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In this article we will discuss about:- 1. History of Photosynthetic Bacteria 2. Classification of Photosynthetic Bacteria 3. Metabolism.
History of Photosynthetic Bacteria:
Before nineteenth century it was considered that the photosynthetic machinery is present in purple bacteria because these bacteria showed movement towards light (phototactic) and growth is also induced by light.
On the other hand, S. Winogradsky, a German botanist observed that some purple bacteria can utilize hydrogen sulphide to sulphate with intracellular deposition of sulphur. C.B. Van Niel (1930) defined various metabolic versions of anoxygenic photosynthesis and demonstrated that it is the characteristic mode of energy yielding metabolism in both purple and green bacteria.
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The photosynthetic purple bacteria use a variety of hydrogen donors in place of water (e.g. H2S or various organic compounds). In some anaerobic photosynthetic bacteria using hydrogen donors other than hydrogen or water (e.g. succinate) not only CO2 is reduced to NADPH2 but also atmospheric nitrogen is reduced to ammonia. Such nitrogen fixation occurs at the expense of photic energy.
This fact is of considerable importance in view of the significance of nitrogen fixation in the economy of nature, therefore, it appeared that they lack photosystem II (PS-II), which among other things, in green plants is involved in O2 production (from OH). The photosynthetic bacteria show no enhancement.
Parson and Cogdell (1975) isolated functional complexes from photosynthetic bacteria. The reaction centre from the purple non-sulphur bacterium, Rhodopseudomonas sphaeroides, contains four molecules of chlorophyll and two molecules of bacteriopheophytin (like b chlorophyll but the Mg replaced by two H+), one or two molecules of ubiquinone and one atom of ferrous iron together with three polypeptides of apparent molecular weight in the region of 28, 24 and 21 KD.
All the photosynthetic bacteria are divided into 35 groups. The group 10 contains anoxygenic phototrophic bacteria, while group 11 belongs to oxygenic phototrophic bacteria.
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The anoxygenic group (no evolution of oxygen) has purple tad green bacteria, while oxygen evolving group has only cyanobacteria. Another type of oxygenic bacteria has recently been discovered and placed under prochlorophyta. Prochlorophyta acts as a bridge between cyanophyta and chlorophyta (or green algae).
Classification of Photosynthetic Bacteria:
The photosynthetic bacteria are divided into two broad groups, Anoxygenic photosynthetic bacteria and oxygenic photosynthetic bacteria. Classification of photosynthetic bacteria is given in Table 13.1.
1. Anoxygenic Photosynthetic Bacteria:
The anoxygenic photosynthesis depends on e– donors such as reduced sulphur compounds, molecular hydrogen or organic compounds. The ammonium salts are generally used as nitrogen source. Nitrogen fixation has been reported in some bacterial species.
Some can grow chemo-auto-trophically under aerobic/micro-aerobic condition. Fatty acids, ethanol and organic acids serve as carbon sources. They are found in fresh water, brackish water, and marine and hyper-saline water.
They may be classified into seven groups:
Sub-group 1:
Globules of sulphur are found inside the cell e.g. Amoebobacter, Chromatium, Lamprobacter, Thiocapsa, Iximprocystis, Thiocystis, Thiodictyon, Thiopedia, Thiospirilliim.
Sub-group 2:
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Globules of sulphur appears outside the cell e.g. Ectothiorhodospira.
Sub-group 3:
Globules of sulphur may appear outside the cell. Most genera depend upon growth factor. Cells of Rhodobacter, Rhodocyclus, Rhodomicrobium, Rhodopila, Rhodopseudomonas, Rhodospirillum grow by photo-assimilation of simple organic substrates.
Sub-group 4:
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Internal membrane systems or chlorosomes are absent. .Spiral shaped cell wall has no lipopolysaccharide. Cells contain bacteriocholophyll g and carotenoids. Reduced sulphur compounds are not utilized. They are photoheterotrophic e.g. Heliobacillus (glider).
Sub-group 5:
Globules of sulphur appears outside but never inside the cell. Bacteriochlorophylls are located in chlorosomes. Simple organic substances are photo-assimilated only in presence of sulphide and bicarbonates e.g. Ancalochloris, Chlorobium, Pelodictyon, Prosthecochloris.
Sub-group 6:
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Cells are arranged in multicellular filaments that show gliding motility and utilize organic substances e.g. Chloroflexus, Chloronema, Heliothrix, Oscillochloris.
Sub-group 7:
Cells grow chemo-heterotrophically under aerobic conditions; no growth occurs under anaerobic conditions. They contain bacteriochlorophyll a and carotenoids e.g. Erythrobacter.
Anoxygenic photosynthetic bacteria have been divided into three groups on the basis of pigmentation; purple bacteria, green bacteria and heliobacteria (Table 13.2).
(i) Purple Bacteria (Proteobacteria):
The anoxygenic phototrophs grow under anaerobic conditions in the presence of light and do not use water as e-donor as in higher plants. The pigment synthesis is repressed by O2.
They grow auto-trophically with CO2 (C source) and hydrogen or reduced sulphur compounds act as e– donor. Photo-heterotrophy (i.e. light as energy source and an organic compound as a carbon source) also supports growth. Some purple bacteria also show chemo-organotrophy i.e. can grow in dark under similar conditions.
The purple bacteria comprise of five sub-groups of Proteobacteria as given below:
i. Alpha group:
Rhodospirillum, Rhodopseudomonas, Rhodobacter, Rhodomicrobium.
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Rhodopila, Rhodovulvum
ii. Beta group:
Rhodocyclus, Rhodoferax, Rubrivivax
iii. Gamma group:
Chromatium, Thiospirillum
iv. Delta group:
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Nil non-phototrophs
v. Epsilon group:
Bdellovibrio, Myxococcus, Campylobacter, Helicobacter
Purple bacteria contain Bchl a, Bchl b and show the photosynthetic membranes in flat sheets (lamellae). Certain bacteria (Chromatium sp.) show membrane as individual vesicle. The colour of the purple bacteria shows brown, pink brown-red, purple-violet based on carotenoid contents. The internal membrane extends to give rise to photosynthetic pigments.
The photosynthetic pigments and internal membrane are influenced by light intensity. At high intensity, photo-apparatus is inhibited, whereas the cells get packed with membranes when grown at low light intensity. Carotenoids give rise to purple colour; mutants lack carotenoids are blue green reflecting the actual colour of BChl a.
Purple bacteria are of two types:
a. purple-sulphur bacteria and
b. purple non-sulphur bacteria.
(a) Purple sulphur bacteria (family; Chromatiaceae):
They are Gram-negative bacteria which contain BChl a and BChl b and grow chemolithotrophically in dark with thiosulphate as e– donor. They are also chemoorganotrophs; utilize acetate, pyruvate and few other compounds. The mole % of G+C varies from 46-70.
The cells of purple-sulphur bacteria are larger than green bacteria and packed with intracellular sulfide deposition. They are found in anoxic zone of lakes and sulphur springs (obligate anaerobe).
They contain vesicles that are enclosed within a thin membrane that is not directly associated with the cell membrane called vesicular thylakoid. They are photolithotrophs and motile in nature e.g. Ectothiorhodospira, Chromatium, Thiocapsa, Thiospirillum, Thiodictyon, Thiopedia etc.
(b) Purple non-sulphur bacteria (family: Ectothiorhodospiraceae old name Rhodospirillaceae):
They also contain BChl a and b and use low concentration of sulphide. The concentration of sulphide utilized by purple-sulphur bacteria proved toxic to this category of bacteria. Earlier, scientists thought that these bacteria are unable to use sulphide as an e– donor for the reduction of CO2 to cell material, hence named them non-sulphur.
They deposit sulphur extracellularly. Some non-sulphur bacteria grow anaerobically in the dark using fermentative metabolism, while the others can grow anaerobically in dark by respiration in which e– donor may be an organic compound/inorganic compound as H2. This group is most versatile energetically due to broad requirements and are photo-organotrophs i.e. use organic acids, amino acids, benzoate and ethanol.
They also grow as chemoorganotrophs and require vitamins. They are heterogenous group due to the presence of both polar and peritrichous flagella. Some can utilize methanol for phototrophic growth when grown anaerobically.
The following reaction occurs inside their cell:
2CH3OH -t- CO2 → 3(CH2O) + H2O
The DNA base composition is 61-73 mole % (G-t-C) and the sulphur granules are formed outside the cell. Examples of purple non-sulphur bacteria are Rhodomicrobium, Rhodopseudomonas, Rhodospirillum, Rhodocyclus, etc.
(ii) Green Bacteria:
Instead of green in colour, these are brown due to the presence of carotenoids components. They are Gram-negative. Hence, colour is not a suitable basis for these bacteria. They contain BChl c, BChl d or BChl e plus small amount of Bchl a.
The photosynthetic apparatus is chlorosomes which consist of a series of cylindrical structures underlying and/or attached to cytoplasmic membrane and are quite different with lamellae. These vesicles are enclosed within a thin membrane devoid of bilayer but consist of transporter proteins located in the cytoplasmic membrane. They do not require vitamins for their growth.
Green bacteria are of two types:
Green sulphur bacteria and
Green non-sulphur bacteria.
(a) Green sulphur bacteria (family: Chlorobiaceae):
They are non-motile, rods, spiral and cocci. Some have appendages i.e. prosthecae. Chlorosomes are present in the cell. They do not possess gas vesicles (Chlorobium) except in Pelodictyon. They are strictly anaerobic and obligate phototroph.
Most of them assimilate simple oxygenic substances for photosynthetic growth if sulphur source is present. They deposit sulphur extracellularly. Some (Chloroflexus) grow chemoorganotrophically, hence they are non-sulphur green type. The mol % G+C is 45-58. Examples of these bacteria are Chlorobium, Prostheochloris, Pelodictyon, Chloroherpeton.
(b) Green non-sulphur bacteria (family: Chloroflexaceae):
The green non-sulphur bacteria are filamentous, gliding bacteria, thermophilic in nature. The pigments are Bchl a, Bchl c, β- and Ƴ- carotenes. The chlorosomes are present when grown anaerobically. They are photoheterotrophic and photoautotrophic and show gliding movement. They do not deposit sulphur. The mol % G+C contents vary 53-55. Example is Chloroflexus.
(iii) Heliobacteria:
Based on 16S rRNA sequencing and other morphological and biochemical characters, helicobacter are quite different with other anoxygenic photosynthetic bacteria. They are Gram-positive, rod shaped, motile either by gliding or by means of flagella. The mol % G+C is between 50 and 55, and at present comprises of three genera and five species such as Heliobacterium, Helophilum and Heliobacillus.
Most of them produce endospores and grow up to 42°C. The heliobacteria are green in colour. The baeteriochlorophyll is associated with the cytoplasmic membrane; hence lamellae and chlorosomes are absent.
The endospores contain a dipicolinic acid similar to Bacillus and Clostridium. Most of the heliobacteria are found in tropical soils of paddy fields. They contain BChl g having vinyl (H2C = CH2) group on ring I of the tetrapyrrol molecules similar to bacteriochlorophyll a but difference lies in ring II of the tetrapyrrole.
2. Oxygenic Photosynthetic Bacteria:
The oxygenic photosynthetic bacteria are unicellular or multicellular and possess bacteriochlorophyll a and carry out oxygenic photosynthesis. They contain phycobilins. One group, prochlorophytes lack phycobilins, but contain both bacteriochlorophyll a and b. They are mostly represented by Gram-negative cyanobacteria having only membrane.
Many possess extracellular sheath called glycocalyx or capsule or merely mucilage or slime. The flagella are not present but they show gliding movement. The light harvesting pigments are phycobilin proteins, phycoerythrin, phycocyanin, bacteriochlorophyll a and carotenoids but sheath capsule may contain yellow pigment called scytonemin or red-blue pigment gloeocapsin which may mask cellular pigmentation.
Phycobilins form phyobilisomes on both surfaces on double unit internal membrane called thylakoids. Bacteriochlorophyll a and carotenoids are part of it. Photosynthesis is oxygenic and autotrophic but chemoautotrophy also occurs. Photosynthates get accumulated in the form of glycogen, polyphosphate granules.
Carboxysomes and gas vesicles are present. Heterocysts have modified thylakoids containing low contents of photosynthetic pigments and lack of photosystem II. Akinetes are thick walled resistant storage cells. Some non symbiotic bacteria lack typical cyanobacteria wall is termed cyanelles.
They are put into the following five sub-groups:
Sub-group I:
They are unicellular or non-filamentous aggregates of cells held together by outer walls or a gel-like matrix. Binary fission occurs on one, two or three planes symmetric or asymmetrically or by budding. Examples of the members are Gloeothece, Synechococcus, Gloeocapsa, Gloeobacter.
Sub-group II:
In the members of this group, reproduction takes place by internal multiple fission with production of daughter cells smaller than Vi the parent or by multiple and binary fission e.g. Dermocorspa, Xenococcus, Pleurocapsa.
Sub-group III:
Binary fission occurs in one plane only. Trichomes are composed of cells which do not differentiate into heterocysts or akinetes e.g. Spirulina, Arthospira, Oscillatropia, Phormidium, Lyngbya.
Sub-group IV:
One of few cells of each trichome differentiate into heterocysts, at least when concentration of external combined nitrogen is low. Some genera also produce akinetes e.g. Cylindrospermum, Anabaena, Nodularia, Calothrix, Nostoc.
Sub-group V:
Binary fission occurs periodically or commonly in more than one plane giving rise to mulitseriate trichomes or trichomes with true branches or both e.g. Stigonema, Cyanobotrytis. Westicella, Loriella, Nostichopsis.
3. Members of Pro-Chlorophyta:
On the other hand, group II in section 19 of Prochloron was first discovered as an extracellular symbiont growing either on the surface or within the cloacal cavity of marine colonial ascidian invertebrates.
These bacteria are unicellular, spherical and 8.30 µm in diameter. The mol. % G+C is 31 to 41. In addition, Prochlorothrix is free living, consists of cylindrical cells that form filaments. It has been found in Dutch lakes. The DNA has a high mol. G+C content (53%).
These bacteria are unicellular or filamentous branched/un-branched. They have bacteriochlorophylls a and b and lack accessory red or blue bilin pigment e.g. Prochloron, Prochlorothrix. The presence of 5S and 16S rRNA sequences shows affinity with the cyanobacteria.
The prochlorophytes bear the following characters:
(i) They possess the characters of both prokaryotes and eukaryotes.
(ii) The prokaryotes contain thylakoid which are spread towards periphery but the prochlorophytes contain paired thylakoids. This shows evolution of thylakoid from single to paired form.
(iii) Prochlorophytes consist of both chlorophyll a and chlorophyll b, whereas cyanobacteria contains only chlorophyll a. The chlorophyta consists of both chlorophyll a and chlorophyll b.
(iv) Phycobihsomes are grass-green in colour because they lack phycobilin pigments.
(v) Their 5S and 16S rRNA show affinities with cyanobacteria.
(vi) Possibly a common ancestor has given rise to prochlorophytes, cyanophyta and plant chloroplasts.
4. Unclassified Bacteria:
There are certain other photosynthetic bacteria isolated recently but did not find any place in Bergey’s manual due to their discovery in recent years. Porphyrobacter neustonensis, an aerobic bacteriochlorophyll synthesizing budding bacterium from fresh water and Roseobacter denitrificans was discovered by Fuert and others in the year 1993.
The genus Erythromicrobium sibericus was isolated by Yukov and his Research team in the year 1991, while it is interesting to note the discovery of one photosynthetic as well as nitrogen fixing Rhizobium B T Ai by Evans and others in 1990. Pseudomonas radiora is again an interesting photosynthetic bacteria.
Metabolism in Photosynthetic Bacteria:
The green bacteria are strictly aerobic organisms that are obligately photosynthetic. They utilize H2S, H2 or thiosulphate as an electron donor (in place of H2O as in cyanobacteria and algae) and CO2 as the carbon source.
The purple bacteria contain two groups: the purple sulphur bacteria (Chromatiaceae), which utilize H2S as an electron donor, and the purple non-sulphur bacteria (Ectothiorhodaceae), which depend on organic compounds for their metabolism. Lipids containing short chain fatty acids are suitable substrate.
Poly-β hydroxybutyrate (C6H6O2)n is the major storage reserve material in such organisms. The photosynthetic bacteria found in deeper water are called meromictic where conditions are anaerobic but light is available. They are helpful in early evolutionary forms of life due to their independence without oxygen. The bacteriochlorophylls shows absorption spectrum in an acetone-methanol mixture near ultra-red spectrum at 770 nm.
Most of the photosynthetic bacteria contain bacteriochlorophyll as given in Table 13.4:
