Lecture Notes on Microbial Habitats

Introduction

• Overview of microbial habitats focusing on mineral, rock, and sediment-hosted places.
• Discussion constrained to specific physical search parameters.

Definition of Microbes

• Microbes: Microscopic organisms requiring a microscope for observation.
  • Historically defined based on technological limits of the time.
• Current NASA definition is broad and may not provide useful ecological relationships.
• Preferred approach: phylogeny and evolutionary history of microbes.

Tree of Life

• Three domains of life: Archaea, Bacteria, and Eukaryotes.
  • Most relevant microbes: Bacteria and Archaea.
  • Prokaryotes (Archaea and Bacteria) considered more primitive in cell organization.

Physical-Chemical Constraints

Key Parameters

1. Temperature

   • Zones of microbial activity:
     • Happy growth zone.
     • Metabolically active but not replicating.
     • Resuscitation potential zone.
     • Point of no return where cells are physically disrupted.
   • Ice can disrupt cells; vitrification helps maintain cell integrity during freezing.
   • Current knowledge limits:
     • Active metabolism observed down to -20°C, with some activity at -25°C.
     • Upper temperature limit around 122°C in high-pressure environments.

2. pH

   • Acidic conditions destabilize proteins and affect ATP formation.
   • Known limits: pH from -2 to 12.5.
   • Adaptations include membrane composition changes and DNA repair mechanisms.

3. Water Activity (Salinity)

   • Accessibility of water varies with salinity levels.
   • Chaotropic vs. cosmotropic salts impact water activity.
   • Halophiles adapt via two strategies: salt-in strategy or compatible solutes.

4. Pressure

   • Adaptations include changes to membrane structure to maintain integrity under high pressures.
   • Relevant analogs on Earth for ocean worlds include Mariana Trench pressures.

Energy Considerations

Chemical Redox Energy

• Fundamental currency fueling life processes.
• Energy derived from electron donor-acceptor pairings.
• Microbes can perform diverse electron transfer reactions beyond simple carbon and oxygen utilization.

Gibbs Energy and Power of Life

• Gibbs energy relation: determines energy available from reactions.
• Power (energy/time) influences viability due to environmental pressures.
• Maintenance energy vs. active metabolism: cells need enough energy to sustain both.

Microbial Interactions and Habitats

Symbiotic Relationships

• Examples of mutualistic relationships among microbes enhancing survival.
• Importance of spatial relationships in microbial communities.

Affinity for Surfaces

• Microbes thrive in boundary zones and chemical gradients for energy access.
• Examples of specific microhabitats like Antarctic endoliths and hydrothermal vents.

Microbial Impact on Mineral Formation

• Microbes can create and degrade mineral structures.
• Relationship with minerals can influence nutrient cycling and habitat diversity.

Future Questions

1. Control of mineral composition by microbes: is it beneficial or a metabolic byproduct?
2. Accessing nutrients within mineral structures.
3. Non-destructive methods for studying microbial-mineral relationships (e.g., Micro CT scanning).

Conclusion

• The lecture provided a broad overview of microbial habitats, their constraints, and interactions with their environment. Further exploration is encouraged.