Microbial Life: Prokaryotes and Protists

Introduction to Microbial Life

Microorganisms are abundant in and on the human body, outnumbering human cells. Disruption of these microbial communities can increase susceptibility to diseases and contribute to various health conditions.

• Microbial communities play essential roles in health and disease.

• Examples of conditions linked to microbial imbalance: asthma, allergies, irritable bowel syndrome, Crohn’s disease, autism.

Prokaryotes: Diversity and Classification

Prokaryotes are single-celled organisms lacking a nucleus. They are classified into two domains: Bacteria and Archaea. Their collective biomass is immense, and they impact both the environment and human health.

• Prokaryotic cells are smaller than eukaryotic cells.

• Domains: Bacteria and Archaea.

• Prokaryotes are widespread and diverse.

Prokaryotic Cell Shapes and External Features

Cell shape is a key identifier for prokaryotes. Common shapes include:

• Cocci: Spherical

• Bacilli: Rod-shaped

• Spirilla: Short, rigid spirals

• Spirochetes: Long, flexible spirals

Most prokaryotes have a cell wall, which can be classified by Gram staining:

• Gram-positive: Thick peptidoglycan layer

• Gram-negative: Thinner wall, stains differently

• Other features: sticky capsule, flagella, fimbriae

Adaptation and Nutritional Diversity

Prokaryotes adapt rapidly due to fast population growth and genetic variation. Some form endospores to survive harsh conditions. They exhibit unparalleled nutritional diversity:

• Phototrophs: Capture energy from sunlight

• Chemotrophs: Harness energy from chemicals

Biofilms and Environmental Impact

Prokaryotes form biofilms—complex communities attached to surfaces. These are difficult to eradicate and can cause medical and environmental issues. Prokaryotes are also used in bioremediation to clean up pollutants.

• Biofilms: Organized microbial communities

• Bioremediation: Use of organisms to remove pollutants

Bacteria and Archaea: Evolution and Diversity

Advances in genetics show that Archaea are more closely related to Eukarya than to Bacteria. Archaea thrive in extreme environments, while Bacteria are diverse and include groups such as Proteobacteria, Gram-positive bacteria, Cyanobacteria, Chlamydias, and Spirochetes.

• Archaea: Extreme halophiles, thermophiles, methanogens

• Bacteria: Proteobacteria, Gram-positive, Cyanobacteria, Chlamydias, Spirochetes

Pathogenic Bacteria

Some bacteria cause disease by producing exotoxins (secreted proteins) or endotoxins (lipid components released upon cell death). Examples include Staphylococcus aureus, Clostridium botulinum, and Treponema pallidum.

• Exotoxins: Secreted proteins causing disease

• Endotoxins: Lipid components released from dead cells

Protists: Diversity and Classification

Protists are mostly unicellular eukaryotes found in aquatic environments. They can be autotrophic (algae), heterotrophic (protozoans), or mixotrophic. Protist diversity is organized into four supergroups: SAR, Excavata, Unikonta, and Archaeplastida.

• SAR: Stramenopila, Alveolata, Rhizaria

• Excavata: Modified mitochondria, includes Giardia, Trichomonas, Trypanosomas

• Unikonta: Amoebozoans, fungi, animals

• Archaeplastida: Red algae, green algae, land plants

The Evolution of Plant and Fungal Diversity

Plant Evolution and Adaptations

Plants evolved from algal ancestors and adapted to life on land. Adaptations include mechanisms to maintain moisture, support the body, reproduce without water, anchor in soil, and obtain resources.

• Opportunities: Sunlight, CO2, few pathogens

• Challenges: Moisture retention, structural support, reproduction, anchorage, resource acquisition

Plant Diversity and Evolutionary History

Three key events mark plant evolution: origin of land plants, vascular plants, and seed plants. Plant groups include nonvascular plants (bryophytes), seedless vascular plants (lycophytes, monilophytes), and seed plants (gymnosperms, angiosperms).

• Bryophytes: Mosses, hornworts, liverworts

• Seedless vascular plants: Club mosses, ferns

• Seed plants: Gymnosperms (cones), Angiosperms (flowers)

Alternation of Generations and Plant Life Cycles

Many plants exhibit alternation of generations, with haploid gametophyte and diploid sporophyte stages. Seedless vascular plants dominated ancient coal forests, contributing to fossil fuel formation.

• Alternation of generations: Life cycle alternates between gametophyte (n) and sporophyte (2n)

• Coal formation: Ancient plants formed peat, which became coal

Pollen, Seeds, and Angiosperm Reproduction

Pollen grains carry sperm-producing cells through the air. Fertilization leads to seed formation, with stored food and protective coat. Flowers are the reproductive centers of angiosperms, housing male and female structures.

• Pollen: Sperm transport

• Seeds: Embryo protection and dispersal

• Flowers: Sites of pollination and fertilization

Fruit Structure and Seed Dispersal

Fruits are ripened ovaries that aid in seed dispersal by wind, animals, or being edible. Angiosperms provide much of our food and spices.

• Seed dispersal: Wind, animal transport, edible fruits

• Food and spices: Grains, fruits, spices from angiosperms

Pollination and Angiosperm Evolution

Animal pollinators are attracted by flower color and scent, receiving nectar and pollen as rewards. This mutualism has influenced angiosperm evolution.

• Pollinators: Bees, birds, insects

• Mutualism: Flowers provide rewards, pollinators aid reproduction

Plant Diversity and Food Supply

Loss of plant biodiversity threatens the world’s food supply. New crops may arise from locally used species, emphasizing the importance of conservation.

• Major crops: Rice, wheat, corn, soybeans

• Biodiversity: Essential for future food security

Diversity of Fungi

Fungal Nutrition and Structure

Fungi are heterotrophic eukaryotes that absorb nutrients after external digestion. Their bodies consist of hyphae, forming a mycelium. Mycorrhizae are symbiotic associations with plant roots, aiding nutrient absorption.

• Hyphae: Threadlike filaments

• Mycelium: Mass of hyphae

• Mycorrhizae: Symbiosis with plant roots

Fungal Reproduction

Fungi produce spores in both asexual and sexual cycles. Fusion of haploid hyphae leads to a heterokaryotic stage, followed by nuclear fusion and meiosis.

• Asexual reproduction: Spore production and germination

• Sexual reproduction: Fusion of cytoplasm and nuclei, meiosis

Fungal Classification and Ecological Roles

Fungi are classified into five groups: Zygomycetes, Glomeromycetes, Ascomycetes, Basidiomycetes, and Chytrids. They are essential decomposers and symbionts, supplying nutrients and breaking down organic matter.

• Decomposers: Recycle nutrients

• Symbionts: Mycorrhizae, lichens

• Bioremediation: Digest petroleum products

Practical Uses and Harmful Effects of Fungi

Fungi are used in food production, antibiotics, and biotechnology. Some are parasitic, causing diseases in plants and animals.

• Food: Mushrooms, cheeses, bread, alcohol

• Antibiotics: Penicillin, streptomycin

• Diseases: Ringworm, athlete’s foot, yeast infections

Lichens and Mycorrhizae

Lichens are symbiotic associations of fungi and photosynthetic organisms. Mycorrhizae may have helped plants colonize land, as evidenced by present-day relationships, fossils, and molecular genetics.

• Lichens: Fungi + algae/cyanobacteria

• Mycorrhizae: Fungi + plant roots

Additional info: These notes expand on brief points with academic context, definitions, and examples to provide a comprehensive study guide for General Biology students.