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 600-nm 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 mumol photon m(-2) s(-1), increased in 'blue' light, and was not influenced by UVB less than or equal to 0.0162 W m(-2). 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-2. 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.