Hi -
No, I'm afraid "plankton" is not the answer. That's small animals and plants floating in water -- the animals will be one or more links up the chain.
There's a confusion here between "low" organisms, meaning simple ones, and organisms low down in the food chain.
So the answer is actually plants -- whether single-celled plankton, lettuces or huge trees. They take light energy, carbon dioxide and water, and make sugars and other useful things.
Plants are eaten by animals, and also by decomposing fungi, bacteria etc, all the way up the food chain.
A few bacteria are able to get energy from other sources than light, such as chemicals in the "black smoker" deep-ocean hot springs. These support a special chemical-energy based food-chain.