photosynthesis in bacteria - … · web viewthere are three groups of photosynthetic bacteria:...

PHOTOSYNTHESIS IN BACTERIA1

PHOTOSYNTHESIS IN BACTERIA

There are three groups of photosynthetic bacteria: the purple bacteria, the green bacteria, and the cyana bacteria. The cyanobacteria differ from the green and purple photosynthetic bacteria as they are able to carry out oxygenic photosynthesis. They use water as an electron donor and generate oxygen during photosynthesis. Purple and green bacteria use anoxygenic photosynthesis. They employ reduced molecules such as hydrogen sulfide, sulfur, hydrogen, and organic matter as their electron source for the generation of NADH and NADPH. Consequently, purple and green bacteria do not produce oxygen but many form sulfur granules. Purple sulfur bacteria accumulate granules within their cells, whereas green sulfur bacteria deposit the sulfur granules outside their cells. The purple nonsulfur bacteria use organic molecules as an electron source. There also are differences in photosynthetic pigments, the organization of photosynthetic membranes, nutritional requirements, and oxygen relationships.The purple and green bacteria differ from the cyanobacteria in having bacteriochlorophylls rather than chlorophyll a. Normally green and purple bacteria are anaerobic and use H2S and other reduced electron donors during photosynthesis. Because these bacteria grow best in deeper anaerobic zones of aquatic habitats, they cannot effectively use parts of the visible spectrum normally employed by photosynthetic organisms..Cyanobacteria and algae in lakes and ponds absorbs a large amount of blue and red light. The bacteriochlorophyll pigments of purple and green bacteria absorb longer wavelength, far-red not used by other photosynthesizers .

The second edition of Bergey’s Manual places photosynthetic bacteria into six major groups. The phylum Chloroflexi contains the green nonsulfur bacteria, and the phylum Chlorobi, the green sulfur bacteria. The cyanobacteria are placed in their own phylum, Cyanobacteria. Purple bacteria are divided between three groups.Purple sulfur bacteria are placed in the _-proteobacteria, families Chromatiaceae and Ectothiorhodospiraceae. The purple nonsulfur bacteria are distributed between the -proteobacteria (five different families) and one family of the -proteobacteria.

Photosynthetic Bacteria -The prokaryotic phototrophs include the purple bacteria (suborder Rhodospirillineae) the green bacteria (suborder Chlorobiineae), the blue, green bacteria (Cyanobacteria) and the Prochlorophyta.

The purple bacteria include the families Rhodospirillaceae (purple non sulphur bacteria) and the Chromatiacae (purple sulphur bacteria). The green bacteria include the families Chlorobiaceae (non-motile green and brown sulphur bacteria) and the Chloroflexaceae (filamentous gliding green bacteria). The Cyanobacteria and the Prochloraphyta show certain characteristics of the eukaryotic photosynthesis system.

Rhodospirillaceae (Formerly Athiorhodaceae ) Purple Non Sulphur Bacteria - The purple non-sulphur bacteria do not form globules of elemental sulphur inside the cells. There are some purple bacteria which oxidize sulphide to extracellular sulphur and further to sulphate. Pfennig-(1977) has placed such species in the non-sulphur bacteria. The principal photosynthetic pigments are bacteriochlorophylls a and b and carotenoids of groups I, 2 arid 4. There is one photocentre.The sources of hydrogen are mainly organic molecules. In some cases hydrogen sulphide serves a the source of hydrogens, but not sulphur.

The purple non-sulphur bacteria differ from the Chromatiaceae the Chlorobiaceae in that they can grow either anaerobically, in light (photosynthesis) or aerobically in the dark (oxidative respiration). This switch from photophosphorylation to oxidative phosphorylation requires the presence of both electron transport systems.

Light is absorbed by the bacteriochlorophyll molecule P800, and transferred to the reaction centre that is P890 in


Rhodospirillum rubrum or P870 in Rhodopseudomonas sphaeroides. The ratio between P800 and the reaction centre molecule is 2: 1.

Chromatiaceae (Formerly Thiorhodaceae ) Purple Sulphur Bacteria - The purple sulphur bacteria form globules of elemental sulphur from sulphide inside cells. There is one photocentre. Hydrogen sulphide, sulphur, thiosulpbate or molecular hydrogen are sources of hydrogens.Organic molecules are utilized in some cases. The principal photosynthesis pigments are bacteriochlorophylls a and b which show absorbtion bands at 375 and 590 nm in the visible region and 800, 850 and 890 in the infrared region for their chlorophylls. A large amount of carotenoids of groups 1, 3 and 4 or tetrahydrospirilloxanthin are present. The absorption peaks of the carotenoids are between 400 and 600 nm.The primary photochemical reaction centre from the bacteriochlorophyll in Chromatium chromatophores is P890 (equivalent to P870 in photosynthetic strains). P800, is the light-harvesting cytochrome, c553, related to or identical with c555 of Chlorobium..

Chlorobiaceae (Formerly Chlorobacteriaceae) Non Motile Green and Brown Sulphur Bacteria - The Chlorobiaceae are strictly anaerobic. There is one photocentre. Hydrogen sulphide, sulphur, thiosulphate or molecular hydrogen are the sources of reducing hydrogen, and sulphate is given off as a byproduct, The Chlorobiaceae are characterized by H2S-, CO-2 and light depending metabolism, which takes place under strictly anaerobic conditions. CO2 assimilation does not apparently take place vi the reductive pentose phosphate cycle.

Elemental sulphur is formed as an intermediate product of oxidation, and is excreted into the medium to form sulphur globules. Sulphur is then more or less completely oxidized to sulphate under conditions of limiting sulphide

The Chlorobiaceae characterized by the presence of chlorobium chlorophylls BChl, c or d which form the major components. In addition small amounts of B.Chl a are present. The reaction centre pigment is BChl a Carotenoids of group 5 form accessory pigments. In 1968 Pfenning identified two brown species, Chlorobium phaeobacteroides and Chlorobium, phaeovibrioides, and since then several brown species have been isolated.

In almost all genera of the green sulphur bacteria there are green and brown coloured species that resemble each other morphologically and physiologically, but differ in their photosynthetic pigments. Separate species have been established for the green and brown counterparts of each genus.

Photosynthetic Pigments in Green and Brown Species of Chlorobiaceae -

Colour Light harvesting Pigment Reaction centre pigment Carotenoids

Green spp BChl c or d BChlap ChlorobacteneOH-chlorobactene

Brown spp BChl c BChlap Isorenierateneβ-isorenieratene

Chloroflexaceae Filamentous Gliding Green Bacteria - Pierson and Castenholz (1974) discovered Chloroflexus aurantiacus, a new genus and species from hot springs in USA, Guatemala, Iceland and New Zealand. Chloroflexus is primarily organotrophic. It can grow chemoorganotrophically under aerobic conditions in the dark or light.


The major bacteriochlorophyll is BChl c. This bacteriochlorophyll differs from BChl c of green sulphur bacteria in being esterified at the C-7'", side chain with steorylalcohol (Chl C5) instead of farnesol. The major carotenoids are β and γ carotene and their derivatives, There is one photocentre.

The sources of hydrogens are hydrogen sulphide and, organic molecules. The major oxidation product is elemental sulphur, and not sulphate. Dubinina and Gorlenko (1975) established a new genus, Chloronema, with two species, C . giganteum and C . spiroideum. These are planktonic forms in fresh water lakes, and contain gas vacuoles. 'Chlorobium vesicles are present' and contain BChl d. Like all green filamentous bacteria, the species are photoorganotrophic and facultatively anaerobic.

Cyanobacteria (Formerly Blue Green Algae) Blue Green Bacteria - The Cyanobacteria form the largest group of photosynthetic prokaryotes. They are widely distributed and show great diversity. Their photosynthetic apparatus is structurally and functionally similar to the eukaryotic chloroplast. The light harvesting pigments of the Cyanobacteria are chlorophyll a and phycobiliproteins.

The Cyanobacteria thus differ from the green and purple bacteria which contain bacteriochlorophylls as their photosynthetic pigments. They also differ from other photosynthetic bacteria in having two photosystems instead of one and in their ability to perform oxygenic photosynthesis (photosynthesis with evolution of oxygen).

The source of hydrogen is water. Respiration is aerobic. The Cyanobacteria are gram negative. Many are motile. Motility is of the gliding type as in motile green bacteria. The photosynthetic apparatus is located in a series of thylakoids or membranous sacs which contain chlorophyll a and carotenoids, the 'photochemical reaction' centres and the photosynthetic electron transfer chain.

The thylakoid membranes are functionally distinct from the cell membrane: the hitter never has phycobilisome attached to it. Components of the Photosynthetic electron transport system (e.g. ferredoxin, plastocyanin and cytochrome f) have been located in the thylakoid membrane. The only chlorophyll found in the Cyanobacteria is chlorophyll a. This is an important homology with chloroplasts. There are two photosystems I and II. Photosystem I almost exclusively receives the light energy harvested by chlorophyll, and only cyclic photophosphorylation takes place, β-carotene is always present, and is usually accompanied by either or both oxycarotenoids, zeanthin and echinone.In addition most Cyanobacteria synthesize group specific pigments, notably carotenoid glycosides. Phycobiliproteins are the major light-harvesting pigments of the Cyanobacteria. (These are also found in the red algae). Light energy absorbed by the PBPs is transfered mainly to photosystem II. Each pap is composed of two different polypeptide chains α and β, each carrying at least one bilin chromatophore attached io it covalently. The α and β monomers form the native protein oligomers, (αβ) n.Cyanobacteria contain at least, three different types 6f PBPs phycocyanin (PC) (A max 620 nm), allophycocyanin (AP) (λ max 650 nm) and allophycocyanin B (APB) (A max 670 nmnm).

None of these glycolipids occur in purple bacteria. Green bacteria contain only MGDG, which is localized in the chlorobium vesicle, the light harvesting organelle.

Prochlorophyta - The prochlophyta is a newly established group. Like the Cyanobacteria the group has oxygenic photosynthesis, with two linked photosystems. The electron donor is water, and oxygen is the end product of oxidation. The photosynthetic pigments are chlorophylls a and b and carotenoids.


CHARCTERISTICS OF THE MAJOR GROUS OF PHOTOSYNTHETIC BACTERIA: Source: Prescott Microbiology Group Chl/BChl Other

Photosynthesis Pigments

Type of Respiration Source of hydrogens

Rhodvspirillaceae BChl a or b Carotenoids of Aerobic: oxygen Usually organic


(Athlorbodaceae)Purple non sulphur

bacteria

groups 1,2 & 4

respiration.1 photocentre

compounds H2S In some cases, but not

sulphur

Chromatiaceae (Thiorbodaceae)

Purple sulphur bacteria

BChl a or b Carotenoids of groups1,3 &4

Anaerobic1 photocentre

H2S S, thiosulphate & molecular hydrogen

Chlorobiaceae (Chlorobacteriaceae)

Green sulphur bacteria

Chlorobium chlorophylls(BChl c or d--

majorBChl a-- major

)

Carotenoids of group 5

Strictly anaerobic H2S, CO2 & light dependent

metabolism

H2S, S, thiosulphate or molecular hydrogen.

Sulphate as byproduct.

ChloroflexaceaeGliding filamentous

green bacteria

BChl c-- major BChl a-- major

β + γ caotenes&their

dervatives

Aerobic: oxygen respiration

H2S & organic molecules

CyanobacteriaBlue green bacteria

Chl a CarotenoidsGreen SpeciesChlorobactene

OH-ChlorobacteneBrown speciesIsorenieratene

β-isorenieratene

Aerobic: oxygen respiration.

Oxygenic photosynthesis

Water.Emit oxygen as by

product

Prochlorophyta Chl a& b Aerobic: oxygen respiration.


Water

Higher plants Chl a and bChl c in brown

algae

Carotenoids Aerobic: oxygen respiration.


Water

DISTRIBUTION OF PHOTOSYNTHETIC BACTERIA:

Photosynthetic bacteria are present in anaerobic zones of many aquatic locations in shallow ponds, slowly flowing waters, lakes and estuaries.


Types of Bacterial Photosynthesis - Bacteria which use the radiant energy of light in nutrition arc called phototrophs, while those utilizing chemical bond energy are called chemotrophs.

Phototrophs normally utilize CO2 as the source of carbon. The energy required for the reduction of CO2 during its assimilation may be obtained from inorganic or organic substrates.

Metabolism in which inorganic substances like H2S or thiosulphate are the sources of reducing power is called photolithotrophy. When the oxidation of organic compounds like malate and succinate yields reducing power, the process is called photoorganotrophy.The general equation for photosynthesis is : 2H2A + CO2-->C(H2O) + H2O + 2A

In green plants growing aerobically 10 light H2A is H2O. In photosynthetic bacteria that are obligatory anaerobes H2A is hydrogen sulphide (H.S) or organic molecules. In the Chromatiaceae and Chlorobiaceae the reducing substrate for replacing electrons are hydrogen sulphide or, hlosulphate.

In the Rhodospirillaceae the substrates are organic molecules like malate or succinate. Many members of the Rhodospirillaceae can grow in the dark. They apparently obtain their energy by electron, transport dependent phosphorylation linked to oxidation of organic molecules. An outline of the different types of reactions in bacterial photosynthesis is given below:

(i) Purple sulphur bacteria, e.g. Chromatium, utilize H2S as the reducing substrate instead of H2O in photosynthesis. Sulphur(S) is produced instead of oxygen.CO2 + 2 H2S -->C(H2O) + 2 S + H2O(ii) Purple sulphur bacteria can also use thiosulphate as a reductant. 2 CO2 + Na2S2O3 + 5 H2O -->2 C(H2O) + 2 H2O -I- 2 NaHSO4

(iii) In non-sulphur bacteria like Rhodospirillum rubrum, the electron donor & are organic compounds like ethanol, isopropanol or succinate.CO2 + 3C2H5OH-->C(H2O) + CH3CHO + H2O

It will be seen that in green plants there is evolution of oxygen during photosynthesis, while in bacteria no oxygen is evolved.The cyanobacteria differ from purple and green bacteria in the nature of their photosynthetic pigment system and in their ability for oxygenic photosynthesis

The purple and green bacteria carry out anoxygenic photosynthesis, i.e. there is no evolution of oxygen. There is only one photosystem involved in photosynthesis. The electron donors are sulphur, reduced sulphur compounds, molecular hydrogen or simple organic compounds.

These are substances with lower redox potentials than water. It should be noted that even in cyanobacteria there may be anoxygenic photosynthesis with only one photosystem when H2S is the electron donor.

Photosynthesis Pigments - There are three main classes of photosynthesis pigments, chlorophylls (Chl) (including bacteriochlorophylls, BChl), carotenoids and phycobilins (phycobiliproteins: PBPs).


All species capable of carrying out photosynthesis contain one or more types of chlorophyll or bacteriochlorophyll and carotenoids.

Phycobilins are found only in the red algae (Rhodophyta: eukaryotes) and the blue green bacteria (Cyanobacteria: prokaryotes). All phototrophic bacteria, except the Cyanobacteria, contain bacteriochlorophylls and carotenoids.

Chlorophyll - Green algae and higher plants contain two types of chlorophylls, Chl a and Chl b. Both are soluble in organic solvents. Chl a is present in all photosynthetic organisms in which there is evolution of oxygen during photosynthesis.

The following types of Chl a are known:

Chl a 660, Chl a 670, Chi a 680, Chl a 685, Chl a 690 and Chl a 700-720. The short wavelength forms of Chi a are mainly present in photosystem II, while the long wavelength forms are mainly present in photosystem I.

Chi b is present in all green algae and higher plants. Most of the Chl b is present in PSII. There are two forms of Chl b, Chl b 640 and Chl b 650.

Chl c. Brown algae contain a compound called Chl c which is related to chlorophyll. Chi d has been reported in the red algae.

Structure of Chlorophyll - The emperical formula of chlorophyll a is C55H72O5N4Mg. Chlorophyll a is a blue-green microcrystalline solid, consisting of a 'head' and a 'tail'.

The head consists of a porphyrin ring or tetrapyrrole nucleus, from which extends a tail made up of a 20-carbon grouping called the phytol.

The porphyrins are complex carbon nitrogen molecules that usually surround a metal. In chlorophyll the porphyrin, surrounds a magnesium ion while in haemoglobin it surrounds an iron, ion.

The cytochromes of the electron transport system also have porphyrin rings. The basic unit of the porphyrin ring is the porphobilinogen molecule. Four such pyrroles make up units of the tetrapyrrole structure.

Phytol is a long straight chain alcohol containing a double bond. It may be regarded as a hydrogenated carotene (vitamin A). Its formula is C20H39.


Chlorophyll b has the emperical formula C55H70O6N4Mg, It is a green black microcrystalline solid. It differs from chlorophyll a in having an aldehyde (CHO) group attached to carbon atom 3, instead of a methyl (CHs) group.

Carotenoids -The carotenoids are found in almost all photosynthetic organisms. They are yellow and orange pigments which are soluble in organic solvents.

There are two types of carotenoids, carotenes and carotenols. Carotenes, e.g. β-carotene, are hydrocarbons. Most of the carotenes are present in photo system I.

Carotenols (xanthophylls) are alcohols. Fucoxanthol is present in diatoms and other brown algae. Most, of the xanthophylls are present in photosystem II.

Carotenoids Groups of Phototrophic Bacteria -

Group Name Major Components

1 Normal Spirilloxanthin Series LycopeneRhodopin

2 Alternative Spirilloxanthin Series and Ketocarotenoids SpheroideneHydroxyspheroidene

3 Okenone Series Okenone

4 Rhodopinal Series LycopenalLycopenol

5 Chlorobactene Series ChlorobacteneHydroxychlorobactene

Phycobilins - Phycobilins are water soluble open chain tetrapyrroles which are present in red algae and blue green bacteria


(Cyanobacteria).

There are two kinds of phycobilins, phycocyanins and phycoerythrins.Phycocyanins are predominate in the blue green bacteria, while phycoerythrins predominate in the red algae. Phycobilins are mainly present in PSII, but are also present in PS I.

LOCATION OF PHOTSYNTHETIC PIGMENTS: The photo synthetic pigments in purple bacteria are bound to the intra cytoplasmic membranes (thylakoides) of the chloroplast.. These originates as vesicular, tubular invaginations of the cytoplasmic membrane into the interior or remain connected. The membranes of different species occur in different forms and can be present as tubular vesicles and as concentrically or cylindrically arranged lamellar strands occupying most of the cell interior.In Chlorobinea , the photosynthetic pigments are bound to two separate cell structure The antenna pigments are localized in chlorosomes.

In Green sulphur bacteria , the photosynthetic pigments are located in ellipsoidal vesicles called chlorosomes or chlorobium vesicles, which are attached to the plasma membrane but are not continuous with it. The chlorosome membrane is not a normal lipid bilayer or unit membrane .Chlorosomes contain accessory bacteriochlorophyll pigments, but the reaction center bacteriochlorophyll is located in the plasma membrane and must be able to obtain energy from chlorosome pigments.

In Cyanobacteria Photosynthetic pigments and electron transport chain components are located in thylakoid membranes lined with particles called phycobilisomes. These contain phycobilin pigments, particularly phycocyanin, and transfer energy to photosystem II.

PHOTOSYNTHETIC APPARATUS IN PHOTOSYNTHETIC BACTERIA:

The three major groups of phototrophic bacteria produce distinctive structures in which photochemical apparatus is located.

In Purple bacteria these are extensive invaginations of cytoplasmic membrane which may be tubular, lamellar or vesicular in form. All components of the photochemical apparatus Iantenna chlorophyll, reaction centre and electron transport chain are located in these membrane structures.

The cytoplasmic membrane of Green bacteria is characteristically not invaginated and it houses only the reaction centre(containing bacteriochlorophyll- a and the electron transport system). The antenna chlorophylls are tightly joined to the cell membrane through a base plate containing a special class of bacteriochlorophyll - a that is not part of the antenna , but rather functions in energy transfer from antenna to reaction centre. Chlorosomes are

surrounded by monolayered membrane about 4nm thick composed of lipid and protein.

Cyanobacteria contain specialized unit membrane bound sacs termed thylakoids that are structurally distinct from the cell membrane. They house the reaction centres and electron transport chain as well as antenna chlorophyll-a; major antenna pigments .


Phycobilin proteins are aggregated into hemispherecal structures termed Phycobilisomes that are attached to the cytoplasmic structural surface of thylakoid membranes. Within phycobilisomes, the various individual phycobilin proteins are arranged in a precise order with hallophycocyanin closet to the reaction centre and phycoerythrin farthest from it. This arrangement ensures that energy transfer within phycobilisome towards the reaction centre because the excited state of phycoerthrin which absorbs short wavelength of light is more energetic than the excited state of phycocyanin which absorbs light of long wavelength.

STRUCTURE OF PHOTOSYNTHETIC APPARATUS:

The photosynthetic apparatus of all organisms consists of three essential components.:

1.An antenna of light harvesting pigments.

2. A photosynthetic reaction centre and

3. An electron transport chain.

1.AN ANTENNA OF LIGHT HARVESTING PIGMENTS: This includes chlorophylls, carotenoids and phycobilins. Their function is to absorb light energy and to transfer it to the reaction centre. . Chlorophylls and accessory pigments are assembled in highly organized arrays called antennas, whose purpose is to create a large surface area to trap as many photons as possible. An antenna has about 300 chlorophyll molecules. Light energy is captured in an antenna and transferred from chlorophyll to chlorophyll until it reaches a special reaction-center chlorophyll directly involved in photosynthetic electron transport .The particular set of light harvesting pigments that comprise an antenna system are group specific and their cumulative light

absorptive properties determine the range of wavelength of light over which photosynthesis occurs and therefore the habitat of the phototrophs.As a consequence of these variations in the composition of antenna system, phototrophs are collectively capable of utilizing radiant energy that falls in the wavelengths of visible and near infra-red light. At least, seven kinds of e chrlophylls occur in various groups absorb light in two regions of spectrum-violet around 400nm, red and infra-red around 600-1,100nm. The region of peak absorption within the larger wavelength varies with the particular chlorophylls of the species and the proteins with which they are associated.

2. Photosynthetic reaction centre: The photochemical reaction centre contains the site where a molecule of chlorophyll becomes photo activated and oxidized by donating an electron to a carrier molecule. Chlorophyll molecules in the reaction centre differ from those in antenna in two important respects:


i) They are associated with certain proteins that interact with them in a manner that decreases energy required to raise them to their activated state.

ii) they are in close proximity with carrier molecules that can accept an electron from them when they are activated.

CI-----------> CI-----------> C+I- -----------> C+I

e- -----------> A -----------> A-

C-Reaction centre chlorophyllI-Immediate electron acceptorA-First stable reduced electron carrier.3. An electron transport chain/Photosynthetic Electron Transport System :The electron transport system of photosynthetic bacteria differs from that of aerobic chemoorganotrophic bacteria.

Cytochrome a and other types of cytochrome oxidase are not present in the photosynthetic electron, transport system, because photosynthesis takes place under anaerobic conditions. Hence there is no need of a cytochrome which interacts with

molecular oxygen. The electron transport system consists of an intermediate electron acceptor (I). a primary electron acceptor (X), a secondary acceptor (Y), generally believed to be ubiquinone (UQ), and b- and c-type cytochromes.The electron transport carriers are asymmetrically located in the membrane. This is necessary for setting up the hydrogen ion gradient. Immunological studies suggest that the reaction centre spans the membrane of the chromotophore. It is probably located beneath the ATPase complex.

THE ELEMENTARY PROCES OF PHOTOSYNTHESIS:Photosynthesis is defined as the conversion of light energy to biochemically utilizable energy and reducing power (NADH and NADPH) in the cells of phototrophic organisms for the synthesis of cell material.Photosynthesis in bacteria is basically quite different from that of higher plants.


The purple bacteria and the green bacteria which are collected in the order Rhodospiralles can be regarded as relics from the earliest period in the evolution of photosynthesis. They are :

i) Not able to utilize water as the hydrogen donor,ii) Able to utilize hydrogen donors such as hydrogen sulphide, molecular hydrogen or organic compounds.iii) Not able to evolve oxygen during photosynthesis,iv) Typical aquatic bacteria, widely distributed in fresh water and sea water,v) Unicellular with red,orange or green pigmentation depending on their content of bacteriochlorophylls and

carotenoids.The Cyanobacteria utilize water as hydrogen donor and evolve oxygen in light reaction. Their pigment system includes chlorophylla a ,carotenoids and phycobilins.

The general equation for photosynthesis is : 2H2A + CO2-->C(H2O) + H2O + 2A

In green plants growing aerobically in light H2A is H2O.The different types of reactions in bacterial photosynthesis is given below:

(i) Purple sulphur bacteria, e.g. Chromatium, utilize H2S as the reducing substrate instead of H2O in photosynthesis. Sulphur(S) is produced instead of oxygen.CO2 + 2 H2S -->C(H2O) + 2 S + H2O

(ii) Purple sulphur bacteria can also use thiosulphate as a reductant. 2 CO2 + Na2S2O3 + 5 H2O -->2 C(H2O) + 2 H2O -I- 2 NaHSO4

(iii) In non-sulphur bacteria like Rhodospirillum rubrum, the electron donor & are organic compounds like ethanol, isopropanol or succinate.CO2 + 3C2H5OH-->C(H2O) + CH3CHO + H2O

In green plants there is evolution of oxygen during photosynthesis, while in bacteria no oxygen is evolved.

The cyanobacteria differ from purple and green bacteria in the nature of their photosynthetic pigment system and in their ability for oxygenic photosynthesis.

The purple and green bacteria carry out anoxygenic photosynthesis.

Oxygenic Photosynthesis:

i) In the Cyanobacteria and Prochlorophyta photosynthesis is oxygenic, i.e. there is evolution of oxygen.

ii) There are two linked photosystems involved in photosynthesis.

iii) The electron donor is H2O, and oxygen is the ultimate product of oxidation. They are, therefore, aerobic phototrophs.

iv) The photosynthetic apparatus of the Cyanobacteria is remarkably similar in structure and function to the eukaryote chloroplast.


v) Their light harvesting pigments, Chl a and phycobiliproteins, are homologous to those of the chloroplast of Rhodophyta (red algae).

Anoxygenic Photosynthesis:

i) The purple and green bacteria carry out anoxygenic photosynthesis

ii) There is no evolution of oxygen.

iii) There is only one photosystem involved in photosynthesis.

iv) The electron donors are sulphur, reduced sulphur compounds, molecular hydrogen or simple organic compounds.

v) Even in cyanobacteria there may be anoxygenic photosynthesis with only one photosystem when H2S is the electron donor.

OUTLINES OF PHOTOSYNTHETIC REACTIONS: There are two reactions involved in photosynthesis. The first reaction requires light and is called light reaction or HILL reaction. Light reaction is photochemical reactionThe second reaction does not require light and is called dark reaction. It is thermochemical reaction. The unit of photosynthesis consists of two types of centres. Photosystem-I and Photosystem-II.These are excited by two different wavelengths of light. They are linked by redox catalyst. The light reaction involves two processes. i) Photophosphorylation and ii) photolysis of water in green plants. In photophosphorylation light is converted to chemical energy. Photophosphorylation is of two types- i) cyclic photophosphorylation and ii) non-cyclic photophosphorylation.The dark reaction takes place through a series of steps known as Calvin Benson cycle.

Oxidative Photophosphorylation:The electron transfer reaction of oxidative photophosphorylation involves a series of membrane bound carriers collectively known as photosystems, i.e. a pathway of electron transfer. In case of anaerobic green or purple sulphur photosynthetic bacteria ther is only one photo system known as Photosystem-I or cyclic oxidative photophosphorylation.N Cyanobacteria, algae and green plants, two photosystems referred to as Photosystems I and II or non cyclic oxidative phosphorylation is present.Cyclic Phosphorylation (Anoxygenic Photosynthesis - The Light Reaction in Green and Purple Bacteria):In green and purple bacteria possess slightly different photosynthetic pigments, bacteriochlorophylls many with absorption maxima at longer wavelengths. Bacteriochlorophylls a and b have maxima in ether at 775 and 790 nm, respectively. In vivo maxima are about 830 to 890 nm (bacteriochlorophyll a) and 1,020 to 1,040 nm (Bchl b). This absorption maxima of the infrared region better adapts these bacteria to their ecological niches.

When the special reaction-center chlorophyll P870 is excited, it donates an electron to bacteriopheophytin. Electrons then flow to quinones and through an electron transport chain back to P870 while driving ATP synthesis. Note that although both green and purple bacteria lack two photosystems, the purple bacteria have a photosynthetic apparatus similar to photosystem II, whereas the green sulfur bacteria have a system similar to photosystem I. Green and purple bacteria face a further problem because they also require NADH or NADPH for CO2 incorporation. They may synthesize NADH in at least three ways. If they are growing in the presence of hydrogen gas, which has a reduction potential more negative than that of NAD+, the hydrogen can be used di-


rectly to produce NADH. Like chemolithotrophs, many photosynthetic purple bacteria use proton motive force to reverse the flow of electrons in an electron transport chain and move them from inorganic or organic donors to NAD+ . Green sulfur bacteria such as Chlorobium appear to carry out a simple form of noncyclic photosynthetic electron flow to reduce NAD+.

In this system , the chlorophyll molecule acts as an internal electron acceptor and donor for photosystem I . In the course of returning the excited electron to the oxidized chlorophyll molecule,a sufficient energy gradient is developed to permit the synthesis of one ATP molecule.

The photosynthetic electron transfer system appears to be organized asymmetrically across the intracytoplasmic membrane in the purple bacteria. The cyclic transfer system is compatible with the basic mechanisms of chemiosmosis.

At low light intensities, cyanobacteria can carry out anoxygenic photosynthesis. While carrying out anoxygenic photosynthesis; the cyanobacteria derive their reducing power from the oxidation of hydrogen sulphide. By oxidizing hydrogen sulphide, cyanobacteria can form elemental sulphur granules that are deposited outside the cells.Non cyclic Photophosphorylation (Oxygenic Photosynthesis= The Light Reaction in Eucaryotes and Cyanobacteria):


In eucaryotic cells and cyanobacteria, there are two kinds of antennas associated with two different photosystems.Photosystem I absorbs longer wavelength light (≥680 nm) and funnels the energy to a special chlorophyll a molecule called P700. The term P700 signifies that this molecule most effectively absorbs light at a wavelength of 700 nm. Photosystem II traps light at shorter wavelengths (≤680 nm) and transfers its energy to the special chlorophyll P680. When the photosystem I antenna transfers light energy to the reaction-center P700 chlorophyll, P700 absorbs the energy and is excited; its reduction potential becomes very negative. It then donates its excited or high-energy electron to a specific acceptor, probably a special chlorophyll a molecule (A) or an iron-sulfur protein . The electron is eventually transferred to ferredoxin and can then travel in either of two directions.

In the cyclic pathway (the dashed lines in figure ), the electron moves in a cyclic route through a series of electron carriers and back to the oxidized P700. The pathway is termed cyclic because the electron from P700 returns to P700 after traveling through the photosynthetic electron transport chain. PMF (proto motive force) is formed during cyclic electron transport in the region of cytochrome b6 and used to synthesize ATP. This process is called cyclic photophosphorylation because electrons travel in a cyclic pathway and ATP is formed. Only photosystem I participates.

Electrons also can travel in a non cyclic pathway involving both photosystems. P700 is excited and donates electrons to ferredoxin as before. In the non cyclic route, however, reduced ferredoxin reduces NADP + to NADPH .

2Fd (reduced) +2H+NADP+ ---------------- > 2Fd (Oxidized) + NADPH + H+


Because the electrons contributed to NADP+ cannot be used to reduce oxidize P700, photosystem II participation is required. It donates electrons to oxidized P700 and generates ATP in the process. The photosystem II antenna absorbs light energy and excites P680, which then reduces pheophytin a. Pheophytin a is chlorophyll a in which two hydrogen atoms have replaced the central magnesium. Electrons subsequently travel to Q (plastoquinone) and down the electron transport chain to P700. OxidizedP680 then obtains an electron from the oxidation of water to O2.

2H2O ------------------- > 4H+ + 4e- + O2

Thus electrons flow from water all the way to NADP+ with the aid of energy from two photosystems, and ATP is synthesized by non cyclic photophosphorylation. It appears that one ATP and one NADPH are formed when two electrons travel through the non cyclic pathway.

Just as is true of mitochondrial electron transport, photosynthetic electron transport takes place within a membrane. Chloroplast granal membranes contain both photosystems and their antennas.

Cyclic versus Non cyclic Electron Flow: The ‘Z’ pathway however, can become cyclic when the energized electron at the first electron carrier P430 of the photosystem- I returns to fill electron hole in the reaction centre P700. This shunt pathway involves the electron carriers in the photosystem-II. Thus ATP is formed but not NADPH and oxygen is not used. This is used when the cell has an ample supply of NADPH but requires additional ATP.


PHOTOSYNTHESIS IN HALOBACTERIUM:

Some archaea are able to use light as a source of energy. Instead of using chlorophyll, these microbes use a membrane protein called bacteriorhodopsin (more correctly called archaeorhodopsin). One such archaeon is the halophile Halobacterium salinarum. H. salinarum normally depends on aerobic respiration for the release of energy from an organic energy source. It cannot grow anaerobically by anaerobic respiration or fermentation. However, under conditions of low oxygen and high light intensity, it synthesizes bacteriorhodopsin, a deep-purple pigment that closely resembles the rhodopsin found in the rods and cones of vertebrate eyes. Bacteriorhodopsin’s chromophore is retinal, a type of carotenoid. The chromophore is covalently attached to the pigment protein, which is embedded in the plasma membrane in such a way that the retinal is in the center of the membrane. Bacteriorhodopsin functions as a light-driven proton pump. When retinal absorbs light, a proton is released and


the bacteriorhodopsin undergoes a sequence of conformation changes that translocate the proton into the periplasmic space . The light-driven proton pumping generates a pH gradient that can be used to power the synthesis of ATP by chemiosmosis. This phototrophic capacity is particularly useful to Halobacterium because oxygen is not very soluble in concentrated salt solutions and may decrease to an extremely low level in Halobacterium ’s habitat. When the surroundings become temporarily anoxic, the archaeon uses light energy to synthesize suffi cient ATP to survive until oxygen levels rise again. Note that this type of phototrophy does not involve electron transport. It had been thought that rhodopsin- based phototrophy is unique to Archaea. However, proton-pumping rhodopsins have recently been discovered in some proteobacteria (proteorhodopsin) and a fungus.DARK REACTION:Many autotrophs obtain energy by trapping light during the light reactions of photosynthesis, but some derive energy from the oxidation of inorganic electron donors. Autotrophic CO 2 fixation is crucial to life on Earth because it provides the organic matter on which heterotrophs depend.

Four different CO 2 -fixation pathways have been identified in microorganisms. Most autotrophs use the Calvin cycle , which is also called the Calvin-Benson cycle or the reductive pentose phosphate cycle. The Calvin cycle is found in photosynthetic eucaryotes and most photosynthetic bacteria. It is absent in some obligatory anaerobic and microaerophilic bacteria. Autotrophic archaea also use an alternative pathway for CO 2 fixation. CO2 FIXATION PATHWAYS:1.Calvin Cycle The Calvin cycle is also called the reductive pentose phosphate cycle because it is essentially the

reverse of the pentose phosphate pathway. Thus many of the reactions are similar, in particular the sugar transformations. The reactions of the Calvin cycle occur in the chloroplast stroma of eucaryotic autotrophs. In cyanobacteria, some nitrifying bacteria, and thiobacilli (sulfuroxidizing chemolithotrophs), the Calvin cycle is associated with inclusion bodies called carboxysomes . These polyhedral structures contain the enzyme critical to the Calvin cycle and may be the site of CO 2 fixation.


The Calvin cycle is divided into three phases: (i) Carboxylation phase, (ii) Reduction phase, and (iii) Regeneration phase.

(i) Carboxylation phase : During the carboxylation phase, the enzyme ribulose 1,5-bisphosphate carboxylase , also called ribulose bisphosphate carboxylase/oxygenase (Rubisco), catalyzes the addition of CO 2 to the five-carbon molecule ribulose 1,5-bisphosphate (RuBP), forming a six-carbon intermediate that rapidly and spontaneously splits into two molecules of 3-phosphoglycerate (PGA). PGA is an intermediate of the Embden-Meyerhof pathway (EMP), and(ii) Reduction phase: In the reduction phase, PGA is reduced to glyceraldehydes 3-phosphate by two reactions that are essentially the reverse of two EMP reactions. The difference is that the Calvin cycle enzyme glyceraldehyde 3 phosphate dehydrogenase uses NADP + rather than NAD +. (iii) Regeneration phase : Finally, in the regenerationphase, RuBP is regenerated, so that the cycle can repeat. In addition, this phase produces carbohydrates such as glyceraldehydes 3-phosphate, fructose 6-phosphate, and glucose 6-phosphate, all of which are precursor metabolites. This portion of the cycle is similar to the pentose phosphate pathway and to synthesize fructose 6-phosphate or glucose 6-phosphate from CO 2 , the cycle must operate six times to yield the desired hexose and reform the six RuBP molecules.

6RuBP + 6CO 2 → 12PGA → 6RuBP +fructose-6-PThe incorporation of one CO 2 into organic material requires three ATPs and two NADPHs. The formation of glucose from CO 2 may be summarized by the following equation.

6CO 2 + 18ATP + 12NADPH + 12H + 12H 2 O → glucose +18ADP +18Pi +12NADP+

2. Pyruvate Synthetase Pathway (Reductive Carboxylic Acid Cycle) - In the green bacterium Chlorobium thiosulfatophilum Evans et. al (1966) described the pyruvate synthetase pathway for CO2 fixation. CO2 is used to form pyruvate by means of the pyruvate synthetase reaction the ultimate reductant is hydrogen sulphide.The light dependent oxidation of H2S provides the reducing power for the reduction of ferredoxin (Fd). Acetyl CoA then accepts CO2 and is reduced by ferredoxin to yield pyruvate,

1) Formation of pyruvate by pyruvate synthetase is dependent on reduced ferredoxin.

Acetyl-CoA+Co2+Ferredoxin (reduced)---> Pyruvate+CoA+Ferredoxin (oxidized)

(2) Conversion of pyruvate into oxaloacetate.Pyruvate + ATP + CO2--> Oxaloacetate + ADP + PiOxaloacetate then enters a reversed


tricarboxylic acid cycle.(3) Carboxylation of succinyl CoA to yield d-Ketoglutarate (involving reduced ferredoxin).Succinyl CoA + co2 + Ferredoxin (reduced)-->α-Ketoglutarate + CoA + Ferredoxin (oxidized)(4) α -Ketoglutarate is converted into citrate through oxalosuccinate. Citrate then splits into oxaloaceticacid and acetate.α -Ketoglutarate -->Oxalossuccinate --> Citrate --> Oxaloacetic acid + Acetate.The net result of each cycle is that a molecules of CO2 are fixed (reductive fixation) and one equivalent of oxaloacetate is produced.Three molecules of ATP are required: (I) for activation of acetate, (2) for carboxylation of pyruvate and(3) for activation of succinate.The reductive carboxylic acid cycle appears to be particularly suited to provide the carbon skeletons for the main products of bacterial photosynthesis, which are mainly amino acids.

3 . 3-hydroxypropionate cycle : A few archaea genera and the green non sulfur bacteria (another group of anoxygenic phototrophs) use the 3-hydroxypropionate cycle to fi x CO 2 .

4. Acetyl-CoA pathway: Methanogens use portions of the acetyl-CoA pathway for carbon fixation. Acetogens use the pathway in its entirety. Both the acetyl- CoA pathway and methanogenesis involve the activity of a number of unusual enzymes and coenzymes.

Pass word:photosyn

photosynthesis in bacteria - … · web viewthere are three groups of photosynthetic bacteria:...

Documents