高摄食压力下球形棕囊藻凝聚体的形成

棕囊藻包含囊体和游离单细胞两种生活史阶段。囊体是棕囊藻藻华爆发时的优势形态，藻华衰退时囊体能够形成凝聚体，但是棕囊藻游离单细胞的凝聚体极少被发现。本次研究将球形棕囊藻单细胞和高密度的海洋弯曲甲藻共同培养，使球形棕囊藻的生长承受高摄食压力，通过观察摄食者和棕囊藻的生长、凝聚体的数量和形态，阐明单细胞凝聚体的形成以及与摄食压力的关系。当球形棕囊藻进入衰退期时，高摄食压力引发游离单细胞聚合形成凝聚体，无摄食压力情况下，单细胞不形成凝聚体。凝聚体由无鞭毛细胞组成，细胞排列紧密，近似球体。凝聚体形成伊始，凝聚体内部可见凝胶状物质将细胞互相粘结，并且粘附了纤维等物质。凝聚体的体积和粘附的细胞数量逐渐提高，细胞排列愈加紧密，凝聚体内部形态和结构不易分辨。凝聚体的形成有效保护了部分单细胞免受摄食压力的影响，减少了摄食死亡率。凝聚体的形成是球形棕囊藻面临高摄食压力时采取的主动的防御策略。球形棕囊藻能够频繁引发大规模藻华的原因可能在于其在生长的各个阶段中均具有优越的竞争策略。

Formation of aggregation by Phaeocystis globosa (Prymnesiophyceae) in response to high grazing pressure

Marine snow is a ubiquitous feature of the ocean and is considered to be a key mechanism for transporting carbon to the deep ocean. Although not the only source of marine snow, aggregation of phytoplankton is one of its major contributors. Large fluxes of aggregates are typically coupled to primary productivity and are associated with the termination of phytoplankton blooms. Phaeocystis is a cosmopolitan bloom-forming alga that is often recognized as both a nuisance alga and an ecologically important member of the phytoplankton. Phaeocystis has physiological capability to transform between solitary cells and colonial life cycle stages. The predominant form during the bloom is the colonial phase. Aggregations of senescent colonies were also observed occasionally at the end of the bloom. However, there is yet no direct evidence with regard to the development of aggregation of Phaeocystis solitary cells. In this current study we conducted experiments to test the hypothesis that aggregation formation by P. globosa can be initiated by the introduction of grazers (Oxyrrhis marina). Phaeocystis globosa solitary cells were distributed into twelve 250 mL culture flasks at an initial concentration of 4000 cells/mL. O. marina was added into six of the flasks (initial concentration 0.2 cells ml-1) that were used as the grazing treatments, and the other six flasks containing P. globosa solitary cells only served as the controls. We also investigate the effect of turbulence on the aggregation formation by P. globosa solitary cells. Our results demonstrated that the aggregations were formed only in the presence of grazers, which clearly suppressed the accumulation of solitary cells. These results strongly suggest that the development of colonies was initiated by grazing. These aggregations also had a very unique structure: cells without flagella closely clumped together to form an approximately spherical structure. Microscopic observations showed that these colonies lacked the mucus envelop that is characteristic of Phaeocystis colonies, and that naked cells were seen clearly on the colony surface. It was unable to tell whether the interior of the colonies was hollow or not, as the cells were packed tightly and the colonies appeared opaque. In our experiments the overwhelmingly high grazer abundances may have greatly impeded the solitary cells to produce polysaccharide material and construct a mucilaginous colony via growth and cell division. Aggregation size and cell numbers per aggregation increased with time, and the development of aggregations effectively protected the cells from grazing. The grazer abundance increased rapidly in the first twelve days, but then it began to fluctuate, coinciding with the time when colonies began to occur. We speculate that the formation of aggregations may represent an energetically inexpensive means to increase the collective size against grazing. Through a combination of life cycle stages and possibly grazer-activated defenses, Phaeocystis appears to have a greatly reduced mortality compared to competing phytoplankton.