1.19: Prokaryotes

1.1 Introduction to Bacterial and Archaeal Diversity

• Prokaryotes are single-celled organisms characterized by the absence of a distinct nucleus and membrane-bound organelles, setting them apart from eukaryotic cells.

• There are two main groups of prokaryotes: Bacteria and Archaea. Despite their morphological similarities, they occupy separate domains of life, leading to significant biochemical and genetic differences between them.

• The evolutionary significance of prokaryotes is underscored by the fact that an ancestor of Archaea is thought to have given rise to Eukarya, suggesting a shared lineage that is pivotal in the tree of life.

• Genetic comparisons, particularly focusing on ribosomal 16S RNA, illustrate the evolutionary relationships between bacteria, archaea, and eukaryotes, establishing a framework for understanding biological diversity.

1.2 Fundamental Differences

• Cellular Structure:

  • Bacteria and Archaea display substantial differences in cellular structure that include:

    • Membrane lipid structure: Bacteria have ester-linked lipids, whereas Archaea possess ether-linked lipids, allowing them to survive in extreme environments.

    • Cell wall composition: Bacteria typically have peptidoglycan in their cell walls, whereas Archaea can have various structures, reflecting their adaptation to different ecological niches.

    • Genetic information processing: Variations exist in processes like replication and transcription; for example, Archaea share some similarities with eukaryotic mechanisms.

• Both groups of prokaryotes are crucial to Earth’s ecosystems, contributing significantly to nutrient cycling, decomposition, and even influencing climate change. Notably, bacteria outnumber human cells in the human body by a factor of up to 10, playing pivotal roles in digestion and immunity.

1.3 The First Inhabitants of Earth

• The Last Universal Common Ancestor (LUCA): This ancient organism is believed to have emerged long before multicellular life, existing for billions of years and serving as a precursor to all life forms.

• The early Earth, approximately 4.54 billion years ago, presented a harsher atmosphere with minimal oxygen and high levels of radiation, creating a challenging environment for early life.

• Early prokaryotes likely thrived in protected habitats such as deep oceanic vents or subsurface areas shielded from harsh conditions, using metabolic strategies adapted to high volcanic activity.

1.4 Evolution of Bacteria and Archaea

• Fossil evidence is crucial for understanding the evolution of bacteria and archaea, with ancient microbes often leaving traces as bubbles or stromatolites in rock formations.

• Genetics and molecular clocks assist in deciphering the timeline of evolutionary divergence:

  • Bacterial divergence: estimated to have occurred between 2.5 to 3.2 billion years ago.

  • Archaeal divergence: believed to have taken place between 3.1 to 4.1 billion years ago.

• Key evolutionary milestones include the emergence of cyanobacteria, vital for producing oxygen through photosynthesis and significantly contributing to atmospheric oxygen levels during the Great Oxygenation Event.

1.5 Microbial Mats and Stromatolites

• Microbial Mats:

  • Layers of microbial communities primarily composed of bacteria, found in diverse environments, including hydrothermal vents, that likely represent some of the earliest life forms (evidence dates back to 3.5 billion years ago).

  • These mats showcase immense metabolic diversity and strategies for energy acquisition, playing critical roles in various biogeochemical cycles.

• Stromatolites:

  • These structures are sedimentary formations produced by the trapping and binding of sediment by microbial mats, providing fossilized evidence that aids in understanding early life on Earth. Modern stromatolites can still be found today, particularly in regions such as Australia.

1.6 The Ancient Atmosphere

• Early Earth’s atmosphere was predominantly anoxic and supported the existence of anaerobic organisms that thrived without oxygen.

• Within a billion years, autotrophic phototrophs emerged, and later, cyanobacteria, around 2.7 billion years ago, catalyzed the initiation of a significant oxygenation event.

• This transformational change in atmospheric composition facilitated the development of energy-efficient metabolic pathways and increased the potential for land colonization, which was supported by the formation of ozone, shielding the planet from damaging UV radiation.

1.7 Adaptability of Bacteria and Archaea

• Prokaryotes exhibit remarkable adaptability, found in diverse and often extreme environments, employing several strategies:

  • Extremophiles: Organisms that thrive in extreme conditions: high salinity, acidity, temperature, and radiation, showcasing the resilience and versatility of prokaryotic life.

  • Many bacterial species have developed protective cellular structures such as robust cell walls that help them survive varying osmotic conditions.

  • Some bacteria can form endospores, a dormant form that enhances survival during adverse conditions such as droughts or extreme temperatures, allowing them to endure until favorable conditions return.

1.8 Constraints of Cell Size in Bacteria and Archaea

• Prokaryotes typically range from 0.2 to 1.0 µm in diameter, with some exceptions like Epulopiscium fishelsoni, which can grow up to 80 µm.

• Cellular constraints arise due to the reliance on diffusion for transport of nutrients and removal of waste products. As cell size increases, efficiency declines unless advanced transport mechanisms are developed, a hallmark feature in eukaryotes due to cellular compartmentalization.

1.9 Conclusion

• Prokaryotes encompass a vast array of biological diversity and exhibit extraordinary adaptability within different ecological niches.

• Ongoing research into their evolutionary history and unique characteristics is crucial for understanding ecosystems and developing biotechnological applications, revealing the essential roles prokaryotes play in maintaining life on Earth.