Concept preview

What you will be able to do

1. 01Explain how phytoplankton support aquatic food webs and produce oxygen.
2. 02Describe important partnerships (symbioses) between protists and other organisms.
3. 03Describe the malaria life cycle and other major protist diseases.
4. 04Explain why protist diseases are hard to prevent and how they are controlled.

The protists are easy to overlook — most are invisible, and they live out of sight in water and soil — yet their collective effect on the planet is out of all proportion to their size. In this final protist topic we pull back from the single cell to see what these organisms do in the wider world: how they feed the oceans and make much of our oxygen, how they form partnerships that build entire ecosystems, and how a handful of them cause some of the most devastating diseases in human history. By the end you should see the protists not as a biological curiosity but as one of the quiet powers that runs the living Earth — for better and for worse.