Symbiont-bearing foraminifera: harbingers of global change?

Rapidly increasing human populations are altering the Earth's environments at unprecedented rates. Major categories of anthropogenic change include increasing input of anthropogenic nutrients to aquatic systems, increasing concentrations of greenhouse gases, and ozone depletion. Foraminifera have recorded countless global change events in the geologic record, ranging from the subtle to mass extinction events. Taxa suspected to have harbored algal endosymbionts, particularly the larger benthic foraminifera and planktonic foraminifera characteristic of warm, shallow surface waters of the pelagic realm, have typically responded dramatically to environmental changes. The purpose of this paper is to explore why these foraminifera should be particularly susceptible to ongoing anthropogenically-induced global change, to examine some of the evidence that they are responding, and to make some predictions as to how their assemblages may respond in the 21st century and beyond. Benthic foraminiferal assemblages are known to be sensitive to coastal nutrification; large, symbiont-bearing foraminifera lose dominance to small, fast-growing herbivorous and detritivorous species when nutrient supply increases in tropical reef-associated environments. Symbiont-bearing benthic foraminifera also appear to be sensitive to increasing intensities of biologically-damaging ultraviolet radiation, exhibiting damage to symbionts, calcification and reproduction, as well as increased susceptibility to infestation and predation. On the other hand, the larger rotaliid and globigerinid taxa, which secrete low-Mg calcite shells, may fare well as atmospheric CO2 concentrations increase, at least relative to the high-Mg calcite miliolid foraminifera and aragonitic corals, as falling pH of surface waters increases energetic expenditures for calcification.