Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 28.6 Protists Play Key Roles in Ecological Communities
Protists are fundamental players in ecological systems—especially aquatic ones—due to their roles as symbionts and producers. Their impact spans from coral reefs to the global carbon cycle, and they respond sensitively to environmental changes such as nutrient levels and climate change.
Protists as Symbionts
- Mutualistic protists benefit their hosts:
- Dinoflagellates live inside coral polyps, nourishing reefs and supporting biodiversity.
- Hypermastigotes in termite guts digest cellulose, enabling wood consumption.
- Parasitic protists can be harmful:
- Plasmodium: causes malaria, lowering economic development.
- Pfiesteria shumwayae: a fish-killing dinoflagellate.
- Phytophthora ramorum: causes sudden oak death.
Protists as Producers
- Photosynthetic protists form the base of aquatic food webs.
- Convert CO₂ into organic matter, supporting all trophic levels.
- Key contributors:
- Diatoms, dinoflagellates, multicellular algae: ~30% of global photosynthesis
- Photosynthetic prokaryotes: ~20%
- Plants: ~50%
- Nutrient availability (N, P, Fe) regulates protist populations. Excess nutrients from human activity can cause overgrowth and ocean dead zones.
Impact of Climate Change
- Global warming is lowering chlorophyll levels (a biomass proxy) in oceans.
- Warm water hinders nutrient upwelling, starving protists.
- Declines in photosynthetic protists threaten:
- Marine food webs
- Fishery yields
- The global carbon cycle
- On land, plants dominate photosynthesis, but aquatic systems rely on protists.
In a Nutshell
Protists are essential components of ecosystems. As symbionts, they can help or harm their hosts; as producers, they fuel entire aquatic food webs. However, their productivity is tightly linked to nutrient availability and climate conditions, making them critical indicators—and victims—of environmental change.