Stephanosphaera pluvialis (Volvocales, Chlorophyta), a rare freshwater species of the northwestern Russia

Abstract

Stephanosphaera pluvialis Cohn is considered a rare alga, a habitant of rock pools. There are no signs of its findings after the mid 1930s in Russia, and it has never been seen in the Russian White Sea islands or in the Leningrad Region of Russia. In mid July, 2007, however, it dominated in two small pools in the Kandalaksha Bay of the White Sea (66°19′40ʺ N, 33°52′10ʺ E) and in early September, 2013 in two very small puddles on the western coast of Lake Ladoga (60°34′15ʺ N, 30°41′9ʺ E). The cenobia and cells from both Ladoga probes were studied under the light microscope during long-time cultivation in a mixed culture, using liquid mineral medium supplemented by vitamins B6 and B12. In fresh material, the greater part of revealed 8- and 4-celled coenobia and cells were larger than it was described by Cohn, namely the sizes of coenobia in the first puddle were 62–66 μm, those of the cells 22–28 μm; the coenobia in the second one were 70–85.5 μm, the cells 26–30 μm. After 1.5–2 months cultivation, there were observed single coenobia of spherical cells organized in tetrades or giant (100 μm) coenobia with dying cells. In that period and later, single cells with thickened walls (cysts or zygotes) of different sizes were revealed. After 5 months of cultivation, few single cells with short appendix or very small young coenobia with rounded cells were observed.