Diversity of Microbial Eukarya: Organelles, Phylogeny, and Endosymbiosis

Organelles and Phylogeny of Microbial Eukarya

Introduction to Microbial Eukarya

Microbial members of the domain Eukarya are highly genetically and biologically diverse, often more so than larger eukaryotes. They display a wide range of morphological and ecological complexity, but their metabolic diversity is relatively limited.

• Metabolic Types: Most microbial eukaryotes are either chemoorganotrophic (obtaining energy from organic compounds) or phototrophic (using light as an energy source).

• Oxygen Requirement: Many species are obligate aerobes, requiring oxygen for survival.

• Classification: Microbial eukaryotes are often classified morphologically as protists, algae, or fungi.

• Cell Structure: All Eukarya share a common ancestor with complex cell structure and true organelles.

Endosymbioses and the Eukaryotic Cell

Primary Endosymbiosis

Primary endosymbiosis refers to the process by which a bacterial symbiotic partner was acquired directly by the ancestor of Eukarya. This event was foundational in the evolution of the eukaryotic cell.

• Mitochondria: Evolved when an ancestral eukaryote formed a symbiotic relationship with a bacterium capable of respiration.

• Over time, the bacterial endosymbiont and its eukaryotic host evolved mutual dependency, eventually becoming a single cell.

• Mitochondria are derived from Bacteria and contain their own bacterial DNA.

• Chloroplasts: Acquired when a eukaryotic cell (already containing mitochondria) engulfed a phototrophic cyanobacterium.

• All phototrophic eukaryotes (plants and algae) require chloroplasts for photosynthesis.

Secondary Endosymbiosis

Secondary endosymbiosis is the process by which a eukaryotic cell engulfs another eukaryotic cell (such as a green or red alga), retaining its chloroplast and becoming phototrophic.

• Examples:

  • Euglenids and chlorarachniophytes (from green algae)

  • Alveolates and stramenopiles (from red algae)

• Secondary endosymbioses are common and ongoing in evolution.

Diagram: Endosymbiosis Pathways

Figure 18.2 (described): The diagram illustrates the evolutionary pathways of primary and secondary endosymbiosis. Primary endosymbiosis involves the engulfment of a bacterium (leading to mitochondria and chloroplasts), while secondary endosymbiosis shows eukaryotic cells engulfing other eukaryotic algae, resulting in diverse phototrophic lineages such as dinoflagellates, apicomplexans, stramenopiles, euglenids, and chlorarachniophytes.

Key Terms and Concepts

• Chemoorganotroph: An organism that obtains energy by oxidizing organic compounds.

• Phototroph: An organism that uses light as its primary energy source.

• Obligate Aerobe: An organism that requires oxygen for growth and survival.

• Endosymbiosis: A symbiotic relationship in which one organism lives inside the cell or body of another organism.

• Primary Endosymbiosis: The direct engulfment of a prokaryote by a eukaryotic ancestor, leading to organelles such as mitochondria and chloroplasts.

• Secondary Endosymbiosis: The engulfment of a eukaryotic alga by another eukaryote, resulting in additional complexity in organelle structure.

Example: Mitochondrial Origin

Mitochondria in all eukaryotic cells are believed to have originated from an ancestral alpha-proteobacterium through primary endosymbiosis. This event allowed eukaryotes to efficiently utilize oxygen for energy production.

Additional info:

• Secondary endosymbiosis explains the presence of multiple membranes around chloroplasts in some protists, reflecting their complex evolutionary history.

• Endosymbiotic theory is supported by the presence of circular DNA and prokaryote-like ribosomes in mitochondria and chloroplasts.