Microorganisms and aqueous biogeochemistry

What environmental factors influence microbial distribution?

• Availability of PEDs and TEAs

• redox potential

• pH

• temperature

• nutrients

How do microbes alter their environment chemically?

• By consuming/producing O2 and CO2

• oxidising/reducing elements

• transforming chemical species

What determines the order of microbial zones in sediments and water?

• Redox potential of PED-TEA pairs and light availability

What types of environments show zonation patterns?

Soils, aquifers, marine sediments, stratified lakes, microbial mats.

What causes variation in chemical species in pore water?

• Inputs from:

  • sediments

  • water column

  • atmosphere

  • microbial metabolism

Why are Mn2+ and Fe2+ more common in suboxic zones? 

They’re released from decaying matter and minerals; insoluble in oxic zones

What forms when hydrogen sulfide reacts with iron?

Iron sulfides like pyrite

Why do autotrophs yield less net energy than heterotrophs?

Autotrophs must fix inorganic carbon which consumes energy

What factors influence ∆G in microbial respiration?

• pH

• temperature

• pressure

• specific PED/TEA combinations

What is the photic zone?

The layer where light penetrates enough for photosynthesis

What is the euphotic zone?

The layer with 0.1%-1% surface light - typically 50-200m deep

What role does symbiosis play in zonation?

It helps microbes share resources and adapt to environmental gradients

What makes microbial mats self-sufficient ecosystems?

• Micro-scale gradients → light, O2 → pH → redox-active elements

• gene transfer

• resource pooling

Why do microbial mats thrive in extreme environments?

Fewer predators and high adaptability.

Why does colour banding in microbial mats indicate?

It indicates the type of microbe and the depth

• Green: Cyanobacteria, GNS, GSB

• White: S-oxidisers

• Purple: PSB, PNB

• Orange: GNB, Fe-oxidisers

• Black: SRB, methanogens

What causes laminations in microbial mats?

Recurring disturbance and upward displacement of microbial communities.

What are stromatolites?

Lithified microbial mats that reflect environmental conditions (sedimentation, salinity, seasons and growth patterns) – useful for studying past climates and ecosystems

How are stromatolites formed?

• Diurnal and tidal disturbance cause mineral (mostly CaCO3) accumulation in microbial mats.

• Community responds by creating new mat on top, leaving organic+mineral-rich laminations below.

What is the definition of solubility?

The maximum amount of solute that can dissolve in a solvent at a specific temperature and pressure.

What is the K_sp in solubility?

The solubility product constant - an equilibrium constant for dissolution reactions

How does the K value affect solubility>

• Higher K → more products → increased solubility

• Lower K → more reactants → decreased solubility

What effect does pK value have on solubility?

• Higher pK → more reactants → decreased solubility

• Lower pK → more products → incraesed solubility

What buffers the pH of natural waters?

Reactions with rocks and atmospheric gases

How do minerals affect pH?

• Salts of weak acids and strong bases → raise pH

• Salts of weak bases and strong acids → lower pH

What is the common ion effect?

when multiple reactions share the same ion, it alters solubility due to charge balance.

What are common ligands that form complexes with metals?

Cl⁻, HS⁻, OH⁻, HCO₃⁻, acetate (CH₃COO⁻)

How does complexation affect solubility?

It increases solubility by stabilising metal ions in solution

What is E in redox chemistry?

Measured reduction potential of a half-reaction

What is Eh?

Reduction potential normalised to the standard hydrogen electrode (SHE)

What doe Eh-pH diagrams show?

Speciation of elements based on redox and pH conditions.

What is the ion activity product (IAP or Q)?

The product of actual measured ion concentrations in solution

What is the saturation index used for?

To compare actual ion concentrations to equilibrium values and assess saturation state.

What are the 5 controls on solubility?

1. MIneral characteristics

2. pH

3. Common ion effect

4. Complexation

5. REdox

What is E⁰?

The standard electrode potential which is the Eh a system would have if all the species involved were in their standard states (activity = 1)