescence measurements were performed in situ under natural irradiances for colonies of the coral Stylophora pistillata growing in shallow (2 m) and deeper (11 m) waters of the Red Sea. The effective quantum yield (DF/F ) showed a diurnal pattern inversely related to that of the incident 9 m downwelling photosynthetically available radiation (PAR), but this pattern was skewed for the shallow colonies such that the values were always lower in the afternoon than during morning hours when measured at similar irradiances. Accordingly, the relative photosynthetic electron transport rate (rETR, 5 DF/F 3 incident PAR) for 9m those colonies also showed lower values in the afternoons than in the mornings at equal irradiances. The rETRs also saturated well before maximal midday irradiances occurred. At the same time, nonphotochemical quenching (NPQ, measured as [Fm 2 F ]/F ) was higher in the afternoons than during the mornings under similar incident 9 9 m m PAR values. These results indicate an afternoon loss in photosynthetic capacity from photoinhibition. Such photoinhibition was not apparent during the course of the day in the corals growing in deeper water. The latter also showed no saturation of photosynthesis at the highest incident PAR values at noon (which were approximately a third of those at the shallower site). In addition, the shallow-growing corals showed significantly lower maximum quantum yields (DF/F measured during nighttime, equivalent to Fv/Fm) than the deeper growing ones. This is most 9m likely caused by an additional, more chronic, photoinhibition in the shallow-growing corals but could also be due to some form of photoacclimation or to factors other than light. Photoinhibition has been defined as reduced photosynthetic efficiency (quantum yield, Henley et al. 1991; Franklin and Foster 1997) or capacity (photosynthetic rate, Kok 1956; Kirk 1994), or both, at excess irradiance. Until at least a decade ago, such photoinhibition had not been reported for corals, and the existence of this phenomenon in the coral reef environment was questionable (Muscatine 1980; Dubinsky et al. 1990; Falkowski et al. 1990; Franklin et al. 1996). With the continuing improvements of nonintrusive techniques for measuring quantum yields, photoinhibition has recently been reported to occur in some corals. Measuring chlorophyll fluorescence of in hospite zooxanthellae with an underwater pulse amplitude–modulated (PAM) fluorometer, Brown et al. (1999) and Hoegh-Guldberg and Jones (1999) reported on dynamic photoinhibition based on midday depressions of maximum quantum yields (Fv/Fm, measured after periods of dark adaptation). However, both those studies were performed on corals held in aquariums, albeit under sunlight conditions, and such confinements could have imposed stressful changes in the organisms and complicated 1 Corresponding author ([email protected]). extrapolations of the results to natural, in situ, conditions. With the development of the Diving-PAM, measurements of chlorophyll fluorescence became applicable also for in situ research of coral photosynthesis (Beer et al. 1998; Ralph et al. 1999). However, those in situ works did not make use of the ambient irradiance during the measurements. Rather, the Diving-PAM’s internal halogen light source was used to irradiate the corals, thus generating so-called ‘‘rapid light curves.’’ Using submersible fluorometers, it is also possible to perform photosynthetic measurements of corals in situ under ambient irradiances. The only measurements performed so far under such conditions were done with a fast repetition rate (FRR) fluorometer (Gorbunov et al. 2001; Lesser and Gorbunov 2001). They showed midday depressions of DF/F for several Caribbean species, and Gorbunov 9m et al. (2001) concluded most of this to be due to dynamic photoinhibition. However, neither of these studies, nor the ones by Brown et al. (1999) and Hoegh-Guldberg and Jones (1999), showed loss of photochemical capacity, which would fit the more classic definition of photoinhibition (e.g., Kirk 1994). In this work, we describe diel changes in DF/F and the 9m 1389 Photoinhibition in shallow-water corals Fig. 1. Average diurnal cycle (6SE) of downwelling photosynthetically active radiation (PARL), measured at 2 m water depth using a Li-Cor underwater quantum sensor, for 15 d within the period 16 September–4 October 2000. The PARL values were recorded every minute. relative electron transport rate (rETR) of zooxanthellae residing within shallow and deeper growing colonies of Stylophora pistillata as measured in situ under natural irradiances. In shallow-growing colonies, photoinhibition was expressed both as decreased rETRs and concomitantly increased values of nonphotochemical quenching (NPQ) in the afternoons relative to the morning hours, when compared under similar incident photosynthetically available radiation (PAR) values. In addition, the shallow colonies showed lower Fv/Fm values than those measured for the deeper growing ones. Materials and methods Measurements were carried out during September and October 2000 on colonies of S. pistillata, a dominant coral species of the Red Sea. The colonies investigated grew at water depths of 2 and 11 m in front of the Inter University Institute (IUI) located on the northwestern shore of the Gulf of Aqaba (Red Sea), just south of Eilat, Israel (298309N, 348559E). The water temperature during the measurement period was 25.5 6 1.08C. All fluorescence measurements were performed in situ using an underwater PAM fluorometer (Diving-PAM, Walz). Several 24-h measurement series were done for both shallow (n 5 10) and deep (n 5 8) colonies at their horizontal tips. The Diving-PAM itself was placed in a plastic basket secured to the bottom, and a specially built flexible holder attached to a tripod held its optical fiber in place ;1 cm from the coral surface. These measurements were always made in such a way that the measured (top-facing) section of the coral branch never became shaded during the course of the day. In situ downwelling photosynthetically active (400–700 nm) radiation (PARL) was measured at 1-min intervals using an underwater quantum sensor (LI-192SA, LiCor, hence the subscript L in PARL) located about 50 cm beside the corals and connected to a Datalogger (LI-1000, Li-Cor). In addition, the downwelling irradiance closer to the particular coral being investigated was measured by the Diving-PAM’s quantum sensor (hence, PARD), which had been calibrated against a LI-189 quantum sensor (Li-Cor). This sensor was placed horizontally on the flexible holder, close to the tip of the Diving-PAM’s optical fiber. Measurements of fluorescence parameters and PARD were done automatically every 30 min over 24-h periods as set by the Diving-PAM’s internal timer. The water temperature was recorded by the Diving-PAM’s internal thermometer. The effective quantum yield of photosystem II (PSII) (DF/ F ) was measured as (F 2 F)/F (5DF/F ) (Genty et al. 9 9 9 9 m m m m 1989; Schreiber et al. 1994), where F is the fluorescence yield under ambient light (steady state fluorescence) and F is the maximum fluorescence yield sampled after a 9m 0.8-s period of saturating light (;10,000 mmol photons m22 s21). The effective quantum yield as measured during nighttime is actually equivalent to the maximum quantum yield, frequently termed Fv/Fm, where Fm is the maximum fluorescence yield in a dark-adapted plant following a short period of saturating light and Fv is the variable fluorescence (Fv 5 Fm 2 F0; F0 is the minimum steady state fluorescence yield measured in close-to-darkness). rETRs were calculated as DF/F 3 PARD. The rETR values given here are relative 9m because we did not consider the fraction of light absorbed by the photosynthetic pigments of the zooxanthellae within the coral tissue (see also Hoegh-Guldberg and Jones 1999) or the distribution of absorbed light between pigments associated with the two photosystems. NPQ was calculated as (Fm 2 F )/F (cf. Hoeg-Guldberg 9 9 m m and Jones 1999). The Fm values used were taken from the highest value measured during the night.