Lecture 13: Redox potentials

What drives oxygen profiles in extreme oligotrophic lakes?

Oxygen concentrations in the hypolimnion are driven by physical processes (temperature)

What drives oxygen profiles in extreme eutrophic lakes?

Oxygen concentrations in the hypolimnion are driven by biological processes

• O2 consumption via respiration > physical effects of temperature on O2 concentration

How does the Epilimnion: Hypolimnion (E:H) ratio influence oxygen profiles?

Large E:H

• Likely to experience anoxic hypolimnia (winterkill) under ice

Small E:H

• Oxygen depletion is less likely to occur

• Epilimnetic PP is more likely to dilute in the hypolimnion

What is a heterograde oxygen profile?

A vertical distribution of dissolved oxygen (DO) in a stratified lake or reservoir where the concentration does not simply increase or decrease steadily with depth, but instead shows a distinct peak or dip—usually located within the metalimnetict (thermocline) layer.

What is a positive heterograde profile?

A DO maximum occurs at the metalimnion

What is a negative heterograde profile?

A DO minimum occurs at the metalimnion

How does stream order influence Oxygen profiles?

Low order streams = high O2 saturation as a result of cold temperatures from shading, and concentrations near saturation because  of turbulent mixing

→ The importance of turbulence decreases with increasing stream order; biological processes become more important (inputs of organic matter)

What is oxidation vs reduction?

Substances that Lose Electrons became Oxidized

Substances that Gain Electrons become Reduced

LEO the lion says GER

What is reduced vs oxidized in photosynthesis?

Reduction of CO₂ → Organic matter

Oxidation of H₂O → O₂

→ Electrons are transferred from H₂O to organic matter

What is reduced vs oxidized in respiration? (when oxugen is present)

Organic matter is oxidized to CO₂

O₂ is reduced to H₂O

→Energy is generated within cells by the transfer of electrons

T/F oxidized lake water has high redox potential?

True

What is redox potential (E_h)?

How oxidized a substance is …

→ Will easily accept electrons

Oxygen is not the only possible electron acceptor!

How is redox potential measured?

Using a probe that measures the flow of electrons in a solution

The voltage necessary to stop the flow of electrons between the electrode and the environment

Highly oxidized solutions therefore have a high Eh and will accept e- easily

Eh is expressed relative to a standard solution of hydrogen ions

How does oxygen decline effect redox reactions?

 Redox conditions can continue using other electron acceptors in lieu of O2

What is an example of redox conditions under anoxic conditions?

Oxidation of organic matter via the coupled reduction of sulphate (SO4) to hydrogen sulfide (H2S) by sulphate reducing bacteria (specialized bacteria capable of metabolism without O2)

What is the redox staircase?

Redox reactions that are undertaken solely by specialized bacteria that can use molecules other than O2 to metabolize organic matter

What is the function of redox reactions in aquatic environments?

Act to generate energy for heterotrophs in environment where O2 replenishment is slow or not possible

Coupled by ecological succession of microbial taxa

What can we use redox conditions to determine?

• Inform what chemical reactions will occur without an input of energy

• Inform what reaction products are favoured to be present in the environments

What occurs when oxygenated conditions reappear during redox reactions?

Once oxygenated conditions appear, reduced species will undergo oxidizing conditions

Reduced species include NH₄ , H₂S and CH₄

If there is energy to be made in the natural environment, there will be a species that exploits this niche

How does chemosythesis return equilibrium?

Chemosynthesis microbes: oxidize reduced molecules to either reduce CO2 to organic matter, or generate energy

• Reaction occurs inn the presence of O2, and return reduced molecules to a more reduced state

What are two examples of chemosynthesis?

• Methanotrophy

• Sulfide oxygenation

T/F redox reactions are not paired?

False

Reduction reactions are typically paired with the oxidation of organic matter; oxidation reactions occur when oxygen becomes present again

What are the 4 implications of changing redox conditions?

1. Changes in chemical presence

2. Changes in nutrient profiles and nutrient availability

3. Changes in methods of organic matter production

4. Changes in organismal composition

What is an example of changing redox conditions and its effects on nutrient profiles and availability?

Oxidized NO₃ vs reduced NH₄

Nitrogen exists in a variety of redox states

Regeneration of phosphorus from lakebed sediments is driven by redox processes

How can organic matter production chnage under redox conditions?

Lack of DO results in other electron acceptors being reduced

Reduced compounds function as electron donors that enable organic matter production under aerobic conditions

What is an example of how organismal composition can change with redox conditions?

• Most organisms cannot exist in lake sediments if the redox potential falls below a threshold value

• Redox potentials at 5cm below the sediment surface is a good indicator of the presence of hexagenia

• Hexagenia aerates sediments via burrowing, but at some point cannot overcome redox conditions