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

Microbial Life: Prokaryotes and Protists

Introduction to Microbial Life

Microorganisms, including prokaryotes and protists, are abundant and play essential roles in both the environment and human health. Disruptions in our microbial communities can increase susceptibility to diseases and contribute to various health conditions.

Prokaryotes

Diversity and Ubiquity of Prokaryotes

Prokaryotes are single-celled organisms lacking a nucleus and are found in nearly every environment on Earth. Their collective biomass far exceeds that of eukaryotes, and they have a profound impact on ecosystems and human health. Prokaryotes are classified into two domains: Bacteria and Archaea.

Prokaryotic Cell Shapes

Microscopic examination reveals three common shapes of prokaryotic cells, which are important for identification:

• Cocci: Spherical cells

• Bacilli: Rod-shaped cells

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

External Features of Prokaryotes

Prokaryotes possess several external structures that contribute to their success:

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

• Capsule: A sticky layer that helps cells adhere to surfaces and evade the immune system.

• Flagella: Used for movement.

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

Genetic Material in Prokaryotes

Prokaryotic cells contain a single, circular chromosome and may also have plasmids—small, independently replicating DNA molecules that often carry advantageous genes, such as antibiotic resistance.

Adaptation and Survival Mechanisms

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

Nutritional Diversity in Prokaryotes

Prokaryotes display remarkable nutritional diversity, allowing them to thrive in 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

Prokaryotes can form biofilms, which are complex, surface-attached communities of microorganisms. Biofilms are highly organized and can be difficult to eradicate, causing medical and environmental problems.

Prokaryotes in Environmental Cleanup

Bioremediation is the use of organisms, especially prokaryotes, to remove pollutants from the environment. Prokaryotes are used in sewage treatment and other waste management processes.

Domains of Prokaryotic Life: Bacteria and Archaea

Molecular genetics has revealed that prokaryotes are divided into two main domains: Bacteria and Archaea. Archaea are often more closely related to eukaryotes than to bacteria.

Key Differences Between Bacteria, Archaea, and Eukarya

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 organisms that thrive in extreme conditions:

• Extreme halophiles: Salt lovers

• Extreme thermophiles: Heat lovers

• Methanogens: Thrive in anaerobic environments and produce methane

Bacterial Diversity

Bacteria are divided into several groups based on genetic sequences:

• Proteobacteria: All gram-negative, diverse metabolism

• Gram-positive bacteria: Thick cell walls, many are important in medicine

• Cyanobacteria: Only prokaryotes with plant-like, oxygen-generating photosynthesis

• Chlamydias: Live inside eukaryotic host cells

• Spirochetes: Spiral-shaped, some are pathogens

Pathogenic Bacteria

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

Examples include Staphylococcus aureus (exotoxins) and Clostridium botulinum (botulinum toxin).

Protists

Diversity of Protists

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

• Autotrophic (algae)

• Heterotrophic (protozoans)

• Mixotrophic (capable of both photosynthesis and heterotrophy)

Protist Supergroups

Protist diversity is organized into four supergroups:

• SAR: Includes stramenopiles, alveolates, and rhizarians

• Excavata: Includes organisms with an "excavated" feeding groove

• Unikonta: Includes amoebozoans, fungi, and animals

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

SAR Supergroup

The SAR supergroup encompasses a wide range of protist diversity:

• Stramenopila: Diatoms, brown algae, water molds

• Alveolata: Dinoflagellates, ciliates, certain parasites

• Rhizaria: Foraminiferans, radiolarians

Excavata

Members of Excavata often have modified mitochondria and include parasitic species such as Giardia and Trichomonas vaginalis, as well as mixotrophs like Euglena.

Unikonta

Unikonts include amoebozoans (amoebas, slime molds) and are closely related to fungi and animals.

Archaeplastida

Archaeplastida includes red algae, green algae, and land plants. Green algae can be unicellular, colonial, or multicellular, and many have complex life cycles involving alternation of generations.

Evolution of Multicellularity

Multicellularity evolved independently in several eukaryotic lineages, including brown algae, fungi, animals, red and green algae, and plants. This evolutionary step allowed for increased complexity and specialization of cells.