Green spheres

Green spheres

Observation - Green spheres. Description: I found these small green spheres, ca 1-2mm diameter on gravelly mud by a reservoir. They are firmly fixed and full of water when they burst. Some presumably collapsed ones can be seen. They don't seem to be fungal, so may be alga or some other lower plan

I found these small green spheres, ca 1-2mm diameter on gravelly mud by a reservoir. They are firmly fixed and full of water when they burst. Some presumably collapsed ones can be seen. They don't seem to be fungal, so may be alga or some other lower plant. Any suggestions would be welcome.