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.2010年深水地平线（Deepwater Horizon）井喷事故，在初始爆炸至油井封井的83天时间里，预估向墨西哥湾泄漏了490万桶原油。事故应急响应过程中，大量使用了Corexit®油基分散剂。尽管洋流使得南佛罗里达幸免于污染暴露，但该事件凸显了针对珊瑚礁开发有效生物测定受试生物（bioassay organisms）的迫切需求。 双盖虫属（Amphistegina spp.）是一类底栖有孔虫（benthic foraminifers），其与硅藻形成的共生关系与珊瑚及其虫黄藻（zooxanthellae）的共生模式高度相似。该属物种几乎遍布全球所有珊瑚礁群落，易于采集且可在实验室培养体系中稳定维持，同时是有孔虫指数（FoRAM Index）的关键组成部分——该指数是近海水体与沉积物质量的重要监测指标。 本研究的核心目标是建立标准化实验方案，用以测试球状双盖虫（A. gibbosa）对潜在有毒有机化学品的急性与慢性响应。初始研究目标为确定美国环境保护署（US Environmental Protection Agency）所定义的Corexit®分散剂两种成分的致死浓度与效应范围。 预实验结果显示，当受试个体暴露于体积分数2%及以上的丙二醇（propylene glycol）环境中48小时后，多数个体呈现死亡状态，计算得到的半致死浓度（LC50）为3%，初始效应范围为2%~4%。将受试个体转移至过滤海水中恢复72小时后，测得的LC50为6%。对所有评估参数（包括亚致死慢性效应：如40天后的生长差异与肉眼可见响应）的分析表明，其效应范围为0.5%~12%，当浓度高于该区间时，受试个体全部死亡。 对于2-丁氧基乙醇（2-butoxyethanol），48小时暴露后的表观LC50为0.2%；经72小时恢复后，LC50升至1%。所有实验均证实，72小时的恢复期足以判定急性毒性效应。一项关键发现为：受试有孔虫在暴露于有毒物质时会出现活动停滞，而在转移至过滤海水后可恢复活力。这一观测结果表明，成年有孔虫在暴露于有毒物质时可能进入休眠状态，该现象此前未见报道。