76 M
.R. Lombardi et al. J. Exp. Mar. Biol. Ecol. 252 2000 75 –84
1. Introduction
In recent years, much attention in tropical marine ecosystems has been directed towards studying a phenomenon in zooxanthellate corals known as ‘‘bleaching’’.
Bleaching results from the expulsion of symbiotic dinoflagellates 5zooxanthellae from the animal host due to one or several environmental stressors such as elevated seawater
temperatures, excess visible or ultraviolet radiation, or water pollution Glynn, 1996. As the zooxanthellae leave, the coral progressively whitens since the remaining polyp is
translucent and the coral skeleton is highly reflective. This discoloration is a common indicator of stress in corals Brown and Howard, 1985. Many investigators now believe
that the zooxanthellae are the primary target of these environmental perturbations Iglesias-Prieto et al., 1992; Lesser, 1997; Warner et al., 1996, 1999, and a number of
mechanisms have been suggested to explain the mechanism by which zooxanthellae leave the host. Among these are exocytosis and host cell detachment Gates et al., 1992.
Zooxanthellae can contribute over 100 of the carbon requirements of the host, via translocated photosynthate, as their contribution to the symbiosis Falkowski et al.,
1990. This relationship permits the assessment of stress or damage on the intact symbiosis through the analysis of parameters associated with the photosynthetic
competency of the zooxanthellae. Common procedures include changes in cell counts, the aerial or per cell concentration of chlorophyll a Iglesias-Prieto et al., 1992, and
indirect calorimetry using oxygen electrodes Lesser, 1997 to measure photosynthesis and respiration, therefore inferring the energetic costs and benefits associated with the
symbiosis. All of these methods require some destructive sampling of the reef community by collecting pieces of coral that also results in damage to neighboring
organisms due to the destructive nature of the collection.
Recent methodologies used to assess stress in corals involve the analysis of the biochemical and biophysical aspects of photosynthesis within the algal symbiont itself,
more specifically of the PSII reaction center. Photosynthetically active radiation PAR:
22 21
400–700 nm in mmol quanta m s
reaches the ocean and is attenuated by scattering and absorption within the water column with the remaining quanta absorbed by
photosynthetic pigments within the zooxnthellae Falkowski and Kolber, 1993. In the PSII reaction center a primary electron donor chlorophyll molecule called P680 is
present, 680 denoting the wavelength nm of peak absorbance for chlorophyll a Campbell, 1996. As photons are absorbed by chlorophyll molecules charge separation
occurs and causes the photochemical oxidation of H O to O Kolber and Falkowski,
2 2
1993. The absorbed light energy is converted to chemical energy via photochemical charge separation, dissipated as thermal energy, or re-emitted as fluorescence Kolber
and Falkowski, 1993; Campbell et al., 1998. Chlorophyll fluorescence, and the changes in fluorescence yields, within PSII can be used to assess photosynthetic activity since
changes in fluorescence yields reflect changes in de-excitation pathways such as electron transfer and heat dissipation Campbell et al., 1998.
In situ studies of coral fluorescence allow researchers to closely monitor coral physiology and changes in physiology during exposure to environmental stress without
destructive sampling. A number of methodologies have recently been used to examine fluorescence yields including pulse amplitude modulated PAM and fast repitition rate
M .R. Lombardi et al. J. Exp. Mar. Biol. Ecol. 252 2000 75 –84
77
FRR fluorescence methods Kolber et al., 1998. New instruments have been developed that incorporate protocols to measure the multiple photochemical turnover
PAM, and single photochemical turnover of PSII FRR in the laboratory and in the field Schreiber et al., 1986; Gorbunov et al., 2000. In particular, PAM measurements of
fluorescence in corals have become widespread Brown et al., 1999; Ralph et al., 1999; Warner et al., 1996, 1999. Previous studies using PAM fluorescence protocols have
indicated that corals exposed to bleaching conditions, such as elevated seawater
9
temperature, yield progressively lower DF 9 F of PSII fluorescence values before they
m
bleach Warner et al., 1999. Similar results have been obtained with FRR fluorometers Lesser and Gorbunov, in press.
Since the quantum yield of PSII fluorescence is an important indicator of functional PSII units we examined the association of color variability corresponding with
9
zooxanthellae loss, or decrease of photosynthetic pigment, with changes in the DF 9 F
m
of PSII fluorescence. In this paper we describe the use of a newly described in situ FRR fluorometer Gorbunov et al., 2000, to examine the fluorescence yields of zooxanthellae
in hospite on corals recovering from a bleaching event due to elevated seawater temperatures.
2. Materials and methods
