Bullseye

Or two actually. That is what I see here - a young bull running around on the green, green grass of his first spring, looking curiously around for the wonders of the world:

But then again, not many people have told me I lack imagination.
Others just call this algae Mesostigma viride. Me too, when counting.

The young bull expression comes from the two pyrenoids on each side of the cell and the ”lightly saddle-like bent” body, as Skuja (1948) describes the form of the cell.

The flatness of the cell supports the impression of the saddle:

What also catches the eye is the square-like-structure on the cell:

For the squares a high N.A. for the objective and condensor is recommended. It is quite small, the whole cell is only about 10 µm "big".

The flagella, on the other hand, are more seldom seen in fixed samples. They emerge from the ventral side of the cell, not apical or from the side, as in most other algae.

Also the stigma is not so clearly seen in lugol samples. The stigma should be a big one and in the middle of the cell - that's why Lauterborn gave it the name Mesostigma.

Mesostigma has earlier been regarded as a member of Prasinophyta, so being in the oldest group of green algae, going back perhaps all the way to PreCambrian (Moestrup 2002). This algae has been a subject to intensive taxonomical studies and a has finally gotten a whole Class of it's own: Mesostigmatophyceae in Streptophyta, (Marin & Melkonian 1999, Leliaert et al 2012). So not in Chlorophyta. AlgaeBASE still uses terms as Prasinophyte and Charophyta, strange.

I have seen these algae in a couple of samples in Finnish waters. Once you get the picture of it in your brain, it's very easy to identify. Although this time the drawings in the books are not very helpful. Even Skuja's drawing is this time not quite accurate:

Skuja, H. (1948).

Try to find it, it is there, waiting for you in many kinds of fresh waters.


Literature:

Lauterborn, R. (1894). Ueber die Winterfauna einiger Gewässer der Oberrheinebene. Mit Beschreibungen neuer Protozoen. Biologische Centralblatt 14: 390-398.
http://www.algaebase.org/pdf/562DFCAF13e03286E3uGv3A26EC8/48540.pdf

Marin, B. & Melkonian, M. (1999). Mesostigmatophyceae, a new class of streptophyte green algae revealed by SSU rRNA sequence comparisons. Protist 150: 399-417, 9 figs, 2 tables.

Skuja, H. (1948). Taxonomie des Phytoplanktons einiger Seen in Uppland, Schweden. Symbolae Botanicae Upsalienses 9 (3). A. -B. Lundequistska Bokhandeln, Uppsala, 399 pp., XXXIX pls.

Leliaert F., Smith D.R., Moreau H., Herron M.D., Verbruggen H., Delwiche C.F. & De Clerck O. Phylogeny and molecular evolution of the green algae. (2012) Critical Reviews in Plant Sciences 31: 1-46.