Biogeochemistry - Microorganisms and aq solutions

What governs where microbes live in an environment?

Availability if PEDs, TEAs, light, pH, temperature, nutrients and energy yield

Why does biogeochemical zonation occur?

Microbes use available energy sources in order of thermodynamic favourability

What happens to oxygen with depth in lakes?

Sharp decline in oxygen as temperature decreases and depth increases

Why do lakes show strong phototrophic influence?

They have greater light penetration than ocean sediments therefore phototrophs have strong influence

Why doe NO3- peak near the oxic-suboxic boundary?

As it is produced by organims and oxidised by the oxygen and/or chemolithotrophs

Why are Fe2+ and Mn2+ more abundant in suboxic/anoxic zones?

Their oxidised forms are more insoluble in oxic conditions

What happens when H2S meets Fe2+?

Formation of iron sulfides such as pyrite minerals

What metabollic process dominates at greatest depth?

Methanogenesis

Why do heterotrophs generally yield more net energy than autotrophs?

Autotrophs have to fix inorganic carbon which requires energy

What is the general order of energy yield for chemolithoautotrophy?

Highest → Hydrogen oxidation
Lowest → Nitrogen oxidation

What is the general order of energy yield for chemoheterotrophy?

Highest → Aerobic respiration

Lowest → Chemoheterotrophic methanogenesis

What happens to sulfur species with depth in stratified lakes?

O2 decreases → sulfide oxidised → sulfate reduction in anoxic waters

Which phototrophs dominate the oxic zone?

Cyanobacteria and algae

Which phototrophs dominate the anoxic zone?

PSB and GSB

What colour of microbial matte layers correspond to specific microbes?

• Green: Cyanobacteria (top), GNS/GSB (lower)

• White: S‑oxidisers

• Purple: PSB, PNB

• Orange: GNB, Fe‑oxidisers

• Black: SRB, methanogens

What causes laminated layers in mats?

Disturbance → community shift upward → new

How do mats shift during the day?

Cyanobacteria produce O2; S-oxidisers move downward to avoid high O2 during the day

What drives stromatolite formation?

Diurnal/tidal disturbance + CaCO₃ precipitation in alkaline waters.

What is chemical equilibrium?

A thermodynamic state where the free energy is minimised and reactants/products no longer change in concentration

How is K related to Gibbs free energy?

ΔG∘=−RTln⁡K

What is Ka?

The acid dissociation for acid-base reactions:

Ka=[H+][A−] / [HA]

What is Kw?

The ionisation constant of water:

Kw=[H+][OH−]

What is Kd / Kg (dissociation constant)?

Constant describing dissociation of strong or weak acids

What is Kf?

Complex formation (association) constant

What is Ksp?

Solubility product constant for dissolution of solids/minerals:

Example for MgCl2:

Ksp=[Mg2+][Cl−]^2

When is pKa equal to pH?

When the concentrations of acid and conjugate base are equal

Total carbonate concentration expression

Ctotal=[CO2]+[HCO3^−]+[CO3^2−]

How does K relate to solubility?

Higher K → more products → more solubility

Lowe K → more reactants → lower solubility

How does pK relate to solubility

Low pK = higher solubility

How does pH affect solubility?

Higher pH → more reactants → lower solubility

Minerals of weak acids + strong bases raise pH

What is the common ion effect?

The presence of a shared ion reduces solubility of a mineral

What is complexation?

Formation of metal-ligand complexes that increase solubility

What is Eh?

The redox potential of a system relative to the standard hydrogen electrode

What does Q < Ksp?

The solution is undersaturated and dissolution occurs

• where Q is the product of the actual measured concentrations

What does Q = Ksp mean?

The system is at equilibrium → saturated

• where Q is the product of the actual measured concentrations

What does Q > Ksp mean?

The solution is supersaturated → precipitation occurs

• where Q is the product of the actual measured concentrations

What is the saturation index (SI)?

SI = log Q/K

How to interpret SI values:

• SI = 0 → equilibrium

• SI < 0 → undersaturated

• SI > 0 → supersaturated