It's important to clarify that "plant-like protist" is not a formal taxonomic term. While protists can share characteristics with plants, such as photosynthesis, they are a diverse group with their own unique classifications.

However, you're right that many living things depend on photosynthetic protists for their survival, and there are a few key reasons for this:

1. Primary Producers: Photosynthetic protists, like algae, are primary producers, meaning they convert sunlight into energy through photosynthesis. This makes them the foundation of many aquatic food webs. Larger organisms, like fish, rely on smaller organisms that eat algae for their sustenance.

2. Oxygen Production: Through photosynthesis, protists release oxygen as a byproduct. This oxygen is essential for the respiration of many organisms, including animals, fungi, and even other protists.

3. Food Source: Some protists, like diatoms, are directly consumed by animals. This is especially important in marine environments where diatoms are a major food source for many zooplankton.

4. Symbiotic Relationships: Photosynthetic protists can also form symbiotic relationships with other organisms. For example, corals depend on a type of algae called zooxanthellae for their food and coloration.

5. Ecological Importance: Protists play a crucial role in the cycling of nutrients in aquatic ecosystems. They help regulate the levels of carbon dioxide and other nutrients, maintaining the balance of the environment.

In summary, the dependence of many living things on photosynthetic protists stems from their critical role in primary production, oxygen production, food webs, and symbiotic relationships. They are essential to the functioning of many ecosystems, particularly aquatic ones.