Pigments in Photosynthetic Bacteria | Microbiology

In this article we will discuss about the presence of pigments in photosynthetic bacteria.

Because of the presence of carotenoids in all photosynthetic tissues their role is anticipated in photosynthesis. The tissue/cells rich in carotenoids devoid of chlorophyll do not photosynthesize. Light energy absorbed by carotenoids appears to be transferred to chlorophyll a or bacteriochlorophyll a and utilized in the photosynthesis.

It was observed if light is absorbed directly by chlorophyll a, generally found to be less efficient in photosynthesis than that of light absorbed by accessory pigments such as phycocyanin or phycoerythrin.

All the photosynthetic bacteria contain chromatophores which have β-carotene, xanthophyll (carotenoids), and phycobilisomes (phycocya­nin and phycoerythrin). Most of the carotenes are present in photosystem I while phycobilisomes are present in photosystem II.

Bacteriochlorophylls:

Photosynthetic bacteria appear in different colour suspension. They are green, purple, violet, red, blue-green, brown coloured due to the presence of photosynthetic pigments in their photosyn­thetic apparatus. The pigments can be measured by their absorption spectra. Generally chlorophyll absorbs maximally at < 450 nm, carotenoid at 400-550 nm and phycobiliproteins of cyanobacteria at 550-650 nm.

Sometimes chlorophyll pigments absorb maximum in infra red region of 650-1000 nm. There are several types of chlorophyll molecules present in bacteria called bacteriochlorophyll (BChl). They differ from each other mainly by the presence or absence of a double bond between carbon atoms 3 and 4 and by the substituents on the porphyrin ring.

The main absorption maxima of chlorophyll a in cyanobacteria is between 680 and 685 nm, whereas bacteriochlorophylls c, d and e present in green sulphur bacteria show absorption maxima between 715-755 nm.

Most of the purple bacteria contains BChl a of 850-890 nm. BChl b shows absorption maxima at 1020 to 1035 nm (Table 13.3). They are found in Rhodopseudomonas viridis, Ectothiorhodospira halochloris and Thiocapsa pfeningii.

Sometimes, type of binding and position of the BChl molecule in the pigment protein complex in photosynthetic apparatus gives rise to different spectral forms to the individual chlorophyll molecule. For instance, BChl a in purple bacteria has four spectral froins: BChl 800, BChl 820, BChl 850 and BChl 870-890 (in Chloroflexus), however, BChl a has two distinct absorption maxima at 808 and 868 nm (Table 13.3).

(i) Carotenoids:

These accessory pigments (400-500 nm, absorption spectrum) are found in phototrophs. They are yellow, green, red-brown in colour and absorb light in blue region. They are insoluble in water and embedded in membrane.

The presence of an oxo or aldehyde group can give them a deep colouration. They have long hydrocarbon chains of C₄₀ compounds (tetraterpenoids) with tertiary hydroxyl or methoxy groups with alternating C-C and C=C bonds (conjugate bonds system).

(Fig. 13.1). The carotenoids do not function directly in ATP synthesis however, transfer energy to reaction center. They are photo-protective and quench singlet oxygen. The members of Chromatiaceae bear okenone, while iso-renceratene is present in Chlorobiaceae. Carotenoids are also reported to be present in certain airborne pigmented bacteria and in halophiles (Halobacter sp.). They have a defensive role in such bacteria.

(ii) Bacteriorhodopsin:

These are present in halophiles in which light-mediated synthesis of ATP not involves BChl but low aeration insert a proton into their membrane. Rhodopsin in retinal carotenoid like protein absorb light and transfer H⁺ across the membrane. They are purple in colour but change occurs in high aeration to O₂ limiting condition.

They absorb light in green region (570 nm) and convert trans form to cis form during absorption of light hence molecules relax and return to more stable form in the dark following uptake of proton from cytoplasm. A separate light driven pump is present which is called halo-rhodopsin to pump C1^– in to the cell as anion for K⁺.

(iii) Phycobilins:

Phycobilins are water soluble proteins which contain covalently bound linear tetra pyrroles (bilins) as chromophores present inside the granules. They occurs on outer surface of thylakoid membrane of cyanobacteria.

Phycobiliproteins are red or blue in colour, contains tetrapyrroles coupled to proteins. Some of the phycobiliproteins are phycocyanin and allophycocyanins. On aggregation,these proteins form a structure called phycobilisomes which are attached to photosynthetic membranes.