유해성 와편모조류 Cochlodinium polykrikoides의 휴면포자 탐색

Abstract

The mixotrophic dinoflagellate Cochlodinium polykrikoides Margalef is a causative agent of the recurring harmful algal blooms (HABs). Many dinoflagellate species have been reported to form resting cysts or to produce other types of resting stages as part of their life cycle. These cysts play a key role in bloom initiation and decline, as well as dispersal and colonization of new areas. Information on cyst numbers and identity is essential for understanding and predicting blooms.

Previous studies have reported the identification of resting cysts of C. polykrikoides from sediments, but the identity of the observed cyst was not fully confirmed. In recent studies, real-time PCR assays have been used for the detection and quantification of harmful dinoflagellate cysts in water and sediment because real-time PCR technique provides highly sensitive, quantitative, and rapid detection. In this study, resting cysts of C. polykrikoides were observed and detected from the coast of the southern Sea of Korea using real-time PCR probes.

In this study, hyaline cysts of C. polykrikoides were observed after the bloom period. Hyaline cysts of C. polykrikoides were similar to vegetative cells in size. On the contrary to vegetative cells, hyaline cysts were pale in color, immobile without chloroplasts. C. polykrikoides vegetative cells regenerated successfully from the hyaline cysts in the favorable conditions. It suggests that hyaline cyst is determined to be one stage of the life cycle of C. polykrikoides. In addition, another kinds of temporary cysts were observed in indoor experimental condition. Temporary cysts (20-30 ㎛) were yellowish brown in color and round shape. Temporary cysts also successfully regenerated to vegetative cells like the hyaline cyst. The germination processes of C. polykrikoides were also observed in field seawater samples. The germling gradually developed into a gymnodinoid cell with the cingulum and sulcus being recognizable as a vegetative cell of C. polykrikoides.

C. polykrikoides rDNA was detected in the sediment before and after the blooming periods. In sediment samples of Jaran Bay, Mulgeon harbor and Mijo, the copy numbers of C. polykrikoides rDNA were opposite from rDNA copy numbers of seawater and direct counting numbers of vegetative cells. In Yokji, the maximum copy number in sediment was 235 copies/cm3 wet sediment (per cm3 of wet sediment obtained from top layer of centrifuged 30 g of sediment). However, the copy numbers of sediment were suddenly decreased before the blooming period, but the vegetative cell numbers were consistently increased. These results showed that C. polykrikoides might be persisted in the sediment as cyst forms. Furthermore, the results of this study will be helpful to predict the potential blooming site and clue for full elucidation of the C. polykrikoides life cycle.