Deeply branching bacteria are ancient microbes that offer a glimpse into Earth's early life. These organisms diverged from the last universal common ancestor long ago, adapting to extreme environments like hot springs and deep-sea vents.

These bacteria have unique traits that help them thrive in harsh conditions. They've developed heat-stable enzymes, acid-resistant cell walls, and efficient nutrient uptake systems. Studying their genomes and metabolic abilities sheds light on how life evolved and diversified over billions of years.

Evolutionary Significance and Adaptations of Deeply Branching Bacteria

Evolutionary significance of deeply branching bacteria

• Deeply branching bacteria earliest lineages to diverge from last universal common ancestor (LUCA)
  • LUCA hypothetical most recent common ancestor of all current life on Earth
  • Deeply branching bacteria provide insights into characteristics and environment of early life forms (Archaea, Bacteria)
• Comparative analysis of deeply branching bacterial genomes helps infer minimal gene set and metabolic capabilities of LUCA
• Presence of deeply branching bacteria in extreme environments suggests LUCA may have inhabited similar conditions (hydrothermal vents, hot springs)
  • Indicates early life on Earth likely evolved in high-temperature, anaerobic, and nutrient-limited settings
• Study of deeply branching bacteria contributes to understanding the structure of the phylogenetic tree of life

Adaptations to extreme environments

• Thermophilic adaptations
  • Heat-stable enzymes and proteins maintain function at high temperatures (90-100℃)
  • Increased membrane stability through unique lipid composition (ether-linked lipids)
  • Enhanced DNA repair mechanisms counteract thermal damage
• Acidophilic adaptations
  • Highly efficient proton pumps maintain intracellular pH homeostasis
  • Specialized cell wall structure withstands acidic conditions (pH < 3)
  • Upregulation of stress response genes and chaperone proteins
• Anaerobic adaptations
  • Utilization of alternative terminal electron acceptors for respiration (sulfur, nitrate)
  • Presence of oxygen-sensitive enzymes and metabolic pathways (nitrogenase, hydrogenase)
  • Specialized cofactors and redox proteins for fermentative metabolism (ferredoxin, rubredoxin)
• Adaptations to nutrient limitation
  • High-affinity nutrient uptake systems for scavenging scarce resources
  • Metabolic versatility to utilize wide range of substrates (H2, CO2, organic acids)
  • Increased surface area-to-volume ratio for efficient nutrient acquisition (filamentous morphology)

Extremophiles and their adaptations

• Hyperthermophiles: organisms thriving in extremely hot environments (>80°C)
• Barophiles: microorganisms adapted to high-pressure environments, such as deep-sea habitats
• Some deeply branching bacteria use chemosynthesis as their primary mode of energy production

Metabolic strategies of bacterial genera

• Aquificae (Aquifex)
  • Chemolithoautotrophic metabolism using hydrogen or reduced sulfur compounds as energy sources
  • Carbon fixation through reductive tricarboxylic acid (rTCA) cycle
  • Oxygen or nitrate as terminal electron acceptors
• Thermotogae (Thermotoga)
  • Fermentative metabolism of sugars and peptides
  • Hydrogen production through oxidation of reduced ferredoxin
  • Sulfur reduction in some species
• Thermodesulfobacteria (Thermodesulfobacterium)
  • Anaerobic sulfate reduction coupled with hydrogen oxidation
  • Autotrophic carbon fixation through acetyl-CoA pathway
• Deinococcus-Thermus (Thermus)
  • Aerobic heterotrophic metabolism
  • Utilization of organic compounds as carbon and energy sources (amino acids, sugars)
  • Some species capable of anaerobic respiration using nitrate or metals as electron acceptors
• Chloroflexi (Chloroflexus)
  • Anoxygenic phototrophy using type II photosynthetic reaction centers
  • Carbon fixation through 3-hydroxypropionate bi-cycle
  • Mixotrophic growth on organic compounds in absence of light (acetate, pyruvate)
  • Some species exhibit horizontal gene transfer, contributing to their metabolic diversity