23.4 Ecology of Protists

Protists in Aquatic Ecosystems

Protists are central players in aquatic ecosystems, functioning as producers, consumers, and decomposers. They form the base of aquatic food webs, drive nutrient cycling, and have a measurable impact on global carbon levels.

Protists as Ecosystem Producers

Photosynthetic protists, collectively called phytoplankton, are the primary producers of aquatic environments. Groups like diatoms, dinoflagellates, and green algae use light energy, water, and $CO_2$ to build organic compounds through photosynthesis.

Phytoplankton account for roughly 50% of all global primary production, which makes them just as important as all land plants combined when it comes to fixing carbon. They absorb atmospheric $CO_2$ during photosynthesis, so they're a major part of the global carbon cycle.

These organisms form the base of aquatic food webs:

• Phytoplankton are consumed by zooplankton (tiny animal-like organisms)
• Zooplankton are then eaten by larger organisms like fish and filter-feeding whales
• This chain of consumption transfers energy upward through trophic levels

When nutrients like nitrogen and phosphorus enter waterways in excess (from agricultural runoff, for example), phytoplankton populations can explode into algal blooms. These blooms block light, deplete dissolved oxygen as they decompose, and can produce toxins, all of which disrupt ecosystem balance.

Ecological Interactions and Nutrient Cycling

Protists don't just sit at one level of the food web. Different species occupy roles as producers, consumers, and decomposers, sometimes within the same ecosystem.

• Many protozoa (heterotrophic protists) form symbiotic relationships with other organisms. Some live in the guts of termites and help digest cellulose, while others partner with coral.
• Protists are key drivers of biogeochemical cycles. They help move carbon, nitrogen, and phosphorus between living organisms and the nonliving environment (water, sediment, atmosphere).
• By consuming bacteria and recycling nutrients back into forms that other organisms can use, protists act as a critical link between the microbial world and larger food webs.

Protist Parasites and Diseases

Some protists are parasites with complex life cycles that allow them to infect animal and plant hosts. Understanding these organisms is essential for developing treatments and prevention strategies.

Life Cycle of Malaria Parasites

Plasmodium is a genus of apicomplexan parasites that causes malaria. It's transmitted by female Anopheles mosquitoes and requires two hosts to complete its life cycle: the mosquito (definitive host, where sexual reproduction occurs) and the human (intermediate host).

The life cycle proceeds through these stages:

1. A mosquito bites a human and injects sporozoites into the bloodstream.
2. Sporozoites travel to the liver, infect liver cells, and multiply to form merozoites.
3. Merozoites are released into the blood, where they infect red blood cells, multiply inside them, and cause the cells to rupture. This releases a new wave of merozoites that infect more red blood cells.
4. Some merozoites differentiate into gametocytes (sexual stage cells) instead of continuing the cycle.
5. When another mosquito feeds on the infected person, it picks up gametocytes. These undergo sexual reproduction in the mosquito gut, eventually producing new sporozoites that migrate to the salivary glands, ready to infect the next human host.

The repeated rupture of red blood cells in step 3 is what causes malaria's characteristic symptoms: cycling fevers, chills, anemia, and in severe cases, organ failure and death. Malaria kills hundreds of thousands of people each year, predominantly in sub-Saharan Africa, and places a heavy economic burden on affected regions through healthcare costs and lost productivity.

Immune Evasion by Protist Parasites

Protist parasites have evolved several strategies to dodge the host immune system, which is a big part of why they're so difficult to treat:

• Antigenic variation: The parasite regularly switches the proteins displayed on its surface, so antibodies the host already made no longer recognize it. Trypanosoma brucei, which causes African sleeping sickness, cycles through a library of over 1,000 different variant surface glycoprotein (VSG) genes to stay ahead of the immune response.
• Intracellular hiding: Some parasites take refuge inside host cells, where circulating immune cells can't reach them. Toxoplasma gondii (the cause of toxoplasmosis) replicates inside specialized vacuoles within host cells that prevent the cell from destroying it.
• Immunosuppression: Certain parasites actively dampen the host's immune response. Plasmodium falciparum produces proteins that inhibit T cell activation and proliferation, weakening the body's ability to fight back.

Beyond immune evasion, protist parasites cause disease by directly damaging host tissues:

• Trypanosoma cruzi invades cells of the heart and digestive system, leading to chronic organ dysfunction known as Chagas disease.
• Phytophthora infestans (technically an oomycete, often studied alongside protists) secretes enzymes that degrade plant cell walls and cause tissue death, resulting in potato late blight, the disease behind the Irish Potato Famine.