Studies of the effect of magnetic fields on diatoms have used a benthic diatom called Amphora coffeaeformis.

"Benthic" organisms live on the floor of seas, rivers and lakes. Amphora coffeaeformis lives in brackish areas like a bay or estuary (where a river meets the sea). In contrast,planktonic diatoms float up in the water, usually near the surface. Because Amphora coffeaeformis is benthic, it has some qualities that made it handy for the study of electromagnetic effects on orientation.

Benthic diatoms in nature crawl about the bottom and leave a slime trail. In the lab they can be placed on an agar plate and their movements tracked with and without a magnetic field.

You can obtain samples of Amphora coffeaeformis and other diatoms from the Culture Collection of Algae at the University of Texas,
utex. It costs about $75/ sample.