Temperature-induced stress leads to bleaching in larger benthic foraminifera hosting endosymbiotic diatoms

Physiological mechanisms of bleaching were studied on larger benthic foraminifera (LBF) hosting endosymbiotic diatoms. Amphistegina radiata, Heterostegina depressa, and Calcarina hispida were exposed to increasing temperatures in static temperature experiments (23 degrees C to 33 degrees C, 6 d). Photosynthetic activity (F-v : F-m, measured with a pulse-amplitude modulated fluorometer), chlorophyll a (a proxy for symbiont biomass), and motility (a proxy for overall fitness of the foraminifera) were reduced in specimens at 32 degrees C to 33 degrees C, and cytoplasm color changes associated with bleaching were observed. A 30-d flow-through experiment at three temperatures (26 degrees C to 31 degrees C) and three levels of inorganic nitrate concentration (0.5 to 1.4 mu mol L-1) confirmed negative effects of temperature at 31 degrees C for A. radiata (including growth) and H. depressa. Another Calcarina species, Calcarina mayorii, was not affected. This suggests that temperature effects are species-specific. However, elevated nutrient concentrations did not affect any of the parameters measured. Temperatures > 30 degrees C stress the foram-diatom endosymbiosis in some LBF species, which may lead to subsequent bleaching of the host. Given that a 2-3 degrees C increase led to rapid bleaching of most species, we propose that, similar to corals, these species are threatened by sea-surface temperature increase predicted for tropical reef waters in the near future.