MORPHOLOGICAL STUDIES OF OCCLUDED PROCESSES IN THALASSIOSIRA SPECIES FROM CHINA

Thalassiosira Cleve is a common diatom genus with high species diversity in coastal waters around the world.Till now,nearly one hundred and fifty taxa belonging to this genus have been reported.Thalassiosira is the main contributor to nano-phytoplankton assemblage because cell sizes of most species are below 20 μm.Previously,some Thalassiosira species were considered as good model organisms of centric diatoms and Bacillariophyta.Many studies concerning genetics,sexual reproduction,feeding pressure,impact on egg producing,hatching of zooplankton,and environmental indicator have been performed.On the other hand,some Thalassiosira species are also important bloom-causing organisms.Algal blooms induced by T.curviseriata and T.diporocyclus have been reported in the Changjiang River Estuary and coast of Hainan Province,respectively.Due to very small cell sizes,electron microscopy(EM) is needed for accurate identification.In order to characterize species diversity and distribution of the genus Thalassiosira along Chinese coastal waters,water samples were collected from different locations.Thalassiosira specimens within these samples as well as several clonal strains isolated from natural samples were studied using scanning electron microscopy.All samples were fixed with Lugo’s solution and were concentrated to a final volume about 2—10 mL by settling overnight for several times.For observation under scanning electron microscope,sub-samples were taken and treated in several steps.Firstly,after the same volume of concentrated H2SO4 was added,these samples were boiled in the water bath for about 10—15 minutes to remove organic materials.Secondly,these samples were rinsed with distilled water several times to rid the acid.Then,these samples were coated by gold and thus could be observed,identified,and photographed under scanning electron microscope.The morphology and pattern of processes,including three types,fultoportula,rimoportula and occluded process,are key taxonomic characteristics for Thalassiosira species,even for some pennatae diatoms.Fultoportula and rimoportula are very common structures in all Thalassiosira taxa,while occluded process is just found in no more than 20 taxa from presently known 150 reported Thalassiosira species.The different features between occluded process and other two types of process is that occluded process does not penetrate the valve,so there is no structure of occluded process on the internal valve face.Previous reports about Thalassiosira species with occluded processes from China coast were rare,and almost no EM photos of occluded processes have been provided.In present study,five Thalassiosira species with occluded processes were observed and they were T.cedarkeyensis Prasad,T.eccentrica(Ehrenberg) Cleve,T.lundiana Fryxell,T.punctigera(Castracane) Hasle and T.tubifera Fryxell.Three of them are newly recorded species for China,T.cedarkeyensis,T.punctigera and T.tubifera.The detailed morphological description and habits of these species were given.Meanwhile,the taxonomic comparison among resembling species were made.Morphological variations of process patterns were discussed and their taxonomic evaluations were analyzed.