This is Halosphaera (probably H. viridis?) from the Prasinophyceae. This "strange" alga is making me a lot of difficulties: it has various forms and also a motil stage (many tiny flagellates).
The shaere on the picture had a diameter of 130 µm.
Two pictures: with Obj. 20x, DIC, (stack with 8 pictures)
with Obj. 40x, DIC, (stack with 2 pictures)