混合营养型甲藻的微捕食行为及机制研究

In the last few decades, harmful algal blooms (HABs) caused by dinoflagellates have been increasing in geographical distribution and frequency. Particularly, studies have shown that blooms of some mixotrophic dinoflagellates have been associated with mass mortalities of shellfish and finfish resulting serious ecological damage and economic loss. Recent studies showed that toxic species Pfiesteria and Prymnesium kill fish and zooplankton by micropredation, which is an ecological strategy to eliminate their predators and competitors while, at the same time, to acquire organic nutrients. This strategy has been considered to possibly play a key role in the formation and maintenance of HABs. Since the four HAB species, Cochlodinium polykrikoides, Akashiwo sanguinea, and Karlodinium veneficum have some similar characteristics with Pfiesteria and Prymnesium, such as mixotrophy (phagotrophy in particular), acute toxicity to aquatic animals and others observed in our preliminary work, we hypothesize that these dinoflagellates may also possess similar micropredation behavior. Therefore, this proposed project will utilize multiple clonal cultures of the abovementioned dinoflagellates established in the laboratory from coastal waters of China, via laboratory simulation and field investigations, to investigate: (1) whether or not these dinoflagellates have micropredation behavior and, if they do, its main characteristics; (2) the possible role of algal toxins in micropredation; and (3) the regulating functions of environmental factors in the toxicity and micropredation behavior of the dinoflagellates. The anticipated findings will not only considerably enhance our understanding of the ecological strategies adopted by these important mixotrophic and HAB-forming dinoflagellates and the ecological functions of algal toxins, but also provide scientific basis for the prevention and control of HABs.

甲藻藻华在世界各地沿海呈显著增加态势，与大量鱼类死亡事件相关，导致严重的经济损失和生态灾害。近期的研究发现，某些有害藻类如费氏藻通过“微捕食”行为杀死鱼类和浮游动物。藻类的微捕食行为不仅能够获取有机营养，而且可能通过抑制甚至消除捕食者和竞争者从而在藻华的形成和维持过程中发挥重要作用。藻类微捕食是一个新的研究领域，其主要特征及生态学机制目前还不清楚。由于多环旋沟藻、红色赤潮藻、剧毒卡罗藻等有害藻类与费氏藻有许多共同特征，如吞噬营养能力、对水生动物的急性毒性等，因此很可能也具有类似的微捕食行为。为此，本项目利用上述3种甲藻纯培养，通过室内和野外试验相结合的方法，考察其是否具有微捕食行为，研究藻毒素在微捕食过程中的作用，进而探索环境因子对甲藻产毒与微捕食行为的调控机制。项目的实施将有助于阐明混合营养型甲藻的生态竞争策略和藻毒素的生态学功能，并可为防控藻华提供科学依据。

混合营养型甲藻红色赤潮藻等是世界范围内暴发频率高、覆盖范围广、危害严重的有害藻华种类。然而，其关键生态竞争策略和藻华形成机制目前并不清楚。本项目调查了珠江口海域典型混合营养型甲藻红色赤潮藻、剧毒卡罗藻、双胞旋沟藻的微捕食行为及其主要特征，研究了甲藻毒素在微捕食过程中的功能与效应，并探索了环境因子对混合营养型甲藻产毒及微捕食行为的调控机制。研究结果表明：（1）红色赤潮藻、剧毒卡罗藻和双胞旋沟藻藻液及其毒素粗提物对共存浮游植物具有显著毒性效应。红色赤潮藻能够在6 h内裂解红胞藻、锥状斯氏藻和球形棕囊藻，并利用裂解细胞释放的营养物质加速生长。红色赤潮藻毒素粗提物对水生动物褶皱臂尾轮虫和海水青鳉鱼仔鱼具有急性毒性效应，表现为行为异常至死亡。（2）红色赤潮藻毒性具有显著株间差异，中国株（JX13、JX14、CCMA256）比美国株（AS2、ASNP6）表现出更强的毒性。红色赤潮藻毒性与其营养利用方式具有密切联系：无毒（低毒）藻株不能裂解浮游植物细胞，也不能利用有机营养促进生长。有毒藻株能够显著抑制褶皱臂尾轮虫繁殖，降低海水青鳉鱼鱼卵孵化率，而无毒（低毒）藻株更易被水生动物捕食。（3）红色赤潮藻和剧毒卡罗藻毒素的释放过程需要细胞接触。在细胞直接接触条件下，红色赤潮藻和剧毒卡罗藻对目标藻（红胞藻）的生长具有显著抑制效应，同时红胞藻对红色赤潮藻和剧毒卡罗藻的生长具有显著促进作用（48 h时红色赤潮藻细胞密度增加50%）。（4）盐度、氮源、营养盐浓度和比率等环境因子对红色赤潮藻和剧毒卡罗藻毒性具有调控作用。尿素、高浓度氮磷、氮磷比失衡条件能够刺激红色赤潮藻和剧毒卡罗藻产生更多的毒素。研究提示：红色赤潮藻等甲藻能够利用毒素支持微捕食，是这些混合营养型藻类获取营养物质的重要生态策略，同时，通过毒素去除竞争者和捕食者也可能是藻华形成和维持的重要机制。