No organism lives entirely alone. Protists interact with other protists, animals, and with bacteria, sometimes as predators and sometimes as prey.
Nematode feeding on Euglena
An unidentified nematode worm feeds on an Euglena. Note the starch grain from the Euglena moving down the gut of the worm in this sequence of images. The broken pellicle and ruptured cell membrane has caused the cell to swell up.
Ciliate feeding frenzy
Unidentified ciliates squirming in the carcass of a dead planktonic crustacean. The bright droplets are oil globules, which the ciliates are feeding on (ingested oil globules are also visible within their cytoplasm). Outside the animal carcass you can see another ciliate which has been left out of the feeding frenzy, and is trying to find its way in….
Another feeding-frenzy on an insect carcass has been filmed in higher quality below:
Epiphytic green algae
Germinating green alga spore, anchored to the cell wall of a dead cyanobacterial filament. The green alga is not a parasite, merely using the cyanobacterium as a substrate for attachment. After all, its erstwhile host is dead.
It is possible that these may be dwarf male filaments from some species of Oedigoniales, a phenomenon called nannandry. In such species, smaller male filaments attach to and grow on the larger female filaments, and participate in sexual reproduction at maturity.
Ciliate feeding on “noodle”
This ciliate is slurping up a noodly filamentous bacterium, which is also straining to escape by gliding motility. Watch the ciliate ratchet up the bacterium, while struggling to find leverage in the surrounding environment.
Heliozoan trapping flagellates
A heliozoan, probably Raphidophrys, uses its long axopods to trap flagellated cells of Ochromonas (a chrysophyte). The whole heliozoan also moves and the axopods appear to bend away from the direction of motion.