Dinoflagellates - Gymnodinium
Gymnodinium belongs to the naked flagellates, which are without cellwall,
Not infrequently they have no chloroplast and live heterotrofic.
The image shows the big nucleus inside the cell and the two grooves with flagella, (which are not to be seen). In the upper part of the image you can see a diatom-chain from the genus Chaetoceros.
The image and the video are taken from a seawater sample, but the same genus is presented frequently in freshwater and lakes. The cell length is about 40-80 my.
Video shows the moving of Gymnodinium