Anoxygenic Photosynthesis in the Archaean World
The Archaean Environment (3.4 bya)
Hydrothermal vents
Stromatolites – gives clues on the
appearance of photosynthetic activity
(3.4 bya) – inferred by morphology, no
direct evidence to support any of the
possible origins (Olson and Blankenship,
2004) but stromatolites suggested
that photosynthetic organisms existed
“redox gradients” – oxidising surface
at the top and an alkaline hot solution
at the bottom (more subject to
debate)
Oxygenic photosynthesis had a large impact on the planet
- Great Oxidation Event: oxygen began to accumulate from ~2.7-2.2 bya (seen from
formation of iron oxides Fe2+ Fe 3+)
- No ozone prior – needed photochemistry and oxygen for about ~450 million
years, ozone layer has been protecting organisms against UV
Photosynthesis
In the 1770s – Joseph Priestly performed experiments showing that plants (sprig of
mint) released a gas that allowed combustion (inadvertently demonstrated that
plants released molecular oxygen – even though he was not aware of this)
Jan Ingenhousz later demonstrated that sunlight was necessary for photosynthesis
and that only green parts of plant could release oxygen
Photosynthesis leads to carbon assimilation, but it is a misnomer to say that is the
hydration of carbon – it is a light-driven redox reaction
Van Niel recognised that some bacteria used hydrogen sulphide rather than oxygen
– concluded that photosynthesis depends on elctron donation and acceptor reactions
and that oxygen released during photosynthesis came from the oxidation of water
Oxygenic Photosynthesis
CO2 + 2H2O + Light Energy (CH2O) + O2 + H2O
Anoxygenic Photosynthesis
2H2S + CO2 - 4e- -> CH2O + H2O + S2
2H2Org (Succinate) + CO2 – 4e- -> CH2O + H2O + Org (Fumarate)
2H2 + CO2 - 4e- -> CH2O + H2O
Van Niel Equation
2H2A + CO2 – 4e- and light -> CH2O + H2O + A2
, Experimental evidence that molecular oxygen came from water was provided by Hill
and Scarisbrick (1940) using isolated chloroplasts where A is an eelectron acceptor
or Hill oxidant
Ruben et al., (1940) also demonstrated this using 18O enriched water
Titration experiments also revealed that there were 2 light reactions (in oxygenic
photosynthesis)
Photosynthetic Energy Transformation
Light reactions = electron and proton transfer
reactions
Light independent reactions = biosynthesis of
carbohydrates from CO2
In more primitive organisms (eg. oxygenic
cyanobacteria, prochlorophytes, anoxygenic
photosynthetic bacteria) lack organelles
- Light reactions occur in complex membrane
system (photosynthetic membrane)
that is made up of protein complexes,
electron carriers, lipid molecules
- Photosynthetic membrane is
surrounded by water and thought of
as a 2-D surface that defines a
closed space with inner and outer
water phase
- Protein complexes embedded creates
an asymmetrical arrangement which allows some energy released during electron
transport to create an electrochemical gradient of protons across the
photosynthetic membrane
Light reaction convert energy into several forms
- First step is conversion of photon to an excited state of an antenna pigment
molecule located in the antenna system
- Antenna system consists of several pigment molecules – chlorophyll,
bacteriochlorophyll, carotenoids
- Antenna system anchored to proteins within photosynthetic membrane and serve a
specialised protein complex known as a reaction centre
In oxygenic photosynthetic organisms – 2 different reaction centres (PSI and PSII)
work concurrently but in series
- PSII feeds electrons to PSI
- Electrons are transferred from PSII to PSI by intermediate carriers
, - Net reaction is the transfer of electrons fronm a water molecule to NADP+,
producing reduced NADP (NADPH)
- Energy stored in NADPH is then used for later reduction of carbon
- Addtionally, the movement of electrons also pumps hydrogen ions across the
membrane producing a electrochemical gradient which is used to generate ATP via
ATP-synthase
In anoxygenic photosynthetic organisms, water is not used as the electron donor
- Electron flow is cyclic and driven by a single photosystem, producing a proton
electrochemical gradient used to provide energy for reduction of NAD+ by an
external H-atom or electron donor (eg. H2S or an organic acid) in a process known
as “reverse electron flow”
- Fixation of CO2 occurs via different pathways in different organisms
Reaction Centres
Oxygenic Organisms Anoxygenic Organisms
- Plants, algae, bacteria - Use light energy to extract electrons from
- Structure of PSII in molecules other than water
these organisms is very - Assumed to be ancient organisms
similar - Purple bacteria, green sulphur bacteria, green
gliding bacteria, gram positive bacteria
Type 1 Reaction Centres
Iron-sulphur type reaction centres
Electron from sulphide succinate, through
metabolic processes MQ which eventually
excites a chlorophyll molecule to emit an
electron
Type 2 Reaction Centres
Quinone type reaction centres
Electron from chlorophyll passed onto quinone
then to ETC (ATP synthesised)
Electron returns to original donor through
metabolism
Primary distinguishing feature between two
types are the early electron acceptor cofactor
- FeS centres in type 1
- Pheophytin/quinone complexes in type 2
The Archaean Environment (3.4 bya)
Hydrothermal vents
Stromatolites – gives clues on the
appearance of photosynthetic activity
(3.4 bya) – inferred by morphology, no
direct evidence to support any of the
possible origins (Olson and Blankenship,
2004) but stromatolites suggested
that photosynthetic organisms existed
“redox gradients” – oxidising surface
at the top and an alkaline hot solution
at the bottom (more subject to
debate)
Oxygenic photosynthesis had a large impact on the planet
- Great Oxidation Event: oxygen began to accumulate from ~2.7-2.2 bya (seen from
formation of iron oxides Fe2+ Fe 3+)
- No ozone prior – needed photochemistry and oxygen for about ~450 million
years, ozone layer has been protecting organisms against UV
Photosynthesis
In the 1770s – Joseph Priestly performed experiments showing that plants (sprig of
mint) released a gas that allowed combustion (inadvertently demonstrated that
plants released molecular oxygen – even though he was not aware of this)
Jan Ingenhousz later demonstrated that sunlight was necessary for photosynthesis
and that only green parts of plant could release oxygen
Photosynthesis leads to carbon assimilation, but it is a misnomer to say that is the
hydration of carbon – it is a light-driven redox reaction
Van Niel recognised that some bacteria used hydrogen sulphide rather than oxygen
– concluded that photosynthesis depends on elctron donation and acceptor reactions
and that oxygen released during photosynthesis came from the oxidation of water
Oxygenic Photosynthesis
CO2 + 2H2O + Light Energy (CH2O) + O2 + H2O
Anoxygenic Photosynthesis
2H2S + CO2 - 4e- -> CH2O + H2O + S2
2H2Org (Succinate) + CO2 – 4e- -> CH2O + H2O + Org (Fumarate)
2H2 + CO2 - 4e- -> CH2O + H2O
Van Niel Equation
2H2A + CO2 – 4e- and light -> CH2O + H2O + A2
, Experimental evidence that molecular oxygen came from water was provided by Hill
and Scarisbrick (1940) using isolated chloroplasts where A is an eelectron acceptor
or Hill oxidant
Ruben et al., (1940) also demonstrated this using 18O enriched water
Titration experiments also revealed that there were 2 light reactions (in oxygenic
photosynthesis)
Photosynthetic Energy Transformation
Light reactions = electron and proton transfer
reactions
Light independent reactions = biosynthesis of
carbohydrates from CO2
In more primitive organisms (eg. oxygenic
cyanobacteria, prochlorophytes, anoxygenic
photosynthetic bacteria) lack organelles
- Light reactions occur in complex membrane
system (photosynthetic membrane)
that is made up of protein complexes,
electron carriers, lipid molecules
- Photosynthetic membrane is
surrounded by water and thought of
as a 2-D surface that defines a
closed space with inner and outer
water phase
- Protein complexes embedded creates
an asymmetrical arrangement which allows some energy released during electron
transport to create an electrochemical gradient of protons across the
photosynthetic membrane
Light reaction convert energy into several forms
- First step is conversion of photon to an excited state of an antenna pigment
molecule located in the antenna system
- Antenna system consists of several pigment molecules – chlorophyll,
bacteriochlorophyll, carotenoids
- Antenna system anchored to proteins within photosynthetic membrane and serve a
specialised protein complex known as a reaction centre
In oxygenic photosynthetic organisms – 2 different reaction centres (PSI and PSII)
work concurrently but in series
- PSII feeds electrons to PSI
- Electrons are transferred from PSII to PSI by intermediate carriers
, - Net reaction is the transfer of electrons fronm a water molecule to NADP+,
producing reduced NADP (NADPH)
- Energy stored in NADPH is then used for later reduction of carbon
- Addtionally, the movement of electrons also pumps hydrogen ions across the
membrane producing a electrochemical gradient which is used to generate ATP via
ATP-synthase
In anoxygenic photosynthetic organisms, water is not used as the electron donor
- Electron flow is cyclic and driven by a single photosystem, producing a proton
electrochemical gradient used to provide energy for reduction of NAD+ by an
external H-atom or electron donor (eg. H2S or an organic acid) in a process known
as “reverse electron flow”
- Fixation of CO2 occurs via different pathways in different organisms
Reaction Centres
Oxygenic Organisms Anoxygenic Organisms
- Plants, algae, bacteria - Use light energy to extract electrons from
- Structure of PSII in molecules other than water
these organisms is very - Assumed to be ancient organisms
similar - Purple bacteria, green sulphur bacteria, green
gliding bacteria, gram positive bacteria
Type 1 Reaction Centres
Iron-sulphur type reaction centres
Electron from sulphide succinate, through
metabolic processes MQ which eventually
excites a chlorophyll molecule to emit an
electron
Type 2 Reaction Centres
Quinone type reaction centres
Electron from chlorophyll passed onto quinone
then to ETC (ATP synthesised)
Electron returns to original donor through
metabolism
Primary distinguishing feature between two
types are the early electron acceptor cofactor
- FeS centres in type 1
- Pheophytin/quinone complexes in type 2
