RESPONSES TO CHEMICAL EXPOSURE BY FORAMINIFERA: DISTINGUISHING DORMANCY FROM MORTALITY

Amphistegina gibbosa are benthic foraminifers that host diatom endosymbionts in a relationship similar to that of corals with zooxanthellae. These foraminifers are abundant along most of the south Florida coast and shelf, and are commonly associated with reef communities throughout the Caribbean. Although more tolerant of temperature changes than most reef-building corals, they are sensitive to light and water-quality stressors, and are useful for experimental studies where small size and relative ease of collection allows for statistically robust sample sizes. Amphistegina spp. can be used to monitor bleaching events and water quality that supports coral growth. With more knowledge of their reactions to chemicals, these species may serve a similar role in monitoring the effects of waterborne pollutants. In the wake of the Deepwater Horizon oil spill in the Gulf of Mexico, and with continuing concerns over oil spills in the area, investigations began with 48-hour exposure experiments on A. gibbosa in test media containing chemical components of the Corexit oil dispersants employed in the gulf, with a goal of establishing acute toxicological measures. Preliminary experiments led to the identification of apparent dormancy in response to toxic exposure, followed by recovery and resumption of normal activities after removal from the test media and a recovery period in clean seawater. This effect confounded what was originally considered to be apparent mortality after exposure and actual mortality following the recovery period. To utilize this unexpected effect, new methods were developed with the goal of allowing the identification of concentrations that cause acute reactions (including dormancy) after 48 hours, as well as more precisely determining target levels of mortality such as lethal concentration for 50% of test specimens (LC-50). These methods provide more accurate interpretation of current results, as well as for future studies of A. gibbosa and other foraminifers as indicators of chemical pollutants. The dormancy effect also made clear the necessity of further exploring sub-lethal effects, including differences in growth rates following exposures to different concentrations of test media, to understand how A. gibbosa reacts to such waterborne pollutants.