Microbial Diversity: Protists and Fungi (General Biology Study Notes)

Microbial Diversity: Protists and Fungi

Introduction

This study guide covers the diversity, evolution, and ecological roles of protists and fungi, as well as their classification and distinguishing features. These topics are essential for understanding microbial life and its impact on ecosystems, as outlined in General Biology curricula.

Protists

Definition and Phylogeny of Protists

Protists are a diverse group of mostly unicellular eukaryotic organisms that are not classified as animals, plants, or fungi. They are found in various environments, especially aquatic and moist terrestrial habitats.

• Phylogeny: Protists are paraphyletic, meaning they do not form a single clade but are grouped for convenience.

• Supergroups: Protists are distributed among several eukaryotic supergroups, reflecting their evolutionary diversity.

Nutrition in Protists

• Autotrophic: Use photosynthesis to produce organic compounds (e.g., algae).

• Heterotrophic: Obtain nutrients by absorbing organic material or ingesting particles (e.g., amoebae, slime molds).

• Mixotrophic: Combine photosynthesis and heterotrophy (e.g., Euglena).

Reproduction in Protists

• Asexual reproduction: Common, often by binary fission or budding.

• Sexual reproduction: Occurs in many groups, with diverse life cycles.

Ecological Roles of Protists

• Producers: Photosynthetic protists (algae) form the base of aquatic food webs.

• Predators: Some protists consume bacteria, fungi, or other protists.

• Symbionts: Can be mutualistic (e.g., in termite guts) or parasitic (e.g., Plasmodium).

• Saprotrophs: Decompose organic matter, recycling nutrients.

Example: Streptococcus tauri is a very small protist with a single nucleus, mitochondrion, and chloroplast, demonstrating the diversity in protist cell structure.

Protists in Ecosystems

• Essential in both aquatic and terrestrial systems, especially where water is present.

• Key components of plankton and soil food webs.

• Participate in the microbial loop, recycling nutrients and energy.

Fungi

Definition and Evolution of Fungi

Fungi are eukaryotic organisms distinct from plants, animals, and protists. They are primarily multicellular (except yeasts) and obtain nutrients by absorption.

• Evolution: Fungi are more closely related to animals than to plants. The earliest undisputed fungal fossils are about 460 million years old.

Structure and Growth of Fungi

• Hyphae: Thread-like filaments that make up the body of multicellular fungi.

• Septa: Cross-walls that divide hyphae into cells; some fungi have septate hyphae, others are coenocytic (without septa).

• Mycelium: A network of hyphae that penetrates substrates for nutrient absorption.

Fungal Nutrition and Lifestyles

• Saprotrophic: Decompose dead organic matter (e.g., wood, leaf litter).

• Symbiotic: Form mutualistic relationships (e.g., mycorrhizae with plants, lichens with algae/cyanobacteria).

• Parasitic: Infect living organisms, sometimes causing disease.

Fungal Cell Structure

• Cell wall: Composed of chitin and glucan (distinct from plant cell walls).

• Membrane sterol: Contains ergosterol (not cholesterol as in animals).

Fungal Reproduction

• Asexual reproduction: Via spores, budding (yeasts), or fragmentation.

• Sexual reproduction: Increases genetic diversity; involves fusion of hyphae and formation of sexual spores.

Advantages of sexual reproduction: Increases genetic diversity and adaptive potential; removes deleterious mutations. Disadvantages: Energy cost, risk of breaking up favorable gene combinations.

Major Groups of Fungi

• Chytridiomycota: Aquatic, produce flagellated spores (zoospores).

• Zygomycetes: Form zygospores; mostly terrestrial.

• Glomeromycetes: Form arbuscular mycorrhizae with plants.

• Ascomycetes: "Sac fungi"; produce spores in asci.

• Basidiomycetes: "Club fungi"; produce spores on basidia (e.g., mushrooms).

Functions and Roles of Fungi

• Saprobes: Decompose complex organic materials, including lignin and cellulose.

• Mutualists: Mycorrhizae (with plants), lichens (with algae/cyanobacteria), and symbioses with insects (e.g., leaf-cutter ants).

• Pathogens/Parasites: Infect plants, animals, and humans; some are opportunistic pathogens.

Symbiotic Relationships

• Mycorrhizae: Mutualistic association between fungi and plant roots; fungi provide water and minerals, plants provide organic carbon.

• Lichens: Symbiosis between a fungus and an alga or cyanobacterium; important for soil formation and as bioindicators.

• Fungal gardens: Cultivated by ants and termites for food.

Comparisons and Classification

Differences between Bacteria, Archaea, and Eukarya

The following table summarizes key differences among the three domains of life:

Trait	Bacteria	Archaea	Eukarya
Linkage of lipids	Ester	Ether	Ester
Backbone of lipids	Glycerol-3-phosphate	Glycerol-1-phosphate	Glycerol-3-phosphate
Cell wall	Bacterial	Bacterial-like	Eukaryotic
Nucleus	No	No	Yes
Membrane-bound organelles	No	No	Yes
Introns	No	Yes	Yes
Chromosome shape	Mostly circular	Circular	Linear
Cell division	Bacterial	Eukaryotic-like	Eukaryotic

Origin of Eukaryotes: Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotic cells as a result of symbiosis between different species of prokaryotes. Mitochondria and chloroplasts are believed to have originated from free-living bacteria that were engulfed by ancestral eukaryotic cells.

• Evidence: Mitochondria and chloroplasts have their own DNA, double membranes, and reproduce independently within the cell.

Summary

• Protists and fungi are diverse, ecologically important groups of eukaryotes.

• Protists exhibit a wide range of nutritional modes and life cycles.

• Fungi play key roles as decomposers, mutualists, and pathogens.

• Understanding the differences between Bacteria, Archaea, and Eukarya is fundamental to biology.

• The endosymbiotic theory is central to understanding the evolution of complex cells.