The redox of life

Biogeochemistry Wk2

What are exergonic reactions?

Reactions that release energy (∆G<0) → used by orgnaisms to gain energy

What are endergonic reactions?

Reactions that require energy input (∆G>0) → used to build complex molecules

What defines an organic compound>

• Contains carbon (often with H and O), C-H or C-C bonds

• Typical oxidation state of carbon is between -4 and +3

What defines an inorganic compound?

• May lack carbon and lacks C-H/C-C bonds

• Typically has C in +4 oxidation state

Examples of organic species

• methane (Ch₄)

• benzene (C₆H₆)

• lactate (C₃H₆O₃)

• glucose

• ATP

• acetic acid (CH

What is metabolism?

All the biochemical processes occurring in an organism

What are the two categories of metabolism?

• Catabolism → breakdown of molecules to release enegry

• Anabolism → synthesis of molecules requiring energy

What molecule carries useable energy in cells?

ATP

How does ATP relate to catabolism and anabolism?

• Catabolism FORMs ATP from ADP

• Anabolism BREAKS ATP into ADP

What is biomass primarily made of?

organic carbon

What is heterotrophy?

Gets organic crabon from the environment

What is autotrophy?

Converts inorganic carbon into organic carbon

What are the three ATP-generating pathways in chemoheterotrophy?

1. Anaerobic respiration

2. Anaerobic respiration

3. fermentation

What is chemolithotrophy?

ATP generation by oxidising inorganic compounds.

What are the components of a catabolic redox reaction

• Primary Electron Donor (PED)

• Terminal Electron Acceptor (TEA)

What are examples of TEAs in anaerobic respiration?

NO₃⁻, Mn⁴⁺, Fe³⁺, SO₄²⁻

Name three anabolic redox reactions.

• Carbon fixation

• Nitrogen fixation

• Biomolecule synthesis

What two criteria define metabolic classification?

• Energy source (light or chemical)

• Carbon source (organic or inorganic)

What are the PED and TEA in aerobic chemoheterotrophy?

• PED: Organic carbon

• TEA: Oxygen

What acts as PEDs in chemolithoautotrophy?

Inorganic compounds (e.g. rocks)

Name three TEAs in anaerobic chemolithoautotrophy.

NO₃⁻, Fe³⁺, SO₄²⁻

Why is photosynthesis anabolic?

It builds molecules using light energy and inorganic carbon

What do chemolithoautotrophs use as PED?

Don’t use carbon

• H₂

• inorganic carbon - CO2 HCO₃

• H₂S

• Fe2+, FeS_2

• Mn2+

• NH₄⁺. NO₂^-

What is the difference between oxygenic and anoxygenic photosynthesis?

Oxygenic photosynthesis produces oxygen wheres anoxygenic photosynthesis does not use or produce oxygen as electron donor

What do anaerobic chemoheterotrophs use as TEA?

• NO₃-, NO₂-

• Mn4+, MnO₂

• Fe3+, Fe(OH)₃

• UO₂²⁺

• SO₄^2-

What does dissimilatory mean in terms of microbial metabolism?

A reaction occurring at the cell membrane or wall , not fully internal to the cell

What are the two varieties of dissimilatory nitrate reduction?

1. Denitrification → converts nitrate to nitrogen gases → removes nitrogen from soils/water

• NO₃^- → NO₂^- → NO → N₂O → N₂

2. Nitrate ammonification → a reduction of nitrogen with a shorter pathway and end point is ammonium rather than gas.

• NO₃^- → NO₂^- → NH₄⁺

What minerals are commonly reduced in soils by bacteria?

Fe(III) and Mn(IV)- hydroxides

Name three environmental roles of Fe(III)-reducing bacteria.

1. Release Fe 2+ in anoxic groundwater

2. Degrade organic matter in deep sediments

3. Form variegated red beds in rocks

Why is metaloid reduction important for bioremediation?

• It helps reduce toxic contaminants like As(V), Se(IV), Cr(VI), U(VI), Tc(VII), Pu(V)

• It also helps oxidise orgnanic C contaminants in anoxic conditions

What is teh typical TEA for sulfate-reducing bacteria (SRB)?

Sulfate (SO_4^-)

Name three environments where SRB’s thrive?

Watterlogged soils, sewage, deep-see hydrothermal vents

What does being ‘metabolically fluid’ mean for SRBs?

Ability to switch between TEAs like NO₃⁻, Fe(III) reduction or chemolithoautotrophy

What organisms perform methanogenesis?

strictly anaerobic archaea

where does methanogenesis occur?

Anoxic, sulfate-deficient environments (swamps, tundra, marine sediments) (extreme environments)

What are the 4 types of anaerobic chemoheterotrophy?

1. Dissimilatory nitrate reduction

2. Dissimilatory metal/metaloid reduction

3. Dissimilatory sulfate reduction

4. methanogenesis

What are typical PEDs in chemolithoautotrophy?

H₂, CH₄, H₂S, S, S₂O₃²⁻, Fe²⁺, Mn²⁺, NH₄⁺, NO₂⁻

What are the common TEAs in chemolithoautotrophy?

O₂, NO₃⁻, Mn(IV), Fe(III), SO₄²⁻, S⁰, U(VI)

Which PED yields the most energy in chemolithoautotrophy?

Hydrogen (H_2)

What is the most widespread chemolithoautotroph group?

H₂ - oxidising bacteria

What TEAs do hydrogen oxidisers use?

• Aerobes: O₂

• Anaerobes: NO₃^-, Mn(IV), Fe(III), SO₂^2-, CO2

What’s the differenec between methanogens and methanotrophs?

• Methanogens PRODUCE methane

• Methanotrophs CONSUME methane

What are the two types of sulfur oxididers?

• Gradient S-oxidisers → neutral pH, low O2

• Acidophilic S oxidisers: acidic environments. H₂S as PED, O2 TEA

What is the equation for chemolithotrophic methanogenesis?

CO₂ + 4H₂ → CH₄ + 2 H₂O

Where do gradient S-oxidisers typically occur?

S-rich springs, microbial mats, low-o2 seawater, sewage polluted freshwater

What slows abiotic oxidation of Fe(II) and Mn(III)?

Suboxic conditions

What are the two types of iron oxidisers?

1. Neutrophillic Fe oxidiers

• require oxygen but not too much (microaerophilic)

2. Acidophilic Fe oxidiers

• usually associated with mine wastes

What conditions favour manganese oxidisers?

Suboxic conditions

What are the two types of nitrogen oxidisers?

• Nitrifying bacteria: NH₄⁺ as PED, O₂ as TEA

• Anammox bacteria: Anaerobic ammonia oxidisers

Where will you most likely find nitrifying bacteria?

Habitats with high NH_4^+

stratified lakes,

streams contaminated with sewage

Where is ammox bacteria common?

• Marine sediments and in anoxic basins

Name and explain 5 examples of anoxygenic phototrophs:

• Green sulfur bacteria (GSB) - Strictly anaerobic, require sulfide-rich environments, e- donors*: sulfides

• Green nonsulfur bacteria (GNB) – Aerobic or anaerobic, auto- or heterotrophs, e- donors*: sulfides, H_2

• Purple sulfur bacteria (PSB) - Red or purple pigments, auto- or heterotrophs, e- donors*: sulfides, H2 , Fe2+

• Purple nonsulfur bacteria (PNB) - Similar to PSB except can’t handle too much sulfide, photoheterotrophs

• Heliobacteria - Obligate anaerobic photoheterotrophs, → usually in very flooded environments .

What langauge to use when referring to environments?

Oxic and anoxic

What language used referring to processes?

oxygenic and anoxygenic

What language is used to refer to organisms or processes?

Aerobic and anaerobic