Protiststs are of two types: heterotrophic and autotrophic. Autotropes can make their own food while heterotrophs have to rely on other organisms for their nutrition.
This places some of the protist at the lowest level of the food chain where they are the source of nutrition for other organisms and some of the protist are at a level higher up in the food chain.
Algae are one group of protists that produce their own food and form an integral part of the marine food chain. There are many types of algae, some of which can be classified as green, red and brown. These protists have features that makes them closest to plants.
At a higher level in the food chain are species of protists that include amoeba and paramecium; these capture and eat other organisms like bacteria and protozoans.