Responses to Chemical Exposure by Foraminifera: Distinguishing Dormancy From Mortality

The Deepwater Horizon blowout in 2010 released an estimated 4.9 million barrels of crude oil into the Gulf of Mexico in the 83 days between the initial explosion and the capping of the well. Response included extensive use of Corexit© oil dispersant. Although South Florida was spared exposure by currents, this event highlights the need for effective bioassay organisms for coral reefs. Amphistegina spp. are benthic foraminifers that host diatom symbionts in a relationship similar to that of coral and their zooxanthellae. Amphistegina spp. occur abundantly in reef communities nearly worldwide, are easily collected and maintained in culture, and are a key component of the FoRAM Index, a indicator of water and sediment quality in coastal waters. The major goals of this project were to develop protocols to test the acute and chronic responses of A. gibbosa to potentially toxic organic chemicals. Initial objectives were to determine lethal concentrations and effects ranges, as defined by the US Environmental Protection Agency, of two components of the Corexit© dispersants. Preliminary experiments indicated that many specimens exposed to propylene glycol (v/v) at concentrations of 2% or higher appeared to be dead following 48-hour exposure, resulting in apparent LC50 of 3% and an initial effects range of 2-4%. When placed in filtered seawater, after 72-hours the observed LC50 was 6%. All parameters assessed, including sub-lethal chronic effects (differences in growth and visible responses after 40 days), revealed an effects range of 0.5% to 12%, above which there was 100% mortality. For 2-butoxyethanol, the apparent LC50 after 48-hour exposure was 0.2%; after 72-hour recovery the LC50 was 1%. In all experiments, a 72-hour recovery period was sufficient to determine acute effects. A key discovery was the observation of inactivity during exposure to toxic substances, followed by recovery when placed in filtered seawater. This observation indicates the potential for dormancy in adult foraminifers exposed to toxic substances that has not previously been reported