Question

With the aid of examples, explain the role of photosynthetic pigments in oxygenic and anoxygenic photosynthetic bacteria

Answer 1

Every organisms need energy for their metabolic and cellular activities.From where does this energy comes from ?By oxidizing organic compounds, organisms obtain the energy for their activities.These organisms are obtained through various means,either by depending on other organisms / by producing them independently.One of such mechanism of producing this organic compounds form inorganic substances is photosynthesis.

• Plants and some microbes are capable of performing photosynthesis.
• Photosynthesis in bacteria can be explained as follows ; synthesis of carbohydrates using chlorophyll,sunlight,water and carbondioxide.Unlike in plants ;in bacteria oxygen is not invloved as a byproduct except cyanobacteria.
• When oxygen involved as a byproduct due to oxidation of water;it is termed as oxygenic photosynthesis; eg - in plants and cyanobacteria.
• But in most of the bacteria water is not oxidized and oxygen is not produced; hence the process known as anoxygenic photosynthesis.
• Thus bacterial photosynthesis can be divided into two - oxygenic and anoxygenic.

ANOXYGENIC PHOTOSYNTHETIC BACTERIA

• Are of ancient origin,considered to be evolved before oxygenic ones.
• Have representatives in 4 phyla ; purple - sulphur bacteria,purple non - sulphur bacteria , green sulphur bacteria, green non sulfur bacteria.
• They use light energy to extract electrons from molecules other than water like reduced sulphur compounds, molecular hydrogen / organic compunds.
• Habitat - fresh water,brackish water ,marine and hyper saline water.
• On the basis of the pigmentation classified into 3 ; purple bacteria,green bacteria and helio bacteria.

a. PURPLE BACTERIA -

• Use hydrogen / reduced sulphur compounds as electron donors and grows under anaerobic conditions(oxygen represses the pigment system of these organisms ).
• photosynthetic pigments present - Bchl a and b(bacteriochlorophyll).
• Based on the carotenoids present they exhibits a range of coloured appearance ranging from purple - violet,pink brown - red, brown etc.
• Photosynthetic pigments depends on light intensity,gets repressed under high intensity.
• 2 types ; purple sulphur and purple non sulphur.
• Purple sulphur ; contains Bchl a and b,uses - thiosulphate,acetate ,puruvate as electron donors depending on the environment they grow ; Eg - Chromatiium,Thiopedia,Thiospirillum,Thiocapsa etc ; cells are packed with intracellular sulphide deposition.
• Purple non - sulphur bacteria ; also contain Bchl a and b ,but uses sulphide in low concentration ; high concentration of sulphide is proved to be toxic for this type of bacteria.Use organic acids,benzoate,ethanol ,amino acids etc as electron donors. Eg - Rhodomicrobium,Rhodospirillum, etc.

b.GREEN BACTERIA -

• They doesnt appear green as per the name,but brownish due to the presence of carotenoid pigments.
• contains Bchl c,d and e ,also small amount of bchl a .
• Photosynthetic apparatus is called - chlorosomes.
• Are of 2 types - green sulphur and green non - sulphur.
• Green sulphur - presence of chlorosomes,anaerobic,obligate phototroph and deposit sulphur extracellularly.Eg - chlorobium,prostheochloris etc,
• Green no sulphur - filamentous and thermophilic nature,Bchl a,c and carotenes are present,chlorosomes when grown anerobiacally and does not deposit sulphur.Eg - chloroflexus.

c. Heliobacteria -

• quite different from other bacteria on observing biochemical and morphological characters,rod shaped.Eg - Heliobacterium,Heliobacillus.They are green in colour and contain Bchl g.

OXYGENIC PHOTOSYNTHETIC BACTERIA

• Unicellular / multicellular, have Bchl a ,photosynthates gets accumulated in the form of glycogen , carrys out oxygenic photosynthesis as the name suggests.

PHOTOSYNTHETIC PIGMENTS

• Carotenoids - with the help of chlorophyll aids in photosynthesis,not independently capable ,carotenoids transfer light energy to bacteriochlorophyll a and helps in photosynthesis.
• chlorophylls and bacteriochlorophylls - main pigments involved in photosynthesis, chlorophyll a is green in colour and shows maximum absorption in red and blue light; there are different types of chlorophylls and bacteriochlorophylls each having a unique absorption spectrum.Prokaryotes mostly contains chlorophyll d . Bacteriochlorophyll a - present in most purple bacteria,absorbs maximally between 800- 925 nm depending on species.
• Carotenoids and phycobilins are accessory pigments - carotenoids have a photoprotective role(both oxygenic and anoxygenic) while phycobilins have role in energy metabolism9in cyanobacteria).