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SEM examination of fine structure of Meringosphaera mediterranea, a microalga of enigmatic taxonomic position

The marine microalgae species from the genus Meringosphaera Lohman 1902 belong to nanoplankton, with the cell diameter <10 µm. The genus contains five described taxons of which two, M. tenerrima and M. mediterranea, were recorded in our investigations along the eastern coast of the Adriatic Sea. The doubtful taxonomic position of this genus comes from the original description of Lohmann [1] who described the chromatophores of this genus to be yellowish-green and the genus has accordingly been placed in the Xanthophyceae. However, in the following investigations by Norris [2] and his observations of the pigments of living cells to have a golden tinge, the affiliation was changed to Chrysophyceae, family Aurosphaeraceae where it is currently positioned. In addition to the further studies on pigment composition, the ultrastructural investigations of the morphological features of the species belonging to the genus are essential before the exact position of the species from this genus can be settled. The aim of this study is to contribute to the knowledge on the fine structure of Meringosphaera mediterranea Lohman, found in the SEM examinations of the samples collected within the study carried out in the southern open Adriatic Sea in May 2009 [3]. The cells are round, with 10-12 long, undulate, tapering spines (Figure 1.). The cell surface is covered primarily with large elliptical smooth silicate scales and on them the spines are found (Figure 2.). The silicate spines are hollow tubes shown to have a round, convex basal disc, flaring and with a regularly dentate margin. They are ornamented with 10-12 barbs distributed in a helicoidal pattern along the length of the spine. The results from electron microscopy analyses provided the further important morphological information on the species from this enigmatic genus.

microalgae; SEM; morphology

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