Exam 2 (Lecture 7) Non-Universal Prokaryotic Structures: Storage Inclusions and Intracellular Structures

Non-Universal Prokaryotic Structures

Storage Inclusions: Intracellular Nutrient Storage

Many prokaryotes possess specialized intracellular structures called inclusions that store nutrients or other important compounds. These inclusions are not found in all prokaryotes, hence they are termed "non-universal." They play key roles in survival, metabolism, and adaptation to environmental changes.

• Carbon Storage Polymers: Prokaryotes often store excess carbon in the form of polymers, which can be used as energy and carbon sources when external supplies are limited.

  • Poly-β-hydroxybutyric acid (PHB): A type of polyhydroxyalkanoate (PHA), PHB is a lipid-like polymer that forms granules within the cell. It is a common carbon and energy storage material in many bacteria.

  • Polyhydroxyalkanoate (PHA): A broader class of carbon storage polymers, including PHB and related compounds.

  • Glycogen: A polymer of glucose, glycogen serves as a carbohydrate reserve. It is structurally similar to the glycogen found in eukaryotes.

  • Function: These polymers allow cells to store carbon and energy for later use, especially during periods of nutrient scarcity.

  • Example: Ralstonia eutropha accumulates PHB granules when grown in excess carbon conditions.

• Polyphosphate Granules: Inorganic phosphate is stored as polyphosphate granules. These serve as a phosphate and energy reserve, and are important for nucleic acid and phospholipid synthesis.

• Elemental Sulfur Granules: Some bacteria, especially sulfur-oxidizing bacteria (e.g., Beggiatoa), store elemental sulfur (S0) in granules. These are used as an energy source when needed.

Key Points:

• Storage inclusions are not present in all prokaryotes.

• They provide a survival advantage by storing essential nutrients.

• Granules can be visualized under the microscope as distinct intracellular bodies.

Gas Vesicles and Magnetosomes: Intracellular Structures for Environmental Control

Some prokaryotes possess specialized structures that help them control their position in the environment, particularly in aquatic habitats.

• Gas Vesicles: Hollow, protein-bound structures that provide buoyancy to aquatic bacteria and archaea, allowing them to float at optimal depths for light and nutrients.

  • Structure: Composed entirely of protein; impermeable to water but permeable to gases.

  • Function: Allow cells to regulate their position in the water column, often to maximize access to light (in phototrophs) or oxygen.

  • Example: Cyanobacteria form gas vesicles to float near the surface during algal blooms.

• Magnetosomes: Membrane-bound crystals of magnetic minerals (usually magnetite, Fe3O4) found in magnetotactic bacteria.

  • Function: Allow bacteria to orient and move along magnetic field lines, helping them locate optimal environments (e.g., low oxygen zones in sediments).

  • Biomineralization: The process by which bacteria form these magnetic crystals inside the cell.

  • Example: Magnetotactic bacteria use magnetosomes to align with Earth's magnetic field and migrate to preferred microenvironments.

Summary Table: Major Non-Universal Prokaryotic Inclusions

Inclusion Type	Main Component	Function	Example Organisms
PHB/PHA Granules	Polyhydroxyalkanoates (lipids)	Carbon and energy storage	Ralstonia eutropha
Glycogen Granules	Glucose polymer	Carbon and energy storage	Many bacteria and archaea
Polyphosphate Granules	Polyphosphate	Phosphate and energy storage	Various bacteria
Elemental Sulfur Granules	Elemental sulfur (S0)	Energy storage (in sulfur oxidizers)	Beggiatoa, Thiobacillus
Gas Vesicles	Protein	Buoyancy regulation	Cyanobacteria, aquatic phototrophs
Magnetosomes	Magnetite (Fe3O4)	Magnetic orientation	Magnetotactic bacteria

Additional info: Storage inclusions and environmental control structures are important for microbial ecology, allowing prokaryotes to survive fluctuating conditions and to occupy diverse ecological niches.