Lecture 3.2 - Protists

Learning Objectives

• Describe some of the key characteristics of protists.
• Give 3 examples of protists.
• Explain the key roles protists play in the environment.

What Are Protists?

• Diversity: Protists are a very diverse group of organisms.
• Defining Characteristics:
  • They are eukaryotes, meaning they have a nucleus.
  • This group includes all eukaryotes that are not land plants, fungi, or animals.
  • Most protists are unicellular and very small.

Discovery of Protists

• Historical Perspective:
  • Discovered by Antonie van Leeuwenhoek, who noted the abundance of living creatures in a drop of water.
  • Examples include Euglena, a common protist found in pond water.

Types of Protists

• Cell Structure:
  • Most protists are unicellular, but some form colonies or simple multicellular structures.
• Nutritional Diversity:
  • Photoautotrophs: Contain chloroplasts (e.g., algae).
  • Heterotrophs: Absorb organic molecules or ingest larger particles.
  • Mixotrophs: Combine photosynthesis with heterotrophic nutrition.
• Reproductive Strategies:
  • Can reproduce sexually or asexually (e.g., via fission).

Protist Phylogeny

• Still a work in progress with four main clades:
  • Excavata
  • SAR (Stramenopiles, Alveolates, Rhizarians)
  • Archaeplastida
  • Unikonta

Clade Excavata

• Characteristics:
  • Monophyletic group, some have an excavated feeding groove.
  • Nutritional modes: predatory, photosynthetic, mixotrophic, and parasitic.
• Example: Giardia intestinalis
  • A flagellated unicellular eukaryote that is a parasite in the human intestine.
• Transmission:
  • Spread through contaminated water and is resistant to chlorine.

Symptoms and Prevention of Giardia

• Symptoms may appear 1-3 weeks post-infection, lasting 1-2 weeks or more:
  • Diarrhea
  • Gas
  • Greasy, floating stools
  • Abdominal cramps, nausea/vomiting, dehydration.
• Prevention includes avoiding contaminated water.

Clade SAR

• Diversity:
  • Monophyletic group characterized by DNA similarities.
  • Includes diatoms, dinoflagellates, brown algae, and more.

Stramenopiles

• Key Organisms:
  • Includes diatoms, brown algae, and important pathogens.
• Diatoms:
  • Unicellular photosynthetic algae with a glass-like silica wall.
  • Important in marine productivity and carbon fixation (~20% globally).

Brown Algae

• Characteristics:
  • Multicellular and primarily marine; largest and most complex algae.
  • Have chlorophyll-a, chlorophyll-c, and carotenoid pigments; store carbohydrates as laminarin.

Brown Algae Lifecycle: Alternation of Generations

1. Sporophyte (2n) grows from zygote.
2. Spores produced by meiosis develop into gametophytes (n).
3. Male gametophytes release sperm; female gametophytes produce eggs, enhancing fertilization chances.

Alveolates

• Defined by: Membrane-enclosed sacs (alveoli) beneath the plasma membrane.
• Examples:
  • Dinoflagellates: Marine photosynthetic organisms; important in phytoplankton; can produce toxic blooms (red tides).
  • Apicomplexans: Nearly all are animal parasites, e.g., Plasmodium (causes malaria).

Malaria and Plasmodium

• In 2015, 210 million cases worldwide, with significant mortality in sub-Saharan Africa.
• Symptoms include fever, headache, and chills; serious if untreated.
• Control strategies include mosquito nets and spraying.

Clade Archaeplastida

• Contains red and green algae, and the ancestors of land plants.

Rhodophyta (Red Algae)

• Mostly marine; possess chlorophyll-a and phycoerythrin.
• Used in sushi and food stabilizers.

Chlorophyta (Green Algae)

• Most plant-like algae, commonly found in freshwater; studied for biotechnology.

Clade Unikonta

• Includes protists closely related to fungi and animals, alongside fungi and animals themselves.

Amoebozoans

• Example: Slime molds, can be plasmodial or cellular.
• Plasmodial slime molds form a multinucleate mass and produce spores in stressful conditions.

Role of Protists in the Environment

• Symbiotic Relationships:
  • Mutualistic relationships benefit both species.
  • Example: Coral relies on zooxanthellae (a dinoflagellate) for energy while offering protection.
• Photosynthesis:
  • Phytoplankton contribute ~30% of global photosynthesis.
• Parasitic Roles:
  • Malaria and other diseases demonstrate the impact of parasitic protists on human health.
  • Pathogens like Phytophthora significantly affect agriculture and ecosystem health.

Summary of Learning

• Protists are a diverse group including various types of eukaryotes.
• Phylogenetically categorized into four main clades:
  • Excavata, SAR, Archaeplastida, and Unikonta.
• Their ecological roles encompass symbiosis, photosynthesis, and parasitism, highlighting their importance in various ecosystems and environments.

Recommended Readings

• Campbell's Biology, 12th Ed., Chapter 28: Protists.