Mg/Ca ratios in stressed foraminifera, Amphistegina gibbosa, from the Florida Keys

Since 1991, significant proportions of Amphistegina populations in the Flor ida Keys and elsewhere have exhibited stress symptoms that include loss of symbiont color ('bleaching'), anomalous shell breakage and reproductive dam age. Previous studies of other taxa have reported elevated Mg/Ca ratios in tests from pollution-stressed foraminiferal populations. The purpose of thi s study was to test the hypothesis that anomalous shell breakage in stresse d Amphistegina gibbosa is the result of loss of control of calcification, r esulting in elevated concentrations of Mg that weaken the crystal structure of the test. Analysis of Mg and Ca concentrations in A. gibbosa tests, using an Inductiv ely-Coupled Plasma Mass Spectrometer, revealed normal Mg/Ca (2-5 mol%) in a ll specimens analyzed, including normal specimens collected in 1982 (prior to the onset of the stress event), and both normal and broken specimens col lected quarterly from afflicted populations in 1996. Analysis of specimens from the high Mg calcite taxon, Archaias angulatus, revealed Mg/Ca of 10-14 mol%. This study, which presents an ICP-MS procedure that can be used to a ssess Mg/Ca in individual foraminifera, does not support the hypothesis tha t shell breakage in stressed Amphistegina results from disruption of calcif ication at the ionic level. (C) 2001 Elsevier Science B.V. All rights reser ved.