Prokaryotes, the Unseen Majority

Prokaryotes, the Unseen Majority

This work is licensed under a Creative Commons Attribution-NonCommercial- ShareAlike 4.0 International License. Slides are adapted from Dr. S. Grayston's scanning electron micrograph of mycorrhizal root tip hyphae coated with bacteria.

Lecture Objectives

By the end of this lecture students will be able to:

  • Describe the main characteristics of prokaryotes and the main differences between the Domains Bacteria and Archaea.

  • Discuss the concept of LUCA and how molecular phylogeny supports it.

  • Briefly describe our current knowledge in Prokaryote diversity.

  • Discuss the place of prokaryotes in our understanding of the history of life on Earth.

  • Describe the distinguishing features of extremophiles and give one example of a Prokaryote that can be considered an extremophile.

  • Explain the methods and difficulties associated with cultivating prokaryotes.

  • Define the term biofilm and briefly describe the different stages of biofilm formation.

  • Describe the different ways in which prokaryotes can be classified.

  • Briefly discuss prokaryote reproduction, as well as the mechanisms used to increase prokaryotic genetic diversity.

  • Describe in detail the different types of interaction between humans and bacteria.

  • Discuss what antibiotic resistance is and how it emerges.

Introduction to Prokaryotes

Overview

  • Prokaryotes are microorganisms that are invisible to the naked eye.

  • The term "Prokaryote" was first used in 1925.

  • Defined by the following characteristics:

    • Single-cell organism (unicellular).

    • Absence of nucleus and other membrane-bound organelles.

    • Presence of a cell wall.

Prokaryotic Domains

  • Prokaryotes are divided into two distinct domains:

    • Bacteria

    • Archaea

Differences Between Domains

  • Transcription and Translation:

    • In Archaea: Very similar to Eukaryotes.

    • In Bacteria: Different from Eukaryotes.

  • Cell Wall Structure:

    • In Archaea: Chemically different from Bacteria; may contain pseudopeptidoglycan, polysaccharides, or proteins.

    • In Bacteria: Contains peptidoglycan.

  • DNA Packaging:

    • Similar in Eukaryotes and Archaea.

The Last Universal Common Ancestor (LUCA)

  • LUCA is defined as the most recent population from which all organisms on Earth share a common descent.

  • Proposed by Woese as a relatively simple entity representing an early stage of cellular organization.

Prokaryotic Diversity

Current Estimates

  • Abundance:

    • 1 teaspoon of soil may contain up to 100 million prokaryotes.

    • 1 ml of human colon contains 300 million prokaryotes.

  • Species Estimates:

    • There are an estimated 2.2-4.3 million different species of prokaryotes.

    • Only 25,000 species (0.8%) have been characterized.

Prokaryotes as Early Life Forms

  • Prokaryotes are the first inhabitants of Earth, with fossilized microbial mats considered the earliest records of life.

  • Age of Earth:

    • Estimated to be 4.5 billion years old.

    • Environment 3.5 billion years ago was inhospitable for animal life (high CO2, low oxygen).

Microbial Mats

  • Defined as multi-layered sheets of prokaryotes, mainly consisting of bacteria and archaea.

  • Today found in environments like hot springs, deep-sea vents, hypersaline lakes, and marine estuaries.

Stromatolites

  • A sedimentary structure formed when minerals are precipitated out of water by prokaryotes in a microbial mat.

  • Living and fossilized stromatolites are important to understanding early life on Earth.