Prokaryotic Diversity and Evolution

Prokaryotic Diversity and Evolution

  • Phylogenetic Tree of Life

    • Three Domains: Bacteria, Archaea, Eukarya

    • Subcategories under Bacteria include:

    • Green filamentous

    • Spirochetes

    • Proteobacteria

    • Cyanobacteria

    • Planctomyces

    • Bacteroides

    • Thermotoga

    • Aquifex

    • Methanobacterium, etc.

  • Characterizing Prokaryotes by Morphology

    • Prokaryotic shapes:

    • Sphere (Coccus)

    • Rod (Bacillus)

    • Spiral/Helical (Spirillus)

    • DNA sequencing improves identification of prokaryotes substantially.

  • Origin of Prokaryotes

    • Origin of Earth: 4.6 billion years ago (mya)

    • First prokaryotes appeared approximately 3.5 billion years ago.

    • Extremophiles developed in harsh early Earth conditions (high temperatures, radiation, no oxygen).

  • Microbial Mats

    • Represent earliest forms of life on Earth.

    • Fossil evidence dates back ~3.5 billion years.

Evolution of Photosynthesis

  • Cyanobacteria

    • First photolithoautotrophs conducting oxygenic photosynthesis.

    • Contributed to the Great Oxygenation Event (~2.5 Ga) which increased atmospheric oxygen and diversified marine life.

  • Processes of Photosynthesis

    • Primary functions:

    • Energy generation

    • Electron donor for cellular processes

    • Carbon for building organic molecules

  • Specialized Metabolisms

    • Anoxygenic Photosynthesis: Uses alternatives to water (like H2S) as electron donors.

    • Chemosynthesis: Utilizes inorganic compounds for energy without sunlight.

    • Methanogenesis: Anaerobic respiration producing methane by Archaea.

  • Cyanobacteria Metabolism

    • Can perform nitrogen fixation but need a microoxic environment due to oxygen's toxicity.

    • Structure: Specialized heterocyst cells maintain low oxygen levels to protect nitrogenase enzyme from oxygen.

Symbiosis and Eukaryotic Evolution

  • Role of Eukaryotes

    • Metabolically limited compared to prokaryotic diversity; often form symbiotic relationships with bacteria/archaea for nutrient acquisition.

    • Examples include gut microbiota comprising archaea, bacteria, and fungi.

  • Key Events in Prokaryotic Evolution

    • Diversification of prokaryotic life forms leading to numerous clades and species still present today.

    • Evolution of oxygenic photosynthesis based on chlorophyll a, about 1.6-2 billion years ago, descended from cyanobacteria.

    • Transition from prokaryotic to eukaryotic life forms, containing more complex cellular structures.

Nitrogen Cycle Involvement

  • Cyanobacteria play a crucial role in nitrogen cycling:

    • Fix atmospheric nitrogen into forms usable by plants.

    • Participate in the decomposition of organic matter, contributing to soil fertility.

  • Symbiotic relationships in legume root nodules with nitrogen-fixing bacteria illustrate cooperative nitrogen fixation, benefiting both parties.