UV-absorbing compounds in the coral PociZZopora dumicomis: Interactive effects of UV radiation, photosynthetically active radiation, and water flow

A direct relationship exists between irradiance of solar ultraviolet (UV) radiation and concentration of UV-absorbing compounds, know as mycosporine-like amino acids (MAAs), in the Hawaiian reef coral Pocilloporu dumicornis. However, MAA concentration is also influenced by flow regime and the irradiance of photosynthetically active radiation (PAR). High irradiances of UV radiation in reef environments are correlated with high PAR and high water velocity, because all three parameters diminish exponentially with increased depth. This correlation can also be found along horizontal gradients on reefs. The clearest water is typically found on outer reefs growing in oceanic water. These ocean reefs typically experience high water velocities due to ocean swell when compared to the more turbid lagoon reefs exposed only to small wind-driven waves. Flow-modulated photosynthetic rate seems to be a major factor affecting the observed changes in MAA concentration when P. dumicornis is grown under different flow regimes and identical h-radiances of UV and PAR radiation. High PAR and high water velocity significantly enhance the effect of increased UV radiation on MAA concentration. Reef corals thrive under irradiances of solar ultraviolet radiation that would kill or severely damage many forms of marine life (Jokiel 1980). Additionally, reef corals are metabolically dependent on intracellular symbiotic dinoflagellates, commonly known as zooxanthellae, and will eventually die if deprived of photosynthetically active radiation (PAR) (Franzisket 1970). Thus it would be advantageous for coral animal tissue to be transparent to useful PAR wavelengths and opaque to potentially damaging UV wavelengths. Early work suggested that an undefined substance termed “S-320” had such characteristics and might act as a protective screen against UV radiation (Shibata 1969). Concentration of this material was shown to decrease with increasing depth, presumably as a result of attenuating UV radiation (Maragos 1972; Dunlap et al. 1986; Shick et al. 1995). Growth of zooxanthellae in vitro is inhibited by solar UV (Lesser and Shick 1989), but zooxanthellae in vivo apparently are not affected (Jokiel and York 1982) unless a nonacclimatized coral is subjected to a sudden increase in UV irradiance (Scelfo 1984; Gleason 1993). The S-320 material in corals was subsequently identified by Dunlap and Chalker (1986) as a group of compounds known as mycosporine-like amino acids (MAAs). This field of inquiry grew rapidly and a great deal is now known about the structure and distribution of MAAs (e.g. Karentz et al. 1991; Shick et al. 1992; Stochaj et al. 1994). A hypothesis implicit in much of this work is that organisms produce MAAs in direct response to UV radiation and that MAAs function to protect the organism from UV radiation damage. This paper describes a series of investigations on distribution and changes in MAA concentration in the reef coral Pocilloporu damicornis L. in relation to UV radiation irradiance, PAR irradiance, and flow regime. Experiments were conducted at the Hawaii Institute of Marine Biology (HIMB) on the Point Reef off Coconut Island in Kaneohe Bay, Oahu, Hawaii. Specimens of P. dumicornis were collected by scuba diving at depths of from 1 to 7 m on the Point Reef during summer 1984 and again during summer 1991. Portions of freshly collected colonies were used for the laboratory analyses of MAAs as well as soluble protein in the experiments described below. Spectral data (300-700 nm) were recorded with a LiCor LI18OOUW scanning spectroradiometer. The cosine-corrected collector and sensors have a nominal bandwidth of 8 nm and were programed to scan from 300 to 700 nm in 2nm intervals. Despite the 8-nm bandwidth, the instrument compares very well with other underwater systems with regard to stray light rejection (Kirk et al. 1994). The instru\ ment was deployed using scuba at six depths after a measurement of surface incident solar irradiance at approximately local noon. All six depths were measured within a 20-min interval. For each depth three scans were taken and the mean reported in units of mW m-2 nmI. Integrated values of unweighted UV radiation (300-400 nm) and UVB radiation (300-320 nm) for each depth are reported in Table 1. Initial experiments (bathymetric study, within colony distribution, and time course experiment) were conducted during summer 1984. The extraction and analysis of MAAs were performed according to the procedures in Dunlap and Chalker (1986) and Dunlap et al. (1986) using high-performance liquid chromatography (HPLC). Coral samples were sequentially extracted in three volumes of 70% aqueous methanol. Extracts were filtered (glass fiber), distilled under vacuum to -20% of the original volume, and passed through a Waters C18 Sep-Pak column to remove the apolar photosynthetic pigments. Volume was restored with 10% methanol and samples were then run on a Brownlee Spheri-5 (25 cm) RP-8 column with an RP-8 guard column and eluted at 0.7 ml min-’ with 10% aqueous methanol in 0.1% acetic acid. Peaks were detected at 3 13 nm. Peaks were baseline corrected and integrated before concentrations were calculated using available molar extinction coefficients (Ito and Hirata 1977; Takano et al. 1978a,b). These concentrations were then corrected using the calculated extraction efficiencies. The three MAAs (mycosporine-glycine, palythine, and palythinol) known at the time of the initial 1984 investigations were identified using co-chromatography with stan-