Bacterial photosynthesis 2020

This PPt deals about bacterial photosynthesis, different types of photosynthetic bacteria, types of photosynthesis-OXygenic and anoxygenic , photosynthetic structures, photosynthetic pigments and also explain the light reactions and dark reactions.in dark reactions, in addition to Calvin cycle, bacteria has one more carbon dioxide fixation (Pyruvate reductase pathway)

1. Bacterial Photosynthesis
K.Sudha Rameshwari
Assistant Professor,
Department of PG Biochemistry,
V.V.Vanniaperumal college for women, Virudhunagar, Tamilnadu,
India
Purple bacteria
Green sulfur bacteria

2. Photosynthesisistheprocessusedbyplants,
algaeandcertainbacteriatoexploitenergyfrom
sunlightandturnitintochemicalenergy.
Thereactionsofphotosynthesisaredividedinto
twotypes:
•Light-dependentreactions(alsocalledlight
reactions):Whenaphotonoflighthitsthe
reactioncenter,apigmentmoleculesuchas
bacteriochlorophyllreleasesanelectron.
•Light-independentreactions(alsocalleddark
reactionsandknownastheCalvincycle):Light
reactionsproduceATPandNADPH,whichare
therichenergysourcesthatdrivedarkreactions.

3. Photosynthesis in prokaryotes
•Thephotosyntheticprokaryotesare
greenbacteria
puplebacteriaand
cyanobacteria
•Theydifferfundamentallyinthepathwaysof
photosyntheticreactions
•Photosyntheticbacteriahavecomparatively
primitivesystemsessentiallytotheactivities
carriedoutbyphotosystemIineukaryoticplants.
•Lackingawatersplittingactivityequivalentto
photosystemII,photosyntheticbacteriacannot
usewaterasanelectrondonoranddonotevolve
oxygeninphotosynthesis.

4. Cyanobacteria
•Oxygenicphotosyntheticbacteriaperformphotosynthesisina
similarmannertoplants.Theycontainlight-harvesting
pigments,absorbcarbondioxide,andreleaseoxygen.
Cyanobacteriatypicallyprokaryoticincellularorganization,
havetwophotosystemsequivalenttoeukaryoticphotosystemsI
andIIandcarryoutphotosynthesisbysamemechanismsas
eukaryoticplants.
•CyanobacteriaorCyanophytaaretheonlyformofoxygenic
photosyntheticbacteria
•Cyanobacteriacanusewaterasaelectrondonorandevolve
oxygeninphotosynthesis.
•Thereare,however,severalspeciesofCyanobacteria.
•Theyareoftenblue-greenincolorandarethoughttohave
contributedtothebiodiversityonEarthbyhelpingtoconvert
theEarth’searlyoxygen-deficientatmospheretoanoxygen-
richenvironment.

5. Photosynthetic bacteria-Purple bacteria
Anoxygenicphotosyntheticbacteriaconsumecarbondioxidebutdonotrelease
oxygen.TheseincludeGreenandPurplebacteriaaswellasFilamentous
AnoxygenicPhototrophs(FAPs),andPhototrophicHeliobacteria.
Purplebacteriaclassifiedintotwotypes
•PurplesulfurbacteriatheChromatiaceaewhichproducesulfurparticles
insidetheircells.ItusesulfurcontainingcompoundssuchasH
2Saselectron
donorsfornoncyclicphotosynthesisiscalledPhotolithotrophy.
•PurplenonsulfurbacteriaEctothiorhodospiraceae,whichproducesulphur
particlesoutsidetheircells.Itusecomplexsulfurfreeorganicsubstancessuch
asmalateandsuccinateaselectrondonors,theprocessiscalled.
photoorganotrophy.Whilethesebacteriacantoleratesmallamountsof
sulfur,theytoleratemuchlessthanpurpleorgreensulfurbacteria,andtoo
muchhydrogensulfideistoxictothem.
Purplebacteriacannotphotosynthesizeinplacesthathaveanabundanceof
oxygen,sotheyaretypicallyfoundineitherstagnantwaterorhotsulfuric
springs.Insteadofusingwatertophotosynthesize,likeplantsand
cyanobacteria,purplesulfurbacteriausehydrogensulfideastheirreducing
agent,whichiswhytheygiveoffsulfurratherthanoxygen.

6. Photosynthetic bacteria-Green bacteria
•Greensulfurbacteriagenerallydonotmove(non-motile),
andcancomeinmultipleshapessuchasspheres,rods,
andspirals.
•Thesebacteriahavebeenfounddeepintheoceanneara
blacksmokerinMexico,wheretheysurvivedoffthelight
ofathermalvent.
•TheyhavealsobeenfoundunderwaternearIndonesia.
•Thesebacteriacansurviveinextremeconditions,likethe
othertypesofphotosyntheticbacteria,suggestingan
evolutionarypotentialforlifeinplacesotherwisethought
uninhabitable.
•Greenbacteriawhichmaybeuseeitherinorganic
sulfatecontainingcompoundsornonsulfurorganic
moleculesaselectrondonorsfornoncyclic
photosynthesis.

7. Other bacteria
•PhototrophicHeliobacteriaarealsofoundinsoils,especiallywater-
saturatedfields,likericepaddies.Theyuseaparticulartypeof
bacteriochlorophyll,labelledg,whichdifferentiatesthemfrom
othertypesofphotosyntheticbacteria.Theyarephotoheterotroph,
whichmeansthattheycannotusecarbondioxideastheirprimary
sourceofcarbon.
•Greenandredfilamentousanoxygenicphototrophs(FAPs)were
previouslycalledgreennon-sulfurbacteria,untilitwasdiscovered
thattheycouldalsousesulfurcomponentstoworkthroughtheir
processes.Thistypeofbacteriausesfilamentstomovearound.The
colordependsonthetypeofbacteriochlorophylltheparticular
organismuses.Whatisalsouniqueaboutthisformofbacteriais
thatitcaneitherbephotoautotrophic,meaningtheycreatetheir
ownenergythroughthesun’senergy;chemoorganotropic,which
requiresasourceofcarbon;orphotoheterotrophic,which,as
explainedabove,meanstheydon’tusecarbondioxidefortheir
carbonsource.

8. Types of bacterial Photosynthesis
There are two types of photosynthetic processes:
•oxygenic photosynthesis
•Anoxygenic photosynthesis
Thegeneralprinciplesofanoxygenicand
oxygenicphotosynthesisareverysimilar,but
oxygenicphotosynthesisisthemostcommon
andisseeninplants,algaeandcyanobacteria.

9. oxygenic photosynthesis
•During oxygenic photosynthesis, light energy
transfers electrons from water (H
2O) to carbon
dioxide (CO
2), to producecarbohydrates.
•In this transfer, the CO
2is "reduced," or receives
electrons, and the water becomes "oxidized," or
loses electrons.
•Ultimately, oxygen is produced along with
carbohydrates.
Oxygenic photosynthesis is written as follows:
•6CO
2+ 12H
2O + Light Energy→ C
6H
12O
6+6O
2+ 6H
2O

10. Anoxygenicphotosynthesis
•Purpleandgreensulfurbacteriacarryoutanoxygenic
photosynthesisi.e.thereisnoevolutionofoxygen.
•Thereisonlyonephotosysteminvolvedin
photosynthesis
•Theelectrondonorssulphur,reducedsulphur
compounds,molecularhydrogenorsimpleorganic
compounds.
•Thesearesubstanceswithlowerredoxpotentialsthan
water.
•Evenincyanobacteria,theremaybeanoxygenic
photosynthesiswithonlyonephotosystemwhen
hydrogensulphideistheelectrondonor.
ThegeneralequationforAnoxygenicphotosynthesisis:
• 2H
2A+CO
2↔C(H
2O)+H
2O+2A

11. Photosynthetic structures
•In eukaryotic cells of
higher plants, multicellular
red, green and brown
alage, dinoflagellates and
diatoms the
photosynthetic structures
are the chlorpplasts.
•Chlorplasts enclose
membraneous sacs called
thylakoidswhich contain
the units of
photosynthesis

12. Photosynthetic structures
•InProkaryotes(bluegreenbacteria,
prochlorphyta,purpleandgreen
bacteria),thephotosyntheticstructures
arecalledChromatophores.
•IntheRhodospirillaceae(purplenon-
sulfurbacteria)andchromatiaceae
(purplesulfurbacteria)thethylakoidsare
extensionsofthecellmembrane.They
maybeintheformofvesicles,tubular
bodiesorlamellae.

13. Inchlorobiaceae(greensulfurbacteria)thesacslikemembraneous
structurescalledchlorosomesformingthephotosyntheticapparatus
arenotcontinuouswiththecellmembranei.e.itisattachedtothe
cytoplasmicsideoftheplasmamembrane
Photosynthetic structures
Naturalandartificialchlorosomalsystems.(a)Schematicofphotosyntheticapparatusesin
photosyntheticgreenbacteria.(b)Molecularstructuresofbacteriochlorophyll-c–fmolecules.(c)
Syntheticchlorophyllderivativesreportedasmodelsofbacteriochlorophyll-d.

14. Photosynthetic pigments
Three main classes of photosynthetic pigments:
•Chlorphylls(Chl)(including bacteriochlorophyll,
BChl),
•Carotenoids and
•phycobilins (Phycobiliproteins,PBPs)

15. Bacateriochlorophylls
•Thephotosyntheticpigmentsofthepurple
andgreenbacteriaarebacteriochlorphylls
a,b,c,doreandavarietyofcarotenoids.
•Bacteriochlorophylls,likechlorphyllsof
eukaryoticplantsarebuiltonaterapyrrole
ringcontainingacentralmagnesiumatom
anddifferonlyinminorsubstitutionsinside
groupsattachedtothering.
•purplebacteriacontainbacteriochlorphylls
aandb.
•Greenbacteriapredominatelyc,dande
andalsocontainsmallquantitiesof
bacteriochlorophyllsa,whichoccurin
reactioncenters
•Bacteriochlorphyllpigmentsabsorblight
moststronglyinthenearUVandfarred
regionsofthespectrumandtransmitmost
oftheUVwavelengths.

16. Carotenoids
•Thedistinctivecolorsofgreenandpurplebacteriacome
primarilyfromdifferentcarotenoidsoccuringasaccessory
pigmentsinassociationwithbacteriochlorophylls.
•Thecarotenoidsarefoundinalmostallphotsynthetic
orgnaisms.
•Theyareyellowandorangepigmentswhicharesolublein
organicsolvents.
•Therearetwotypesofcarotenoids,caroteneand
carotenols.Carotenes,e.g.ß-carotene.
•MostofthecarotenesarepresentinPhotosystemI.
•Carotenols(Xanoathophylls)arealcohols.
•Fucoxantholispresentindiatomsandotherbrownalgae.
MostofthexanthophyllsarepresentinphotosystemII.

17. Phycobilins
•Phycobilinsarewatersolubleopenchain
tetrapyyroleswhicharepresentinredalgae
andbluegreenbacteria(cyanobacteria).
•Therearetwokindsofphycobilins,
phycocyaninsandphycoerythrins.
•Phycocyaninsarepredominateintheblue
greenalgae,whilephycoerythrinspredominate
intheredalgae.
•PhycobilinsaremainlypresentinPSII,butalso
presentinPSI.

18. •The light harvesting complexes of both purple
and green bacteria like the LHC-I and LHC-II
antennas of higher plants, absorb light and
pass excitation energy to the reaction centers
of the photosystem

19. Electron transport carriers in
Bacterial photosynthesis
•TheelectrontransportSystemofphotosynthetic
bacteriadiffersfromthatofaerobicbacteria.
•CytochromeaandothertypesofCytochrome
oxidaseareabsentinthephotosynthetic
electrontransportsystem,because
photosynthesistakesplaceunderanaerobic
conditions.
•Hencethereisnoneedofacytochromewhich
interactswithmolecularoxygen.

20. Electron transport carriers
Theelectrontransportsystemconsists
ofan
•intermediateelectronacceptor(I),
•aprimaryelectronacceptor(X),
•asecondaryacceptor(Y),generally
believedtobeubiquinone(UQ)and
•bandctypecytochrome

21. Electron transport carriers and its location
•Theelectrontransportcarriersareasymmetricallylocatedin
themembrane.Thisisnecessaryforsettingupinthe
hydrogeniongradient.
•Thereactioncentrespansthemembraneofthe
chromatophore.ItislocatedbeneaththeATPasecomplex.
•Theprimaryacceptor(X)isbelievedtobeassociatedwith
thereactioncenterontheoutersideofthemembrane.
•ThesecondaryacceptorY(probablyUQ)takesprotonfrom
themedium.Itisthuslocatedontheoutersideofthe
membrane.
•Thebtypecytochromeisprobablylocatedintheinteriorof
themembrane.
•Thec-typecytochromeinteractswiththereactioncentre
andislocatedontheinsideofthemembrane.

22. Primary acceptor
•TheelectronfromP870isreceivedbyan
intermediateacceptor(I)andtransferredtoa
primaryacceptor(X).
•ThepurifiedP870reactioncentrehasbeen
showntocontainnonhaemeironand
ubiquinone.Thishasledtopossiblybothacts
asprimaryacceptors.
•Xisthereforelikelytobeaniron-sulfurprotein
oriron-quinonecomplex.Itmayormaynotbe
ferredoxin(Clostridiumhaveferredoxin).
•Bacterialferredoxinsareblackishbrownincolor
withabsorptionmaximaaround390nm

23. Quinone
•Thesecondaryacceptor(Y)isgenerally
believedtobeubiquinone(UQ).
•UQhasalsobeenconsideredtobeaprimary
acceptorinthebacterialreactioncentre,
probablyinanironquinonecomplex.
•UQconsistsofa1-4benzoquinonenucleus
withnisoprenoidsidechainatthesecond
carbonatom

24. Cytochrome b
•Cytochromebispresentinthephotosynthetic
electrontransportsystem.
•Itisadjacenttocytochromecinthecyclicsystem.
•Cytochromebmaybepresenteveninorganisms
whereithasbeenpreviouslyreportedtobeabsent.
Smallamountsofcytochromebcouldbemaskedby
othersubstances.
•OnemoleculeofCytochromebpresentper
reactioncentre
•Cytochromebhasaroleincyclicphotosyntheticflow
inthechromatiaceae,Chlorobiaceaeaswellasinthe
Rhodospirillaceae.

25. C-type Cytochrome
•Anumberofdifferentctypecytochromeshavebeenfoundinthe
electrontransportsystemofphotosyntheticbacteria.
•Inthepurplenon-sulphurbacteriumRhodospirillumrubruma
solublec-typecytochromeisassociatedwithP870ofthereaction
centre.Thiscytochromeisreferredtoascytochromec
2andhasa
highmidpointpotenitialofabout+300mv.
•Cytochromec
2istheelectrondonortoP870.
•InPSIofhigherplants,Plastocyanin(PC)istheelectrondonorto
P700.
•Rodospirillumalsocontainscytochromecc’withtwodifferenthaeme
groups
•PurplesulphurbacterialikeChromatiumcontaincytochromec
552in
additiontocytochromec
2andcc’.
•Cytochromec552(MW72000)hastwohaemegroupsandoneFMN.
•ThegreensulphurbacteriumChlorobiumhasthreecytochromesof
C-type.

26. Photophosphorylation
•Photophosphorylationistheprocessofutilizinglight
energyfromphotosynthesistoconvertADPtoATPi.e
lightenergyisconvertedintochemicalenergy.
•Itistheprocessofsynthesizingenergy-richATP
moleculesbytransferringthephosphategroupinto
ADPmoleculeinthepresenceoflight.
•Twophotosystemsareinvolvedineukaryotesbut
onlyonephotosystemisinvolvedinprokaryotes.
Photophosphorylationisoftwotypes:
•CyclicPhotophosphorylation
•Non-cyclicPhotophosphorylation

27. Reaction centre
•Thephotosyntheticunitconsistsof‘antenna’or‘light
harvesting’moleculesforgatheringlightphotonsand
areceptioncentrewhereenergyconversationtakes
place.
•Lightenergyharvestedbytheantennapigmentsis
transferredtothereactioncentre.
•Thepigmentsandproteins,whichconvertlightenergy
tochemicalenergyandbegintheprocessofelectron
transfer,areknownasreactioncenters.
•Inthechloroplastsofgreenplantsofthereaction
centrechlorophyllsareP700(PSI)andP680(PSII).

28. •Ingreenandpurplebacteriasome40ormore
bacteriochlorophyllmoleculesmakesupthe
photosyntheticunit.
•Thereactioncentrecomplexiscommonly
referredtoasP870,althoughthewavelength
mayvaryindifferentspecies.
•ThereactioncentregenerallycontainsBChl
a(2-5%ofthetotal)orrarely,BChlb.
•Inthepurplenonsulfurbacterium
Rhodospirillumrubrumtheprincipallight
harvestingpigmentisBChlaandthereaction
centremoleculeisP890.

29. •In Rhodopseudomonas spheroides the light
harvesting bacteriochlorophyll absorbs
maximally at 850nm and the reaction centre
BChl is P870. in the R-26 mutant of
R.sphaeroides, which lacks carotenoids, the
reaction centre pigment P870 constitutes 5%
of the total pigment.
•In green sulphur bacteria the principal light
harvesting pigment is BChl c (650nm) or BChl
d (660nm) in the green species and BChl e in
the brown species

30. Light reactions in purple bacteria
•The single photosystem of purple bacteria is
built around three membrane spanning
polypeptides known as the light(L),
medium(M), heavy(H) polypeptides.
•These polypeptides organize a reaction center
containing either bacteriochlorophyll a or b
and a short series of electron which is closely
resembles those of photosystem II of green
plants.

31. •InPurplebacteria,thebacteriochlorophyll
molecyulesatthereactioncenterundergoachange
inabsoptionatawavelengthof870or960nm,
dependingonthespecies,astheyundergocyclesof
oxidationandreductioninconnectionswiththe
excitationofelectrons.
•Thereactioncenterbacteriochlorophyllofthese
bacteriaareidentifiedaccordinglyasP870orP960.
•Thereactioncenterconsistsofapairofspecialized
bacteriochlorophyllbmolecules.
•Afterexcitationinthereactioncenter,electrons
flowtothebacteriopheophytinb,whichresembles
bacteriochlorophyllbwithoutacentralmagnesium
atom.

32. •From bacteriopheophytin electrons from through
two quinones Q
Aand Q
B each are associated with
an iron atom.
•At this point electrons pass from the
photosystem to carriers of the electron transport
system.
•Thus, the electron pathway within the R.viridis
photosystem is equivalent to the P680 
pheophytinQ
AQ
B pathway of eukaryotic
photosystem II (& cyanobacterial photosystem II)
Electrons may flow cyclically or noncylically around
the single photosystem of purple sulfur bacteria.

33. Cyclic electron transport in purple bacteria
•In cyclic electron transport (figure 1) , electrons released from
the photosystem enter a quinone pool.
•The electrons are later transferred from the quinone pool to a
b/c
1complex.
•The bacterial b/c
1complex contains a b-type and c-type
cytochrome linked with an iron sulfur protein and a group of
polypeptides.
•Electrons flow through the bacterial b/c
1complex pumps H+
gradient linked to electron transport as in eukaryotic systems.
•In most purple bacteria electrons flow from the b/c
1complex
to another c-type cytochrome c
2 , a peripheral membrane
protein .
•From cytochrome c
2electrons return at lower energy levels to
the reaction center of the single photosystem.
•After another energy boost through light absorption, that may
repeat the cyclic pathway

34. Quinone
pool
b/c
1
complexP870
or
P960
Cyt c
2
Photosystem
Figure 1. Cyclic electron transport in purple photosynthetic bacteria
H
+

35. Noncyclic electron transport in purple bacteria
•Innoncyclicflowinpurplebacteria(Figure2),electrons
derivedfromvarioussulfurornonsulfurdonorsdependingon
theirenergylevelmaybepassedbyacarrier,usuallya
cytochrometothephotosystemandthentothequinone
pool.
•Ineithercaseelectronsinthequinonepoolinitiallycontain
toolittleenergytodirectlyreduceNAD
+
.
•Someelectronsinthepool,howeverreceiveanadditional
energyboostfromthemembranepotentialbuiltupbycyclic
electrontransport
•TransportofH
+
iselectrogenicandcreatesavoltage
differenceacrossthemembraneaswellasanH
+
gradient.The
electronsboostedbythemembranevoltageattainenergy
levelshighenoughtoreduceNAD
+
.

36. •Noncyclicflowresultsintheonewaytransferofelectronsfrom
donorsubstancestoNAD
+
.TheNADHproducedbythereduction
providesasourceofelectronsforreductionsinthecellasinthe
darkreactionsfixingCo
2intocarbohydrates.Alternatively
electronscarriedbyNADHcanenterelectrontransportlinkedto
thesynthesisofATP.
•ThesameF
0F
1ATPaseactiveinoxidativephosphorylationuses
theH+gradientbuiltupbyphotosyntheticelectrontransportas
anenergysourceforATPsynthesisinpurplebacteria.
•ATPproducedbytheF
0F
1ATPasealongwithNADHformedby
noncyclicphotosynthesisprovidesenergyandreducingpower
forCo
2fixationinthedarkreactions.
Themoleculesandcomplexesofthelightreactionsincludinglight
harvestingphotosystemsandelectrontransportcarriers
associatedwithsaclikeinvaginationsoftheplasmamembranein
purplebacteria.

37. b/c1
complex
P870
or
P960
Quinone
pool
Cyt c
2
Sulfur or
nonsulfurdonors
Photosystem
Figure2.Noncyclicelectrontransportinpurplephotosyntheticbacteria.Electrondonors
fornoncyclicphotosynthesismaybesulfurcontainingcompoundssuchashydrogen
sulphideornonsulfurorganicsubstancessuchassuccinate.TheStarindicatesthe
excitedfromthephotosystem.
H
+
NAD

38. Light reactions in Green Bacteria
•The photosynthetic systems of green bacteria
appear to be two fairly well defined groups
with respect to photosystem and electron
transport system.
•One group is anaerobicand possesses a
photosystem resembling photosystem II of
eukaryotic plants.
•Second group is aerobic, with a photosystem
similar to eukaryotic photosystem

39. Anaerobic Green bacteria
•Withinthephotosystemofanaerobicgroupofbacteria,
bacteriochlorophyllamoleculesformingtheareactioncenter
changeinabsorbanceatawavelengthof870nmandareidentified
asP870.
•Followingexcitationinthereactioncenter,electronsflowthrough
agroupofinternalcarriersincludingbacteriopheophytinandiron
associatedquinones.
•Electrontransportaroundthephotosystemofanaerobicgreen
bacteriaappearstobeprimarilybycyclicpathway(figure3).
•Onlyafewdifferentcytochromesareintheelectrontransport
systemofthesebacteriainconnectionbetweenthiselectronflow
andATPsynthesisorreductionofNAD
+
.
•Itisdoubtfulthatelectronsexcitedbythephotolysishaveenough
energytoreduceNADdirectly.
•HoweverNADHmaystillbeformed

40. Figure3:photosyntheticelectronflowinanaerobicbacteriawhich
progressesprimarilyorexclusivelybyacyclicpathway
P870
P870*
Various
cytochromes
Photosystem

41. Aerobic Green bacteria
•The photosystem of aerobic green bacteria
contains specialized bacteriochlorophyll a
molecules absorbing light at 540nm. These
molecules identified as P840, pass electrons to a
primary acceptor and a chain of Fe/S centers
rather than quinones .
•The more complex electron transport systems of
these bacteria may include ferredoxin, a b/c
1
complex and the ferredoxin –NAD oxidoreductase
complex.
Electron carriers are arranged in aerobic bacteria may
be either Cyclicor noncyclicelectron transport

42. Aerobic green bacteria-Cyclicelectron transport
•Incyclictransport(figure4),electronsflowtoferredoxinafter
excitationandmovementthroughtheinternalcarriersofthe
photosystem.
•Thedirectreductionofferredoxinatthefirstcarrierreflectsthe
factthatelectronsexcitedtosignificantlyhigherenergylevelsby
thephotosysteminthesebacteria.
•ElectronsthenflowtoNAD+viatheferredoxin-NADoxidoreductase
complexwhichmaycontainFADasaninternalcarrierasinthe
equivalentcomplexofhigherplants.
•FromNADHelectronsaretransferredviaonormorecytochromes
totheb/c
1complexandthenbacktoP840moleculesatthe
reactioncenterofthephotosystem.
•Transferfromtheb/c
1complextothephotosystemmaybedirect
ormayoccurviaadditionalcytochromes;thecytochromesonthis
sideoftheb/c
1complex,likethosedeliveringelectronstothe
complexfromNADH.
•Becausetheb/c
1complexispresentintheloop,H
+
ispumped
acrossthemembranehousingsystemeachtimeelectronscycle
aroundthephotosystem.

43. Various
cytochromes
b/c
1
complex
P840
Cyt c
2
Photosystem
Figure 4. Cyclic electron transport in aerobic green bacteria
H
+
P840* NAD
FD-NAD
reductase
FD

44. Non-cyclic electron flow in aerobic green bacteria
•Innon-cyclicelectronflowinaerobicgreenbacteria(fig5)useselectrons
removedfrominorganicsulfurcompounds.Thisflowoccursthroughcytochromes
thatvarywidelyindifferentspecies,electronpassfromthedonorsthroughone
ormoreofthesecytochromestoreachtheb/c
1complex.
•Fromthispointelectronsenterthephotosystemand,afterexcitationsare
deliveredathighenergylevelstoferredoxin.
•Theelectronsmayremainwithferredoxinwithferredoxinwhichservesdirectly
asanelectrondonorfordarkreactionstogreenbacteria.
•AlternativelyelectronsmaybedeliveredfromferredoxintoNAD.TheNADH
producedmayprovideelectronsforthedarkreactionsormayenterthe
respiratoryelectrontransportsystemleadingtooxygenasthefinalelectron
acceptor.
•AlternativelytheelectronsmayreenterthephotosyntheticpathwayfromNADH
andtravelcyclicallythroughoneormoreloopsaroundphotosystem.
•ThesameF
0F
1ATPaseactiveinoxidativephosphorylationusestheH+gradient
establishedbyphotosyntheticelectrontransportastheenergysourceforATP
synthesis.
•Allcomponentsofthelightreactionsareassociatedwiththeplasmamembrane
ingreenbacteria.

45. Various
cytochromes
b/c
1
complex
P840
Cyt
c
2
Photosystem
Figure 5.Noncyclic electron transport in aerobic green bacteria
H
+
P840*
FD-NAD
reductase
FD
To dark reactions
NAD

46. Dark phase of photosynthesis : Co2 utilization
In bacteria the reduction of Co2 during
photosynthesis takes place through
two mechanisms:
1. The reductive pentose pathway or Calvin cycle
2. pyruvate synthetase reaction or reductive
carboxylic acid cycle.

47. Calvin Cycle (Dark reactions)
Calvin cycle takes place in three steps:
•Carbon Fixation
•Reduction
•Regeneration

48. Calvin cycle-Carbon Fixation
•Incarbonfixation,aCO
2moleculefromthe
atmospherecombineswithafive-carbon
acceptormoleculecalledribulose-1,5-
bisphosphate(RuBP).
•Theresultingsix-carboncompoundisthen
splitintotwomoleculesofthethree-carbon
compound,3-phosphoglycericacid(3-PGA).
•ThisreactioniscatalyzedbytheenzymeRuBP
carboxylase/oxygenase,alsoknownas
RuBisCO.Duetothekeyroleitplaysin
photosynthesis,RuBisCoisprobablythemost
abundantenzymeonEarth.

49. Calvin cycle -Reduction
•InthesecondstageoftheCalvincycle,the3-PGA
moleculescreatedthroughcarbonfixationareconverted
intomoleculesofasimplesugar–glyceraldehyde-3
phosphate(G3P).
•ThisstageusesenergyfromATPandNADPHcreatedin
thelight-dependentreactionsofphotosynthesis.
•Inthisway,theCalvincyclebecomesthewayinwhich
plantsconvertenergyfromsunlightintolong-term
storagemolecules,suchassugars.
•TheenergyfromtheATPandNADPHistransferredto
thesugars.
•Thisstepiscalled“reduction”becauseNADPHdonates
electronstothe3-phosphoglycericacidmoleculesto
createglyceraldehyde-3phosphate.
•Inchemistry,theprocessofdonatingelectronsiscalled
“reduction,”whiletheprocessoftakingelectronsis
called“oxidation.”

50. Calvin cycle -Regeneration
•Someglyceraldehyde-3phosphatemoleculesgoto
makeglucose,whileothersmustberecycledto
regeneratethefive-carbonRuBPcompoundthatis
usedtoacceptnewcarbonmolecules.
•TheregenerationprocessrequiresATP.Itisa
complexprocessinvolvingmanysteps.
•Becauseittakessixcarbonmoleculestomakea
glucose,thiscyclemustberepeatedsixtimesto
makeasinglemoleculeofglucose.
•Toaccomplishthisequation,fiveoutofsix
glyceraldehyde-3phosphatemoleculesthatare
createdthroughtheCalvincycleareregeneratedto
formRuBPmolecules.
•Thesixthexitsthecycletobecomeonehalfofa
glucosemolecule.

51. Figure 6.Calvin cycle

52. PYRUVATE SYNTHETASE REACTION(REDUCTIVE CARBOXYLIC ACID CYCLE)
•In green bacteriumChlorobium
thiosulfatophilumEvansetal(1966)described
thepyruvatesynthetasepathwayforCO
2
fixation.
•CO
2isusedtoformpyruvatebymeansofthe
pyruvatesynthetasereaction.
•Theultimatereductantishydrogensulphide.
•ThelightdependentoxidationofH2S
providesthereducingpowerforthereduction
offerredoxin(fd).
•AcetylCoAtheacceptsCO
2,andisreducedby
ferredoxintoyieldpyruvate.

53. •Formation of pyruvate by pyruvate synthetase is dependent
on reduced ferredoxin
Acetyl CoA + CO
2+ferredoxin (reduced) ------Pyruvate +CoA + ferredoxin
(oxidized)
•Conversion of pyruvate into oxaloacetate
Pyruvate + ATP+ CO
2 ----------------Oxaloacetate +ADP+ Pi
•Carboxylation of succinyl CoA to yield α-ketoglutarate(involving
reduced ferredoxin)
Succinyl CoA + CO
2+ferredoxin (reduced) ----α-ketoglutarate +CoA +
ferredoxin (oxidized)
•α-ketoglutarate is converted into citrate through oxalosuccinate.
Citrate then splits into oxaloacetate and acetate
α-ketoglutarate -----Oxalosuccinate ------------Citrate----Oxaloaceticacid
+ Acetate

54. Pyruvate
Acetyl CoACoA
Citrate
Oxaloacetate
Malate
α-Ketoglutarate
Succinate
Fumarate
Isocitrate
Co
2
Co
2
Fig 7: Green bacteria can fix Co2 , by
reversing reactions of Pyruvate oxidation and
the Citric acid cycle

55. Thenetresultofeachcycleisthat4moleculesof
CO
2arefixed(reductivefixation)andone
equivalentofoxalosuccinateisproduced.Three
moleculesofATParerequired
(1)foractivationofacetate,
(2)forCarboxylationofpyruvateand
(3)foractivationofsuccinate,thereductive
carboxylicacidcycleappearstobeparticularly
suitetoprovidethecarbonskeletonsforthe
mainproductsofbacterialphotosynthesis,
whicharemainlyaminoacids

56. Photosynthesis in Halobacteria
•HalobacteriaPossesanincompleteand
primitivephotosyntheticmechanismthat
differsfrompurpleandgreenbacteria.
•Halobacterialiveinextremeenvironments
usuallyathighlevelsofsalinityandtoohigh
temperaturetobetoleratedbyotherforms,
havenolightharvestingantennas,no
photosystemsandnolightdrivenelectron
transportsystem

57. •PhotosyntheticmembranesofHalobacterium
containalightabsorbingmoleculeknownas
bacteriorhodopsin(fig.8),consistingofa
polypeptidechainwiththelightabsorbing
unit.
•Thelightabsorbingunitofbacteriorhodopsin
isretinal,amoleculealmostidenticaltothe
visualpigmentofanimals.
•Bacteriorhodopsinrespondstolightby
pumpingH+acrossthemembranecontaining
thecomplex.

58. •Duringapumpingcycletheretinalunit
alternativelypicksupandreleasesahydrogen.
•Thispickupandreleasemaybecombined
withconformationalchangesintheprotein
componentthatexposetheretinalunittothe
cytoplasmduringH
+
bindingandtothecell
exteriorduringH
+
release.
•TheH
+
gradientestablishedthelightinduced
pumpingthroughthebacteriorhodopsin
moleculedrivesATPsynthesisbyF
oF
1ATPase.

59. Fig 8: Halobacterium
photosynthetic system

60. Heliobacteria:
•The reaction centre P798 absorbs the light energy
and photosynthetic electron flow occurs via
modified form of chlorophyll a called hydroxy-
chlorophyll a -Fe-S-Q-bc
1Cyt –Cyt C
553to reaction
centre which is slightly different from green sulphur
bacteria.
•In both the bacteria NADH production is light-
mediated. The primary electron acceptor in such
bacteria has reduction potential of -0.5 V. If it is
reduced, it is able to reduce NAD
+
directly, hence
reverse electron flow does not require for reducing
NAD
+

61. Applications for Photosynthetic Bacteria
•Photosynthetic bacteria are currently being used
in various applications which include water
purification, bio-fertilizers, animal feed and
bioremediation of chemicals among many
others.
•They are used in the treatment of polluted water
since they can grow and utilize toxic substances
such as H
2S or H
2S
20
3.
•Researchers at Harvard’s Wyss Institute have
engineered photosynthetic bacteria to produce
simple sugars and lactic acid.

62. References
•General Microbiology -C.B.Powar
•Molecular and Cellular Biology -StephenL.Wolfe
•http://www.biologydiscussion.com/bacteria/bacterial-
photosynthesis/electron-transport-system-in-bacterial-
photosynthesis-microbiology/65560
•https://www.nature.com/articles/s41598-019-50026-1#Fig1