Answer
Photosynthetic protists form the basis of aquatic food webs, which are some of the most productive on Earth. With life having begun in the sea, they also were crucial in the early switch to an oxygen-rich atmosphere for Earth, leading to life as we know it.
Work Step by Step
The key here is to recall early material about evolution and oxygen in the atmosphere as well as to review Fig. 28.29 to see to see the role of photosynthetic protist producers in some ecosystems.
