Organisms contain compounds such as proteins starches and fats, the chemical bonds in these compounds can be a source of

Only algae, oak trees, and bermuda grass produce atp by photosynthesis i am almost positive

Although i don't know what is the simulated system, starfish are predators that eat mostly  microalgae,  snails, sponges, bivalves, mussels and other small marine animals. removing starfish from any system would definitely increase this animals that they prey.