Bleaching in Amphistegina gibbosa d’Orbigny (Class Foraminifera): observations from laboratory experiments using visible and ultraviolet light

Bleaching (visible loss of symbiont color) in populations of the diatom-bearing foraminifer Amphistegina has been recorded from reefs worldwide since 1991. Field studies and previous laboratory experiments have strongly implicated solar radiation as a factor in bleaching stress. The influence of spectral quality and quantity of photosynthetically active radiation (PAR) and ultraviolet radiation (UV) on growth rates and bleaching in Amphistegina gibbosa was investigated in the laboratory using fluorescent sources of PAR (‘blue’ with a spectral peak at 450 nm and ‘white’ with a 600nm spectral peak) and biologically effective ultraviolet radiation [UVB (280–320 nm)]. Growth rate, as indicated by increase in maximum shell diameter, saturated at a PAR of 6–8 lmol photon m s, increased in ‘blue’ light, and was not influenced by UVB £ 0.0162 W m. Frequency of bleaching increased with increasing PAR photon flux density and with exposure to shorter wavelengths, with or without an increase in total energy. Growth was significantly inhibited by UVB at 0.105 W m. Specimens in treatments exposed to UVB to PAR ratios >0.003 became dark in color, rather than bleaching, which previous cytological studies indicate is a photo-protective response. Implications of these experiments are that environmental factors that affect either the spectral quality or quantity of solar radiation can influence bleaching in Amphistegina. Introduction Amphistegina spp. are foraminifers that bear diatom symbionts and are found abundantly on coral reefs and tropical carbonate shelves worldwide (Langer and Hottinger 2000). These protists live predominantly on hard and phytal substrates at depths determined largely by the penetration limits of visible radiation (Hallock 1999; Hohenegger et al. 1999). Representatives of this genus can exploit a wide depth range, from intertidal to 120 m (Hallock 1999) using phototaxic behavior (Zmiri et al. 1974) and modification of their tests. They secrete hyaline calcite tests, which are typically thicker at shallower depths, reducing penetration of solar radiation into the test (Hallock et al. 1986). Other adaptive mechanisms, such as symbiont diversity (Lee et al. 1995) or photoprotective compounds, particularly micosporine-like amino acids (MAA) (Dunlap and Shick 1998), may also contribute, though data are lacking. Bleaching, the visible loss of symbiont color, was first documented in field populations of A. gibbosa d’Orbigny in the Florida Keys (USA) in 1991 (Hallock et al. 1993). Similar bleaching was originally observed in the laboratory by Hallock et al. (1986) during experiments to assess growth rates in response to photosynthetically active radiation (PAR) at 400–700 nm under fluorescent sources. Those experiments revealed that individuals grown for 90 days at PAR photon flux densities (PFD) of 14 and 40 lmol photon m s were not significantly different in size or shape but exhibited partial loss of symbiont color, while individuals grown at 6 lmol photon m s were not significantly different in maximum diameter, but produced thinner tests and maintained a healthy golden-brown color. Cytological studies of bleaching in these protists has revealed that visible loss of color results from the deterioration of the diatom endosymbionts, followed by deterioration of the host endoplasm (Hallock et al. 1993; Talge and Hallock 1995). Comparison of partly bleached specimens collected from field populations with specimens partly Communicated by P.W. Sammarco, Chauvin D. E. Williams (&) Æ P. Hallock College of Marine Science, University of South Florida, 140 7th Avenue South, Saint Petersburg, FL 33701, USA E-mail: [email protected] Tel.: +1-3053614569 Fax: +1-305-3614499 Present address: D. E. Williams NOAA/NMFS Southeast Fisheries Science Center, 75 Virginia Beach Drive, Miami, FL 33129, USA Marine Biology (2004) 145: 641–649 DOI 10.1007/s00227-004-1351-5 bleached in laboratory experiments demonstrated that cytological responses are statistically identical (Talge and Hallock 2003). Field observations of symptoms accompanying bleaching, including reproductive failure (Hallock et al. 1995), increased susceptibility to predation (Hallock and Talge 1994) and shell breakage (Toler and Hallock 1998), clearly indicate that bleaching is a stress response. The 1991 onset of bleaching in Florida Keys populations of A. gibbosa coincided with global stratospheric ozone depletion following the eruption of Mount Pinatubo (Randel et al. 1995), an event that Shick et al. (1996) described as a ‘natural experiment’ on the effects of ozone depletion on reef organisms. Hallock et al. (1993) postulated that bleaching in field populations is a symptom of stress induced by exposure to ultraviolet B radiation [UVB (280–320 nm)], particularly, increases in UVB relative to PAR. To pursue this hypothesis, Hallock et al. (1995) exposed healthy individuals to three treatments of PAR (from a ‘white’ fluorescent source) and ultraviolet radiation [UV (300– 400 nm)]: 16 lmol photon m s PAR plus 0.005 W m UV, 16 lmol photon m s PAR with no UV, and 8.8 lmol photon m s PAR with no UV. Results indicated that growth rate, as indicated by increase in shell diameter, was not influenced by these PAR or UV dosages, but that bleaching increased with both PAR PFD and UV dosage. Thus, the optimal PAR PFD for A. gibbosa in laboratory culture has been determined to be between 6 and 14 lmol photon m s. However, the spectral quality and quantity used in previous laboratory experiments (Hallock et al. 1986, 1995) were measured using broadband PAR and UV sensors. Moreover, the spectral quality of fluorescent PAR sources used in these experiments differed substantially from that of solar radiation penetrating clear tropical waters. Most notably, the longer wavelengths between 550 and 700 nm are rapidly attenuated by seawater (Kirk 1994), resulting in a relative increase in the blue and green wavelengths with depth. Fluorescent sources described as ‘white’, which are typically used in culture chambers, deliver longer wavelengths, peaking around 650 nm or are spectrally ‘flat’ between 500 and 650 nm. Because a photon of longer wavelength delivers less energy than a photon of shorter wavelength (e.g. Kirk 1994), foraminifers maintained under a white fluorescent source may respond differently than specimens maintained under a source emitting higher energy, shorter wavelengths. For example, Fitt and Warner (1995) found that a ‘blue’ source of PAR reduced photosynthetic potential more in Montastraea annularis (Ellis and Solander) than did a ‘white’ source. Experiments presented here build upon previous research to refine understanding of A. gibbosa’s PAR requirements for growth, as well as to assess the role of spectral quality in bleaching. Specifically, the first experiment compares responses to blue and white fluorescent PAR sources to investigate the relationship between spectral quantity (PFD) and quality (wavelength) on growth and bleaching. Subsequent experiments compare responses to PAR PFD, with and without UVB