Week 5 - Photolithotrophy and Photoorganotrophy Flashcards
Photolithotrophy and photoorganotrophy are
modes of bacterial metabolism
All living things need
- a source of raw materials
- a source of energy
- a source of reducing power (electrons)
Living things can be classified
according to their metabolism, on the basis of where they get these things from
• in the case of “raw materials” classification is usually on the basis of carbon source
Examples of bacterial metabolism
- chemoorganotrophs
- photoautotrophs
- photolithotrophs
- chemolithotrophs
Phototrophs
“light eaters”
• obtain energy from sunlight
• photoorganotrophs and photolithotrophs
Photoorganotrophs
obtain energy from sunlight
• electrons and carbon from organic sources
Photolithotrophs
aka photoautotrophs, photolithoautotrophs
obtain energy from sunlight
• electrons from inorganic sources
• carbon from CO2
Cyanobacteria (and plants) are one sort of
photolithotroph
• their inorganic source of electrons is water
Many bacteria can
switch between modes of growth
photolithotrophs and photoorganotrophs?
Phototrophy
- photosynthesis
- chlorophylls and bacteriochlorophylls
- carotenoids and phycobilins
- anoxygenic photosynthesis
- oxygenic photosynthesis
Photosynthesis
the conversion of light energy to chemical energy
• phototrophs carry out photosynthesis
• most phototrophs are also autotrophs
• photosynthesis requires light-sensitive pigments called chlorophylls
• photoautotrophy requires ATP production and CO2 reduction
… carry out photosynthesis
phototrophs
Most phototrophs are also
autotrophs
Phototrophs
all use light as energy source
use CO2 = photoautotrphs
use organic carbon = photoheterotrophs
Chlorophylls and bacteriochlorophylls
• organisms must produce some form of chlorophyll (or bacteriochlorophyll) to be photosynthetic
• chlorophyll is related to porphyrins
• number of different types of chlorophyll exist
- different chlorophylls have different absorption spectra
Chlorophyll is related to
porphyrins
Different chlorophylls have different
absorption spectra
Photoautotrophy
- oxidation of H2O produces O2 = oxygenic photosynthesis
* oxygen not produced = anoxygenic photosynthesis
Oxygenic photosynthesis
oxidation of H2O produces O2
Anoxygenic photosynthesis
oxygen not produced
Cyanobacteria produce
chlorophyll a
Prochlorophytes produce
chlorophyll a and b
Anoxygenic phototrophs produce
bacteriochlorophylls
Chlorophylls arelocated within special membranes
- in eukaryotes called thylakoids
* in prokaryotes, pigments are integrated into cytoplasmic membrane
Pigment of purple bacteria
Bchl a and b
Pigment of green sulfur bacteria
Bchl c, d, e
Pigment of green nonsulfur bacteria
Bchl cs
Pigment of heliobateria
Bchl g
Reaction centers
participate directly in the conversion of light energy to ATP
Antenna pigments
funnel light energy to reaction centers
Chlorosomes
function as massive antenna complexes • found in green sulfur bacteria and green nonsulfur bacteria
Phototrophic organisms have accessory pigments in addition to chlorophyll, including
carotenoids and phycobiliproteins
Carotenoids
- always found in phototrophic organisms
- typically yellow, red, brown, or green
- energy absorbed by carotenoids can be transferred to a reaction center
- prevent photooxidative damage to cells
Energy absorbed by carotenoids can be transferred to
a reaction center
Carotenoids prevent
photooxidative damage to cells
Phototrophs:
purple and green bacteria
anoxygenic
• reducing power from H2S (e-) –> S0 (e-) –> SO4 2-
• energy from ADP + light = ATP (to carbon source)
• carbon from CO2 + electrons from H2S + energy from ATP == (CH2O)n
(n subscript)
Phototrophs:
cyanobacteria, algae, green plants
oxygenic
• reducing power from H2O (e-) –> 1/2 O2
• energy from ADP + light = ATP (to carbon source)
• carbon from CO2 + electrons from H2O + energy from ATP == (CH2O)n
(n subscript)
Phycobiliproteins
main antenna pigments of cyanobacteria and red algae
• form into aggregates within the cell called phycobilisomes
• allow cells to capture more light energy than chlorophyll alone
Anoxygenic photosynthesis specific
- found in 4 phyla of bacteria
- photosynthesis apparatus embedded in membranes
- electron transport reactions occur in the reaction center of anoxygenic phototrophs
Anoxygenic photosynthesis
reducing power
reducing power for CO2 fixation comes from reductants present in the environment
(H2S, Fe2+, or NO^2-)
• requires reverse electron transport for NADH production in purple phototrophs
• electrons are transported in the membrane through a series of proteins and cytochromes
Anoxygenic photosynthesis
• for a purple bacterium to grow autotrophically, the formation of ATP is not enough
- reducing power (NADH) is also necessary
- reduced substances such as H2S are oxidized and the electrons eventually end up in the quinone pool of the photosynthetic membrane
How photosynthesis works
- photons are absorbed by pigments (chromophores)
- the energy from the photons is used to pump protons (H+) across a membrane (also used to drive redox reactions)
- energy stores in proton gradients is used to drive ATP synthesis
Nature has invented photosynthesis twice
a) photosynthesis based on chlorophyll (in eubacteria)
b) photosynthesis based on bacteriorhodopsin (best characterized in archaea, but now known in some eubacteria)
Oxygenic photosynthesis specifics
- oxygenic phototrophs use light to generate ATP and NADPH
- the 2 light reactions are called photosystem I and photosystem II
- Z scheme of photosynthesis (photosystem II transfers energy to photosystem I)
- ATP can also be produced in CYCLIC PHOTOPHOSPHORYLATION
Oxygenic phototrophs use light to generate
ATP and NADPH
Z scheme of photosynthesis
photosystem II transfers energy to photosystem I
in oxygenic photosynthesis
Chlorophyll photosynthesis
in bacteria • proteobacteria • gram positive bacteria • cyanobacteria • chloroplast
Bacteriorhodopsin photosynthesis
in archaea
• extreme halophiles
Photosynthetic bacteria and where they get their electrons from:
photolithotrophs
- cyanobacteria: electrons from water, H2O
- purple sulfur bacteria: electrons from sulfides eg H2S
- green sulfur bacteria: electrons from sulfides
Photosynthetic bacteria and where they get their electrons from:
photoorganotrophs
all get electrons from organic molecules • purple non-sulfur bacteria • green non-sulfur bacteria • heliobacteria • Halobacterium (archaebacterium, bacteriorhodopsin)
Winogradsky column - redox gradient
aerobic
microaerophilic (water-soil junction)
anaerobic
H2S
Winogradsky column - microbial zones
- water (aerobic) = algae, cyanobacterium, aerobic heterotrophs
- junction (microaerophilic) = H2S oxidizers, facultative anaerobes
- red-brown (anaerobic-ish) = purple non-sulfur photoheterotrophs
- red-violet (anaerobic) = purple sulfur bacteria
- green-gray (anaerobic) = green sulfur bacteria
- bottom sediment (H2S) sulfate reducters, fermentnative heterotrophs
Photoorganotrophs obtain their electrons and some energy
by oxidizing organic compounds
• if you have access to organic compounds as a foodstuff, why bother with photosynthesis?
- depends of availability of oxygenic
photoorganotrophs are either
a) obligate anaerboes (eg Heliobacteria)
b) chemoorganotrophs when oxygen is present - they switch to photosynthesis under anaerobic conditions (eg Halobacteria, purple-nonsulfur bacteria, green non-sulfur bacteria)
Photoorganotrophs are either
a) obligate anaerboes (eg Heliobacteria)
b) chemoorganotrophs when oxygen is present - they switch to photosynthesis under anaerobic conditions (eg Halobacteria, purple-nonsulfur bacteria, green non-sulfur bacteria)
Photoorganotrophs
in he absence of oxygen
only very limited energy can be obtained by breaking down organic molecules (see chemoorganotrophy and fermenation)
• therefore under anaerobic conditions a huge advantage to be able to obtain extra energy from light
Halobacterium salinarum
- photoorganotroph that is chemoorganotroph when oxygen is present, switch to photosynthesis under anaerobic conditions
- make water red
When oxygen is present, Halobacterium grow as
a chemoorganotroph
Under anaerobic conditions, Halobacterium
synthesizes large quantities of bacteriorhodopsin, a plasma membrane protein with a retinal chromophore
• when it absorbs a photon, the retinal changes conformation - this drives changes in protein structure, leading to pumping of protons across the membrane
Purple non-sulfur bacteria
gram-negative eubacteria
• eg Rhodospirillium rubrum
• lives in muddy sediments in lakes and ponds
• adjusts its metabolism according to the availability of light, oxygen, organic compounds and sulfide
• it can be a photoorganotroph, an aerobic chemoorganotroph, or an anaerobic chemoorganotroph, or a lithotroph that uses the Calvin cycle to fix CO2
Under aerobic conditions, Rhodospirillum is
colorless and grows as a chemoorganotroph
• under anaerobic conditions, cells become purple (mainly due to carotenoids). at the same time, it synthesizes chromatophore membranes containing photosynthetic proteins which bind bacteriochlorophyll
Bacteriochlorophylls absorb light energy strongly in the near
infra-red
• the key protein complex is a reaction center which uses light-energy to drive movement of electrons
• electron flow is coupled to proton translocation across the chromatophore membrane
• see reaction center cytochrome bc complex
electron flow is cyclic - normally no net generation of oxidant or reductant
The reactin center of purple non-sulfur bacteria is a
Type 2 reaction center
• it shows some resemblance to Photosystem II of cyanobacteria and plants
Green non-sulfur bacteria
- Chloroflexus and relatives
- Chloroflexus forms thick mats, often in warm springs
- a motile, filamentous, gliding bacterium
- grows as a chemoorganotroph in the dark, switches to photoorganotrophy under anaerobic conditions in the light
- like Rhodospirillum, Chloroflexus has a Type 2 reaction center but it doesn’t make chromatophores
- the reaction center is in the plasma membrane, and is associated with chlorosomes
Forms thick mats, often in warm springs
Chloroflexus
green non-sulfur bacteria
Like Rhodospirillum, Chloroflexus has a
Type 2 reaction center
BUT
doesn’t make chromatophores
• reaction center is in the plasma membrane and is associated with chlorosomes
Heliobacteria
obligate anaerobes found in sediments in paddy fields
• gram-positive cell structure - not closely related to any phototrophs
• photosynthesis based on a Type 1 reaction center
(shows resemblance to Photosystem I of cyanoabcteria and plants)
Photolithotrophs
- purple sulfur bacteria (electrons from sulfides, eg H2S)
- green sulfur bacteria (electrons from sulfides)
- cyanobacteria (electrons from water, H2O)
Purple sulfur bacteria get electrons from
sulfides
eg H2S
Green Sulfur bacteria get electrons from
sulfides
Cyanobacteria get electrons from
water H2O
Purple sulfur bacteria
eg Chromatium vinosum
• strict anaerobes
• found in sulfur springs, in anaerobic lake sediments
• photosynthetic apparatus very similar to purple non-sulfur bacteria
but pathways of electron flow are different
• instead of cyclic electron flow, electrons are extracted to form sulfides (oxidized to sulfur, and then to sulphate)
and are used to reduce NAD+ to NADH which is used to fix CO2 via the Calvin cycle
• often deposit sulfur granules inside the cell
Green sulfur bacteria
eg Chlorobium
• filamentous, gliding bacteria
• strict anaerobes
• found in anaerboic zones of lakes, plus sulfur springs
• photosynthesis based on a Type 1 reaction center but have chlorosomes (like Chloroflexus)
• electrons extracted from sulfides, used to reduce NAD+ to NADH
• carbon obtained from CO2 (but not fixed via the Calvin cycle)
Cyanobacteria (blue-green algae)
- electrons from water
- used eventually to reduce NADP to NADPH
- ATP and NADPH used (eg) in the Calvin cycle, which fixes CO2
- extracting electrons from water, and passing them to NADP requires some difficult chemistry, and a very high energy input
- cyanobacteria do this by using 2 kinds of reaction centers acting in series (Z scheme)
Photosystem II
a Type 2 reaction center
• resemblance to the reaction centers of purple bacteria and Chloroflexus - but with an extra unit that oxidizes water
Photosystem I
a Type 1 reaction center
• resemblance to the reaction centers of green sulfur bacteria and heliobacteria
Synechocystis 6803
thylakoid membranes
phycobilisomes
Cyanobacteria are related to chloroplasts of
green plants and algae
• green plants retain Photosystem II and Photosystem I but have lost phycobilisomes
Cyanobacteria probably evolved 3-3.5 billion years ago and
were responsible for generating the oxygen-rich atmosphere
How did cyanobacteria evolve in the first place?
purple and green bacteria have a simpler photosynthetic apparatus, and are assumed to have evolved first
• but cyanobacteria combine a Type II reaction center and a Type I reaction center
Cyanobacteria are very
widespread - lakes, rivers, oceans
wide range of different lifestyles and morphologies
• some form mats, some float
range from small single cells to complex filaments
Prochlorococcus marinus and
Anabaena cylindrica are both
cyanobacteria
• Prochlorococcus (1.67 MB, 1696 genes) has small, simple cells
• Anabaena (6.37 MB, 6132 genes) filamentous, multiple cell types
Metabolic versatility in cyanobacteria
eg Oscillatoria limnetica
• a filamentous cyanobacterium found in a warm, sulfide-rich spring in New Zealand
• during the night: sulfide builds up in the spring
• in the morning: Oscillatoria grows like a green sulfur bacterium, using sulfide as a source of electrons, and only Photosystem I
• later in the day: all the sulfide gets used up, and Oscillatoria grows like a normal cyanobacterium, using Photosystem II to extract electrons from water
Strucutral complexity in cyanobacteria
eg Anabaena variabilis
• heterocyst
• vegetative cells
Heterosystes are one solution to the problem of
fixing nitrogen while doing oxygenic photosynthesis
• nitrogen is fixed by nitrogenase
electrons + ATP + H+ + N2 –> NH3 + H2
• the problem is NITROGENASE IS INACTIVATED BY OXYGEN
So nitrogen fixation and oxygen evolution must
be separated, either in time or space
Heterocysts
fix nitrogen
• they keep the environment inside the cell anaerobic by having no photosystem II, by having a high content of respiratory oxidases, and by having a thick, oxygen-impermeable cell wall
Heterocysts and vegetative cells
depend on each other
• vegetative cells make sugars
• heterocysts make nitrogen compounds (amino aids)
• the different chemicals are exchanged between the cells
• so Anabaena is a true multicellular organism
Vegetative cells make
sugars
Heterocysts make
nitrogen compounds (amino acids)
Filamentous cyanobacteria are
multicellular organisms
• morphological development
Anabaena in nitrogen-rich environment = no heterocyst
Anabaena in nitrogen-poor environment = heterocyst forms
cyanobacteria
electrons from water, H2O
purple sulfur bacteria
electrons from sulfides eg H2S
green sulfur bacteria
electrons from sulfides
