海洋生态与环境科学重点实验室知识库

亚历山大藻属中的塔玛亚历山大藻复合种（Alexandrium tamarense species complex）是重要的有害藻华原因种，也是麻痹性贝类毒素（paralytic shellfish toxins, PSTs）的典型产毒藻种。塔玛亚历山大藻复合种最初是对亚历山大藻属中亲缘关系相近、形态特征相似的塔玛亚历山大藻（Alexandrium tamarense）、链状亚历山大藻（A. catenella）和芬迪湾亚历山大藻（A. fundyense）的统称。近年来的研究进一步将塔玛亚历山大藻复合种区分为链状亚历山大藻（A. catenella）、太平洋亚历山大藻（A. pacificum）、塔玛亚历山大藻（A. tamarense）、地中海亚历山大藻（A. mediterraneum）和澳大利亚亚历山大藻（A. australiense）等5个藻种。塔玛亚历山大藻复合种具有多样化的繁殖方式和复杂的生活史过程，休眠孢囊（resting cysts）是其生活史过程中的关键环节，具有耐受环境胁迫、促进藻种传播和影响藻华动态等诸多生态学意义，对于研究有害藻华的形成机理和演变趋势至关重要。

在我国近海，塔玛亚历山大藻复合种中的链状亚历山大藻和太平洋亚历山大藻多次形成有害藻华，对海水养殖业发展、海产品食品安全和人类健康构成了潜在威胁。目前，针对两种亚历山大藻的生理生态特征、产毒状况和有害藻华发生情况进行了大量研究，但对其休眠孢囊的研究相对较少。本研究重点针对这两种亚历山大藻，综合经典的孢囊镜检方法和高特异性的qPCR方法，对2012年、2014年和2015年期间采自渤海（118.95-121.27°E, 37.72-39.32°N）、黄海 (121.33-124.02°E, 32.50-38.75°N)）和东海长江口邻近海域（120.03-125.50°E, 25.50-31.00°N）表层沉积物（共196组）和柱状沉积物样品（共5组）中的孢囊丰度状况进行了分析，研究了黄、东海海域两种亚历山大藻的分布状况和长期变化特征，并结合沉积物粒度和水文要素等环境参数，探讨了影响其分布状况和长期变化的关键因素，以期为深入解析我国近海有害藻华的成因和危害效应提供科学依据。

研究表明，链状亚历山大藻孢囊主要分布在黄、渤海海域，而太平洋亚历山大藻孢囊主要分布在东海长江口邻近海域，在黄海海域偶有检出；两种亚历山大藻孢囊的丰度存在显著差异，黄海海域链状亚历山大藻孢囊的丰度（0-3448 cysts/g DW，平均丰度300 cysts/g DW）显著高于长江口邻近海域太平洋亚历山大藻孢囊的丰度（0-29 cysts/g DW，平均丰度为4 cysts/g DW）。这一结果进一步确认了我国近海链状亚历山大藻和太平洋亚历山大藻相对独立的分布特征，推测与两种亚历山大藻对温度的适应性特征差异，以及长江冲淡水和黄海冷水团对黄、东海水交换的阻隔作用有关。通过研究刻画了两种亚历山大藻孢囊在不同海域的精细分布特征。在黄海海域，高丰度的链状亚历山大藻孢囊主要分布在北黄海中部海域、山东半岛邻近海域及南黄海中部海域，与黄海海域的泥质区基本吻合；在长江口邻近海域，太平洋亚历山大藻孢囊也主要分布在河口外侧泥质区，自西向东呈现出两侧高、中间低的分布态势，这一现象在春季尤为明显。位于调查区中部沿123^oE分布的孢囊丰度低值区与黑潮东海近岸分支（Nearshore Kuroshio Branch Current，NKBC，以下简称东海黑潮分支）的路径基本吻合，显示东海黑潮分支入侵对长江口邻近海域亚历山大藻的孢囊分布及藻华动态具有一定的调控作用。

针对黄、东海海域孢囊高值区采集的5个柱状沉积物样品进行了分析。结果表明，链状亚历山大藻孢囊仅在黄海海域的柱状沉积物（YS-1、YS-2、YS-3、YS-4）中检出，孢囊丰度（0-1254 cysts/g DW）较高；而太平洋亚历山大藻孢囊仅在东海长江口邻近海域的柱状沉积物（ECS-1）中检出，孢囊丰度（0-13 cysts/g DW）较低。其中，黄海海域的柱状沉积物中链状亚历山大藻孢囊丰度近年来呈现波动上升趋势，推测与黄海海域富营养化状况加剧及表层海水温度（sea surface temperature, SST）上升有关。而位于山东半岛近岸海域的YS-3柱状沉积物中，链状亚历山大藻孢囊丰度近10年来有明显下降，可能与黄海海域的绿潮影响有关。在长江口邻近海域的柱状沉积物样品中，太平洋亚历山大藻的孢囊丰度也有波动上升的趋势。

本研究首次综合应用传统的孢囊形态学研究手段和高特异性的qPCR方法，针对我国近海典型海域沉积物中塔玛亚历山大藻复合种中的链状亚历山大藻和太平洋亚历山大藻孢囊进行了定性定量分析，得到以下结论：（1）在黄、渤海及东海沉积物中，链状亚历山大藻和太平洋亚历山大藻的孢囊具有不同的分布特征，其中链状亚历山大藻主要分布在34°N以北的黄、渤海海域，太平洋亚历山大藻孢囊则主要分布在长江口及其邻近海域。（2）黄、东海海域的两种亚历山大藻的孢囊主要分布在泥质区，与亚历山大藻藻华区基本吻合，孢囊的丰度和分布状况与藻种类型、沉积物组成和环流状况密切相关。（3）黄、东海大部分海域链状亚历山大藻和太平洋亚历山大藻孢囊呈现出波动上升的变化趋势，与海域富营养化状况和全球变暖等因素密切相关，其生态效应值得密切关注。研究结果揭示了我国近海两种亚历山大藻孢囊的精细分布状况和长期演变特征，为系统阐明我国近海亚历山大藻藻华的成因，构建亚历山大藻藻华和藻毒素的监测预警体系提供了重要的基础数据和科学认识。

Alexandrium tamarense species complex is a group of important HAB (harmful algal bloom)-causing species and typical producer of paralytic shellfish toxins (PSTs). The species complex was initially a general term referring to three closely related species with similar morphological features in the genus Alexandrium, A. tamarense (M. Lebour) Balech, A. fundyense Balech and A. catenella (Whedon & Kof.) Balech. Recently, the species complex was classified into five species, i.e. A. catenella (Whedon & Kof.) Balech (previously A. fundyense Balech), A. tamarense Balech, A. pacificum Litaker, A. mediterraneum U. John, and A. australiense Sh. Murray. The species complex has different modes of reproduction and a complex life history involving the production of resting cyst, which is important for resilience, population expansion, and bloom dynamics. Therefore, study on the resting cysts is crucial to understanding the mechanisms of the formation and evolution of harmful algal blooms.

In the coastal waters of China, A. catenella and A. pacificum appeared to be the most widely distributed species, and blooms of the two species occurred many times, threatening the aquaculture industry, seafood safety, and human health. Many researches have been carried out on the eco-physiological characteristics, PST production, and bloom dynamics of A. catenella and A. pacificum, while studies on their resting cysts are quite limited. In this study, four cruises in the Bohai Sea (BS, 118.95-121.27°E, 37.72-39.32°N), Yellow Sea (YS, 121.33-124.02°E, 32.50-38.75°N) and East China Sea (ECS, 120.03-125.50°E, 25.50-31.00°N) were carried out in 2012, 2014 and 2015, and 196 surface sediment samples and five core sediment samples were collected. Resting cysts of the two species were identified and counted using both traditional microscope observation and two species-specific qPCR assays. Factors such as grain size of sediment, and hydrological parameters were also taken into account to study the key factors influencing the distribution and long-term variation of the resting cysts.

Studies showed that cysts of A. catenella were mainly found in the sea area north of 34°N in the BS and YS, while those of A. pacificum were mainly distributed in sea area adjacent to the Changjiang River estuary in the ECS, only occasionally detected in the YS. Abundance of A. catenella cysts (0-3448 cysts/g DW, 300 cysts/g DW on average) in the YS was significantly higher than that of A. pacificum cysts (0-29 cysts/g DW, 4 cysts/g DW on average) in the ECS. The results further demonstrated the separated distribution between populations of A. catenella and A. pacificum, probably due to the different adaptive strategies of the two species, and the role of Changjiang River Diluted Water and Yellow Sea Cold Water Mass as natural barriers between the ECS and YS. Resting cysts of A. catenella were mainly found in the central part of the northern YS, the sea area around the Shandong peninsula, and the sea area near Subei Shoal in the southern YS, basically in consistence with the mud patches in the Yellow Sea. Similarly, cysts of A. pacificum were mainly found in the mud patch outside the Changjiang River estuary, exhibiting a “sandwich-like” high-low-high pattern from the west to east. The region with low-abundance of cysts along the latitude 123°E is in accordance with the intrusive pathway of the Nearshore Kuroshio Branch Current (NKBC), suggesting potential impacts of Kuroshio intrusion on the distribution and abundance of A. pacificum cysts.

Five sediment cores were collected in the high abundance zones of resting cysts based on the results of surface sediment investigations in the YS and the ECS, and abundance of A. catenella and A. pacificum cysts in the cores was quantified. Cysts of A. catenella were only detected in the cores from the YS (YS-1, YS-2, YS-3, YS-4) with relatively high abundance (0-1254 cysts/g DW), while cysts of A. pacificum were only detected in the core from the area adjacent to the Changjiang River estuary at much lower abundance (0-13 cysts/g DW). Abundance of resting cysts in cores collected from the YS generally exhibited an increasing trend in association with intensified eutrophication and the warming trend indicated by sea surface temperature (SST). Cyst abundance of A. catenella in the sediment core YS-3 adjacent to the Shandong Peninsula exhibited a dramatic decrease in the last 10 years, probably due to the impacts of intensive green tides sine 2007. In the Changjiang River estuary, cyst abundance in the core ECS-1 also exhibited a gradual increasing trend recently.

In this study, qPCR assays and morphological method were applied to detect resting cysts of A. catenella and A. pacificum in the China seas for the first time. In conclusions: (1) We found strong evidence to support separated distribution patterns of A. catenella and A. pacificum cysts in the China Seas with A. catenella cysts mainly distributed in the sea area north of 34°N in the BS and YS and A. pacificum cysts mainly distributed in the area adjacent to the Changjiang River estuary. (2) The resting cysts of the two species mainly distributed in the mud patches of the YS and ECS, in consistence with the blooming region of Alexandrium spp. Abundance and distribution of resting cysts were influenced by different factors, such as species, grain size of sediment, and circulation pattern. (3) Abundance of A. catenella and A. pacificum cysts in the YS and ECS generally exhibit a gradual increasing trend, which is closely associated with the eutrophication and global warming process. This study revealed the distribution pattern and long-term changes of A. catenella and A. pacificum cysts in the China Seas, which offers important data and knowledge to support future studies on the mechanisms of Alexandrium spp. blooms, and the construction of monitoring and early-warning system on Alexandrium blooms and phycotoxins.