Microbial Life: Prokaryotes and Protists – Structure, Diversity, and Evolution

Microbial Life: Prokaryotes and Protists

Introduction to Microbial Life

Microorganisms, including prokaryotes and protists, are fundamental to life on Earth. They inhabit diverse environments and play crucial roles in health, ecology, and evolution. The human body alone hosts trillions of microbial cells, outnumbering human cells and influencing health and disease.

Prokaryotes

Diversity and Ubiquity of Prokaryotes

Prokaryotes are single-celled organisms lacking a nucleus. They are smaller than eukaryotic cells and collectively possess a biomass at least ten times greater than all eukaryotes combined. Prokaryotes are classified into two domains: Bacteria and Archaea. Their diversity allows them to thrive in nearly every environment on Earth.

• Key Point: Prokaryotes are often unnoticed because they are microscopic and many are harmless or beneficial.

Prokaryotic Cell Shapes and Identification

Prokaryotes are identified by their shapes, which are important for classification and diagnosis:

• Cocci: Spherical cells

• Bacilli: Rod-shaped cells

• Spiral: Includes short, rigid spirilla and long, flexible spirochetes

External Features of Prokaryotes

Prokaryotes possess several external structures that contribute to their survival and identification:

• Cell Wall: Provides structural support and protection. Gram-positive bacteria have thick peptidoglycan layers, while gram-negative bacteria have thinner layers and an outer membrane.

• Capsule: A sticky layer that helps in attachment and protection from the immune system.

• Flagella: Tail-like structures for movement.

• Fimbriae: Hair-like appendages for attachment to surfaces.

Genetic Material in Prokaryotes

Prokaryotic cells contain a single circular chromosome and may have additional small DNA molecules called plasmids, which often carry genes for antibiotic resistance or other functions.

Adaptation and Survival Mechanisms

Prokaryotes adapt rapidly due to fast reproduction and genetic variation. Some can form endospores, which are resistant structures that allow survival in harsh conditions.

Nutritional Diversity in Prokaryotes

Prokaryotes exhibit a wide range of nutritional modes, allowing them to colonize diverse environments. They are classified by their energy and carbon sources:

• Photoautotrophs: Use sunlight and CO2 (e.g., cyanobacteria)

• Chemoautotrophs: Use inorganic chemicals and CO2

• Photoheterotrophs: Use sunlight and organic compounds

• Chemoheterotrophs: Use organic compounds for both energy and carbon

Nutritional Mode	Energy Source	Carbon Source
Photoautotroph	Sunlight	CO2
Chemoautotroph	Inorganic chemicals	CO2
Photoheterotroph	Sunlight	Organic compounds
Chemoheterotroph	Organic compounds	Organic compounds

Biofilms and Environmental Impact

Prokaryotes can form biofilms, which are complex communities attached to surfaces. These biofilms are resistant to removal and can cause medical and environmental issues.

Prokaryotes in Bioremediation

Bioremediation is the use of organisms, especially prokaryotes, to remove pollutants from the environment. They are used in sewage treatment and to clean up oil spills and other contaminants.

Domains of Life: Bacteria, Archaea, and Eukarya

Prokaryotes are divided into two domains: Bacteria and Archaea. Molecular genetics has shown that Archaea are more closely related to Eukarya than to Bacteria. The table below summarizes key differences:

Characteristic	Bacteria	Archaea	Eukarya
rRNA sequences	Some unique to bacteria	Some unique to archaea; some match eukaryotic sequences	Some unique to eukaryotes; some match archaeal sequences
RNA polymerase	One kind; relatively small and simple	Several kinds; complex	Several kinds; complex
Introns	Rare	In some genes	Present
Histones associated with DNA	Absent	Present in some species	Present
Peptidoglycan in cell wall	Present	Absent	Absent

Archaea: Life in Extreme Environments

Archaea include extremophiles such as:

• Extreme halophiles: Thrive in salty environments

• Extreme thermophiles: Thrive in hot environments

• Methanogens: Produce methane in anaerobic conditions

Bacterial Diversity

Bacteria are divided into several groups based on genetic and physiological characteristics:

• Proteobacteria: Gram-negative, diverse metabolism

• Gram-positive bacteria: Thick peptidoglycan cell walls

• Cyanobacteria: Photosynthetic, oxygen-producing

• Chlamydias: Intracellular parasites

• Spirochetes: Spiral-shaped, some are pathogens

Pathogenic Bacteria and Toxins

Some bacteria cause disease by producing toxins:

• Exotoxins: Proteins secreted by bacteria

• Endotoxins: Lipid components of the outer membrane of gram-negative bacteria, released upon cell death

Protists

Diversity and Classification of Protists

Protists are mostly unicellular eukaryotes found in aquatic or moist environments. They can be:

• Autotrophic (algae): Photosynthetic

• Heterotrophic (protozoans): Obtain food by ingestion or absorption

• Mixotrophic: Capable of both photosynthesis and heterotrophy

Protist Supergroups

Protist diversity is organized into four major supergroups:

• SAR: Includes Stramenopila, Alveolata, and Rhizaria

• Excavata: Characterized by an excavated feeding groove

• Unikonta: Includes amoebozoans, fungi, and animals

• Archaeplastida: Includes red algae, green algae, and land plants

Evolutionary Significance of Multicellularity

Multicellularity evolved independently in several eukaryotic lineages, leading to increased complexity and specialization. This evolutionary innovation allowed for the emergence of plants, fungi, and animals from protist ancestors.

*Additional info: Some images and details about protist groups, plant, and fungal diversity were omitted for brevity and focus on the main microbial life topics as per the provided materials.*