Microbial Life: Prokaryotes and Protists – Structure, Diversity, and Ecological Roles

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. The human body hosts trillions of microbial cells, which can influence susceptibility to diseases and various health conditions.

Prokaryotes

Diversity and Ubiquity of Prokaryotes

Prokaryotes are single-celled organisms lacking a nucleus and membrane-bound organelles. 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.

• Prokaryotic diversity is immense, with many species yet to be discovered.

• They inhabit nearly every environment on Earth, from soil and water to extreme environments.

Prokaryotic Cell Shapes

Microscopic examination of cell shape is a key step in identifying prokaryotes. The three most common shapes are:

• 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 outer layer that helps in attachment and protection.

• Flagella: Used for movement.

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

Genetic Material in Prokaryotes

Prokaryotic cells contain a single, circular chromosome and often additional small DNA molecules called plasmids, which can carry advantageous genes such as antibiotic resistance.

Adaptation and Survival Mechanisms

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

Nutritional Diversity in Prokaryotes

Prokaryotes display a wide range of nutritional strategies, allowing them to thrive in diverse environments. They are classified based on 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

Biofilms

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

Prokaryotes in Environmental Cleanup (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

Molecular genetics has revealed that prokaryotes are divided into two domains: Bacteria and Archaea. Archaea are more closely related to eukaryotes 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 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, important in medicine and industry

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

• Chlamydias: Live inside eukaryotic host cells

• 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 when the cell dies

Protists

Introduction to 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 modified mitochondria

• Unikonta: Includes amoebozoans, fungi, and animals

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

SAR Supergroup

The SAR supergroup includes:

• Stramenopila: Diatoms, brown algae, water molds

• Alveolata: Dinoflagellates, ciliates, certain parasites

• Rhizaria: Foraminiferans, radiolarians

Excavata

Excavates are characterized by an "excavated" feeding groove and include anaerobic protists with modified mitochondria, such as Giardia and Trichomonas.

Unikonta

Unikonts include amoebozoans (amoebas, slime molds) and the clade that gave rise 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 life cycles with 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 allowed for increased complexity and specialization of cells.

*Additional info: The above notes are expanded with academic context to ensure completeness and clarity for college-level biology students.*