This electron microscopy study of the morphology of centric diatoms Thalassiosira faurii (Gasse) Hasle emend. Genkal and Th. weissflogii (Grunow) Fryxell et Hasle emend. Genkal et Kulikovskiy from some urban waterbodies of Ukraine (Kiev and Zhitomir) has shown that their quantitative and qualitative characters (valve diameter, number of marginal fultoportulae in 10 µm and number of satellite pores in them, number of central fultoportulae and number of satellite pores in them, structure and arrangement of central and marginal fultoportulae as well as rimoportulae) correspond to the extended diagnosis of these species. At the same time, in one of the Th. faurii samples we have observed a smaller number of satellite pores in central fultoportulae (3) as compared to the description of this species (4). For the first time, patterns of variability in the number of central and marginal processes in 10 μm (an increase in the number of central fultoportulae and decrease in the number of marginal ones in 10 μm with increasing valve diameter) have been revealed in the two species under study. An assessment of the taxonomic value of quantitative features in respect to the coefficient of variation has shown that the number of processes in the valve center is the most variable (CV 18–36%), and the number of marginal processes in 10 μm is the most stable characteristic (8–9%), which coincides with the data on other genera of Centrophyceae (Cyclotella Kütz., Cyclostephanos Round, Discostella Houk et Klee, Handmannia Peragallo, Sceletonema Greville, Stephanodiscus Ehr.).

Keywords: Bacillariophyta, morphological variability, Thalassiosira faurii, Th. weissflogii, Ukraine

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