engleski

The morphological study of Chaetoceros wighamii Brightwell (Bacillariophyceae) from lake Vrana, Croatia

The planktonic diatom genus Chaetoceros Ehrenberg is one of the largest genera among planktonic diatoms, including approximately 170 species. The majority of taxa are marine but several taxa are found in the freshwater/brackish environments. Chaetoceros wighamii Brightwell is among the first species described from the genus. It was discovered from a diatom assemblage found in “a dirty ditch of brackish water”, near the salt water Breydon estuary, England. Since its original description, quite a few other species have been synonymised with C. wighamii, including the freshwater species Chaetoceros amanita Cleve- Euler, which was originally described as resting spores from fossil collections, and several marine species. Here we describe the morphology of a cultured strain designated as Chaetoceros wighamii which was isolated from a water sample collected in the Lake Vrana (Vransko jezero) in Croatia. The polymictic Lake Vrana is a karstic cryptodepression, situated in the central part of the eastern Adriatic coast, connected to the Adriatic Sea by a narrow artificial channel at its southern side. The water is alkaline (pH 7.89– 8.98), with a high conductivity (3870–8760 mS/cm) and a usual salinity range between 2.4 and 6.5 psu. The strain was isolated in April 2011, at a salinity of 2.9 psu, and examined using light, scanning and transmission electron microscopy. The general morphology of the vegetative cells agrees with previous descriptions, including the formation of short and robust chains with very narrow apertures between adjacent cells which are often partially occluded by silica, and the presence of sibling setae fusing at their base. The intercalary setae are characteristically very long, stiff, straight and projecting in various directions from the chain axis. The terminal setae have the same morphology as intercalary ones but they are oriented more parallel to the chain axis, diverging in a broad V-shaped curve towards the end of the chain. The morphological analysis revealed a peculiar ornamentation pattern on the valve face with densely distributed ribs spreading from an irregularly shaped hyaline area, which is positioned slightly eccentrically on the valve face. Other distinctive traits are the lack of any type of process and the absence of a distinct annulus either on intercalary or terminal valves. Neither of the two characters, lack of the process and pattern of valve ornamentation, has been observed in previously described Chaetoceros species. Unfortunately, the cultured strain did not form resting spores. However, characteristic resting spores with spiny dome-shaped primary valves and truncate-conical secondary valves, similar to those described for Chaetoceros wighamii, were found in the sediment from the Lake Vrana. Our observations confirm that C. wighamii is a freshwater/brackish species and that C. amanita is its synonym, whereas the other marine species considered as synonyms of C. wighamii probably correspond to separate taxa. Until now the morphotype corresponding to Chaetoceros wighamii (syn. C. amanita) has been reported from inland waters in USA and Southern and Northern Europe. This study represents the first report of any of Chaetoceros species found in lacustrine environment in Croatia and probably also in Central European habitats.

Chaetoceros wighamii; Chaetoceros amanita; Lake Vrana; morphology

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