Protists play crucial roles in aquatic ecosystems as producers, consumers, and decomposers. They form the foundation of food webs, contribute to nutrient cycling, and impact global carbon levels. Their ecological importance can't be overstated.

Some protists are parasites that cause devastating diseases like malaria. These crafty organisms have evolved complex life cycles and immune evasion strategies. Understanding protist parasites is key to developing treatments and prevention methods.

Protists in Aquatic Ecosystems

Protists as ecosystem producers

• Photosynthetic protists (phytoplankton) serve as primary producers in aquatic environments by utilizing light energy, water, and carbon dioxide to generate organic compounds through photosynthesis (diatoms, dinoflagellates, green algae)
• Phytoplankton form the foundation of aquatic food webs as they are consumed by zooplankton, which in turn are eaten by larger organisms such as fish and whales, thus supporting the growth and survival of higher trophic levels
• Phytoplankton contribute significantly to primary production in oceans, accounting for roughly 50% of global primary production and playing a vital role in the global carbon cycle by absorbing atmospheric carbon dioxide
• In some cases, excessive nutrient input can lead to rapid growth of phytoplankton populations, resulting in algal blooms that can disrupt ecosystem balance

Ecological Interactions and Nutrient Cycling

• Protists participate in various trophic levels within aquatic ecosystems, from primary producers to consumers and decomposers
• Many protists (such as protozoa) form symbiotic relationships with other organisms, contributing to the complex web of interactions in aquatic environments
• Protists play crucial roles in biogeochemical cycles, facilitating the movement of nutrients through ecosystems and between biotic and abiotic components
• Aquatic food webs, which include protists at various levels, illustrate the transfer of energy and nutrients through ecosystems

Protist Parasites and Diseases

Life cycle of malaria parasites

• Plasmodium, a genus of apicomplexan parasites, causes malaria and is transmitted by female Anopheles mosquitoes
• The life cycle of Plasmodium involves two hosts: mosquitoes (definitive host) and humans (intermediate host)

1. Sporozoites are injected into the human bloodstream during a mosquito bite
2. Sporozoites infect liver cells, multiply, and form merozoites
3. Merozoites infect red blood cells, multiply, and cause cell rupture, releasing more merozoites
4. Some merozoites differentiate into gametocytes, which are taken up by mosquitoes during blood meals
5. Gametocytes undergo sexual reproduction in the mosquito gut, forming sporozoites that migrate to salivary glands

• Malaria causes flu-like symptoms, anemia, and potentially fatal complications, resulting in hundreds of thousands of deaths annually, primarily in sub-Saharan Africa, and imposing an economic burden on affected countries due to healthcare costs and reduced productivity

Immune evasion by protist parasites

• Protist parasites have evolved various strategies to evade host immune responses
  • Antigenic variation involves regularly changing surface proteins to avoid antibody recognition (Trypanosoma brucei alters its variant surface glycoprotein (VSG) coat to cause African sleeping sickness)
  • Intracellular hiding allows parasites to reside within host cells to avoid detection by immune cells (Toxoplasma gondii replicates within host cell vacuoles causing toxoplasmosis)
  • Immunosuppression occurs when parasites suppress host immune responses (Plasmodium falciparum produces proteins that inhibit T cell activation and proliferation)
• Protist parasites cause diseases by disrupting host cell functions and inducing tissue damage
  • Trypanosoma cruzi invades heart and digestive system cells, leading to organ dysfunction (Chagas disease)
  • Phytophthora infestans secretes enzymes that degrade plant cell walls and cause necrosis (potato late blight)