A Comparative Study of the Harmful Dinoflagellates Cochlodinium polykrikoides and Gyrodinium impudicum Using Transmission Electron Microscopy, Fatty Acid Composition, Carotenoid Content, DNA Quantification and Gene Sequences

The massive growth of the dinoflagellate alga Cochlodinium polykrikoides in Korean coastal waters was first observed in 1982. Thereafter the blooms associated with massive fish kills have been occurring annually, and C. polykrikoides is now regarded a harmful red tide organism in Korea. Recently, a new chain-forming Gyrodinium impudicum, usually occurring with Cochlodinium polykrikoides, was observed. Thus, we need to identify the toxic C. polykrikoides from the non-toxic Gyrodinium impudicum. Because of a similarity in morphology, it is difficult to distinguish the two species under the light microscope. In this study, comparisons were made using fine-scale features of transverse sections and biochemical analysis including rDNA sequencing targeted ITS regions. The observation of cellular arrangement and structure by transmission electron microscopy showed a similar distribution of the main components in both species, but location of the nucleus and distribution of the chloroplasts were different. Fatty acid composition showed that the long-chain polyunsaturated fatty acid docosahexanoic acid was not present in Cochlodinium polykrikoides. These data suggest that fatty acid composition may be a useful biochemical indicator to distinguish the microalgae. The amount of total carotenoids in Gyrodinium impudicum was approximately 10 times higher than in Cochlodinium polykrikoides. The nuclear DNA content in Cochlodinium polykrikoides was twice as high as that in Gyrodinium impudicum (p < 0.06). Sequence analyses coding for the 5.8S rDNA and ITS regions showed quite different nucleotide alignments with only 55% similarity, the length of the ITS regions also differed. The genetic differences between the two microalgae will allow the development of species specific DNA probes for species differentiation.