Characterizations Of Morphology,Molecular And Life Cycle Of Phaeocystis Globosa In The Southeast China Sea

Blooms of Phaeocystis globosa occur frequently in the Southeast China Sea,which have a significant negative impact on coastal ecology and mariculture.Colonies of P.globosa can even cause nuclear safety problems by blocking the nuclear power plant condensation system.There is still a lack of descriptions about morphological and molecular features of P.globosa in the Southeast China Sea,and its origin remains debate.Some questions of the taxonomy and life cycle of P.globosa are still unsolved.In this paper,the morphology,phylogeny and life cycle of P.globosa had been studied in 7 typical strains isolated from Southeast China Sea,including Guangdong Shantou(SC1),Hainan(SC2),Guangdong Zhuhai(SC3),Guangxi(SC4),Guangdong Maoming(SC5),Guangdong Zhanjiang(SC6)and Fujian(EC7),and 1 strain of P.globosa isolated from Gulf Stream(CCMP629)had been also studied.Based on light microscopy(LM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM),8 strains of P.globosa studied in this paper show the similar morphological characteristics to species P.globosa.Three types of cell can be found in these 8strains,including small flagellates,large flagellates,and colonial cells.However,there are differences in flagellum length of flagellates,and cell size among the 8 strains,and their secretions and extracellular coverings of small flagellates are also variable,which indicates the morphological diversity in P.globosa strains in the Southeast China Sea.Compared with P.globosa strains in coastal water of Europe,there are three main morphometric features in these 8strains.First,the two flagella of flagellates are unequal in length in these 8 strains,with the ratios of 1.1-1.5.Second,there are distinct filamentous coverings on the scales of strain SC5 and SC6,but no scales are observed in other 6 strains.Third,the colonial cells are smaller than P.globosa strains in coastal water of Europe,except strain SC4 and EC7.Although these morphological features are instability,they also imply the distinction between P.globosa strains in the Southeast China Sea and costal water of Europe.Furthermore,the ratios of the length of haptonema to flagella are stable in P.globosa,which can be a referable feature in the identification of Phaeocystis species.Phylogenetic analyses were performed on the 8 strains and the related available sequences in Gen Bank,based on the ribulose-1,5-bisphosphate large subunit(rbc L),16 S,18S and ITS1 DNA sequences.Results show that these 8 strains belong to P.globosa.The rbc L and 16 S sequence are conserved but have the potential to resolve Phaeocystis species;ITS1 region shows sufficient variation;18S r DNA is the best choice to resolve Phaeocystis species in this study.Results of Phylogenetic analyses based on 18 S and ITS1 sequence suggest that P.globosa is a species complex,including warm-water and cold-water genotypes at least.The multiple ITS sequences in warm-water strains suggest that the cryptic species could still hybridize,and the strains from Southeast China Sea may be a strain complex composed of different strains.Life cycle of strains from Southeast China Sea was studied with LM and SEM,and flow cytometry.Most stages in the life cycle of strains studied in this paper are the same as other P.globosa strains,whereas new stages can be found in these strains.Multiple strategies of colony formation can be observed under LM in these strains,and these strategies may contribute to a high abundance of colonies of P.globosa blooms in Southeast China Sea.The colonial cells transformation into small and large flagellates,the mitosis in small flagellates,and the aggregates of non-motile cells adhering to Chaetoceros affinis can be also observed in this study.It is the first time to document the aggregates stage and the meiosis of colonial cells with micrographs in P.globosa.Flow cytometry analysis shows that the large flagellates and colonial cells were diploid,while the haploid small flagellates may also exist in these strains.The life-cycle model pertaining to P.globosa in Southeast China Sea is presented in this paper.In conclusion,morphological,molecular and life cycle features of strains from Southeast China Sea are presented in this paper.Differential of P.globosa strains in Southeast China Sea and Europe coastal water is also demonstrated,which indicates the potential of cryptic species in P.globosa.Moreover,the life-cycle presented in this paper also enriches the stages of P.globosa.This study provides the reference for further research of taxonomy of Phaeocystis,and may contribute to monitor and control the P.globosa bloom in the Southeast China Sea.