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Chapter 28: Protists

Overview

Dr. Adam Hrincevich

This chapter focuses on the diverse group of eukaryotic organisms known as protists, which exhibit significant structural and functional diversity compared to other eukaryotes. Protists are primarily defined by their eukaryotic nature and can be found in a variety of habitats, ranging from freshwater environments to the depths of the ocean.

Learning Objectives

• Understand the process and implications of secondary endosymbiosis in the evolution of eukaryotes.

• Characterize the excavates group of protists and their unique traits.

• Identify and give examples of the SAR group protists, which include Stramenopiles, Alveolates, and Rhizarians.

• Describe the distinctive features and ecological significance of red and green algae.

• Explore the relationships and evolutionary connections between protists, fungi, and animals.

• Discuss the ecological roles that protists play in their environments, including symbiotic relationships and contributions to ecosystems.

• Suggest: Complete a crossword puzzle for terminology comprehension to reinforce vocabulary related to protists.

Concept 28.1: Most Eukaryotes are Single-Celled Organisms

Protists

All protists are defined as eukaryotes and are characterized by their complex cells that contain membrane-bound organelles. Most protists are unicellular, but they can also exhibit a broad range of cellular structures and organization, leading to significant diversity.

Structural and Functional Diversity in Protists

Protists exhibit the highest levels of diversity among eukaryotes, displaying forms that may be single-celled, colonial, or multicellular. This diversity of form corresponds to their various modes of nutrition, which can be categorized into major nutritional types:

• Photoautotrophs: Organisms like algae that contain chloroplasts and can perform photosynthesis to produce their own food.

• Heterotrophs: These protists absorb nutrients from their surroundings or ingest food particles, often acting as predators or parasites.

• Mixotrophs: Capable of photosynthesis and heterotrophy depending on environmental conditions. Reproductive strategies are also diverse, with many protists capable of both sexual and asexual reproduction, which aids in their adaptability.

Four Supergroups of Eukaryotes

Eukaryotes are grouped into four major supergroups based on evolutionary relationships, which include:

1. Excavata

2. SAR (Stramenopiles, Alveolates, Rhizarians)

3. Archaeplastida

4. Unikonta

Endosymbiosis in Eukaryotic Evolution

Endosymbiosis is a crucial mechanism driving protist diversity, where one organism lives within another, leading to significant evolutionary developments. The mitochondria and plastids found in modern eukaryotic cells are believed to have originated from prokaryotic organisms engulfed by ancestral eukaryotic cells. Specific details include:

• Mitochondria: Evolved from an alpha-proteobacterium, which transitioned from a free-living organism to a vital cellular organelle, significantly enhancing the energy efficiency of the host cell.

• Plastids: Evolved from a cyanobacterium, leading to the development of chloroplasts that enable photosynthesis in various protists and plants.

Concept 28.2: Excavates

The excavate group is characterized by unique flagella and modified mitochondria. This group features a distinct cytoskeleton and a feeding groove that assists in the uptake of nutrients. The main groups within Excavata include:

• Diplomonads: Organisms that lack plastids and generally inhabit anaerobic environments. They possess reduced mitochondria known as mitosomes, and typically have two nuclei. An important example is Giardia intestinalis, a well-known parasitic protozoan that can cause gastrointestinal infections in humans.

• Parabasalids: Similar to diplomonads, parabasalids also lack plastids and are adapted to anaerobic conditions. They possess hydrogenosomes, which generate energy anaerobically. Trichomonas vaginalis, causing sexually transmitted infections, is an example of a parabasalid.

Mitochondrion vs. Hydrogenosome vs. Mitosome

• Mitochondrion: Responsible for generating ATP through oxidative phosphorylation in the presence of oxygen.

• Hydrogenosome: Generates ATP anaerobically utilizing alternative compounds, as seen in parabasalids.

• Mitosome: Functions primarily in iron-sulfur cluster biosynthesis without producing ATP, found in some anaerobic protists.

Excavata: Euglenozoans

This group encompasses various organisms, including heterotrophs, autotrophs, mixotrophs, and parasites, all characterized by their unique flagella (often featuring a spiral rod). The clade is subdivided into:

• Kinetoplastids: Characterized by a single large mitochondrion that houses a kinetoplast, a mass of DNA. This group includes free-living species that feed on prokaryotes and notable pathogenic species, such as Trypanosoma, responsible for diseases like African sleeping sickness and Chagas disease.

• Euglenids: Typically possess one or two flagella and are capable of switching between autotrophy (in light) and heterotrophy (in dark conditions), showcasing adaptability.

Concept 28.3: SAR (Stramenopiles, Alveolates, Rhizarians)

The SAR supergroup is monophyletic and defined by molecular data. It comprises a highly diverse collection of protists:

Stramenopiles

This group includes several key photosynthetic organisms, such as diatoms, golden algae, and brown algae. Stramenopiles are characterized by unique flagella with hair-like projections.

Stramenopiles: Diatoms

These unicellular algae are notable for their intricate silica-based cell walls, resembling glass, and play a major role in aquatic ecosystems as significant components of phytoplankton, contributing to global carbon fixation. After diatom blooms, the dead algae can sequester carbon on ocean floors, impacting the global carbon cycle.

Stramenopiles: Brown Algae

Brown algae represent the largest and most complex forms, including common seaweeds. They lack true tissues and organs but have specialized structures such as:

• Holdfast: Anchors the organism.

• Stipe: Supports the main body (or fronds).

• Blade: Photosynthetic surface area. Gas-filled floats, called bladders, enable buoyancy and keep the algae elevated in water for optimal light exposure during photosynthesis.

Alveolates

Defined by the presence of membrane-bound sacs (alveoli) beneath their plasma membranes, the Alveolate group includes:

• Dinoflagellates: Recognized for their unique two-flagella configuration and cellulose plates. Some species cause harmful algal blooms (e.g., red tides), which can produce toxins affecting marine life and human health.

• Apicomplexans: Mostly parasitic organisms that require multiple hosts to complete their life cycles, such as Plasmodium, which causes malaria and has severe public health implications.

• Ciliates: Utilize hair-like structures called cilia for movement and feeding. They are notable for possessing two types of nuclei: a large macronucleus and one or more smaller micronuclei. An example includes Paramecium, widely studied for its complex behaviors and cellular functions.

Rhizarians

Primarily composed of various amoeboid organisms that use pseudopodia for locomotion and feeding; key groups include:

• Radiolarians: Marine protists known for their complex and often beautiful silica skeletons, employing fine pseudopodia to capture prey.

• Foraminiferans (Forams): Recognized by their porous, chambered calcium carbonate tests, foraminifera serve as important indicators of historical ocean temperatures and paleoenvironmental conditions due to their fossil records.

Concept 28.4: Archaeplastida

This supergroup includes red algae, green algae, and land plants, highlighting evolutionary connections between these groups.

Red Algae

Characterized by the presence of phycoerythrin pigment, which gives red algae their distinctive color. Most red algae are multicellular and thrive mainly in tropical coastal waters. They play vital ecological roles, including providing habitat for marine organisms and contributing to reef-building processes.

Green Algae

Green algae are recognized for their chloroplasts and share a closer evolutionary lineage with land plants. This group is considered paraphyletic and is divided into charophytes and chlorophytes, with the latter commonly found in freshwater environments and often forming important primary producers in aquatic ecosystems.

Concept 28.5: Unikonta

The Unikonta supergroup includes animals, fungi, and some protists, specifically amoebozoans and opisthokonts. The timeline for their separation from other eukaryotic lineages remains a topic of ongoing research.

Amoebozoans

Amoebozoans are characterized by their unique lobe- or tube-shaped pseudopodia. Key members include:

• Slime Molds: These organisms exhibit many characteristics similar to fungi due to convergent evolution. They consist of plasmodial slime molds, which have a multicellular feeding stage, and cellular slime molds that exist as single cells and can aggregate in response to starvation.

• Tubulinids: Typically free-living and found in various environments, these amoebas are heterotrophic and contribute to microbial ecology in their habitats.

• Entamoebas: Parasitic amoebas, such as Entamoeba histolytica, known to cause amebic dysentery, posing health risks in many regions of the world.

Concept 28.6: Key Roles in Ecological Communities

Symbiotic Protists

Some protists form symbiotic relationships with their hosts, providing benefits (e.g., corals hosting photosynthetic algae) or causing diseases (e.g., Plasmodium).

Photosynthetic Producer Protists

Protists are critical players in carbon fixation and primary production within aquatic ecosystems. Changes in nutrient levels can significantly influence their population dynamics, ultimately affecting food webs and ecosystem health.