Bacterial Photosynthetic Pigments

1. 2. Photosynthetic pigments in Bacteria
❖ Photosynthetic pigments are coloured light
harvesting (absorbing) molecules – which
absorb specific wavelengths of light. The set
of wavelengths absorbed by a pigment is
called its absorption spectrum.
❖ These compounds, while absorbing specific
wavelengths, reflect specific wavelengths of
light. This reflection of light, makes them
‘colourful’.
❖ Most photosynthetic systems have different
pigments – that enable them to harvest
energy from lights of different wavelengths.

2. ❖ A photosynthetic pigment is present in
chloroplasts or in the cytoplasm of
photosynthetic bacteria which captures
the light energy necessary for
photosynthesis.
❖ The photosynthetic pigments are present
along with proteins in complexes called
Photosystems.

3. Prokaryotes do not contain chloroplasts. Their
photosynthetic pigments are integrated into internal
membrane systems. These systems are present:
(1) As invaginations of the cytoplasmic
membranes (purple bacteria).
(2) In the cytoplasmic membrane itself
(heliobacteria).
(3) In both the cytoplasmic membrane and in
specialised structures – enclosed in
membranes called Chlorosomes (Green
bacteria)
(4) In thylakoid membranes (Cyanobacteria).

4. Photosynthetic pigments commonly
present in bacteria are of the following
types:
1. Bacteriochlorophylls (Bchls)
2. Carotenoids
3. Phycobilins
4. Bacteriorhodopsin

5. 1. Bacteriochlorophylls [Bchls]:
➢ Chlorophyll molecules present in bacteria are called
Bacteriochlorophylls (Bchls).
➢ There are 6 types of chlorophylls in bacteria: They are as follows:
Bacteriochlorophyll a (Bchl a)
Bacteriochlorophyll b (Bchl b)
Bacteriochlorophyll c (Bchl c)
Bacteriochlorophyll d (Bchl d)
Bacteriochlorophyll e (Bchl e) &
Bacteriochlorophyll g (Bchl g)
➢ They differ, based on the presence or absence of a double bond
between the C3 and C4 atoms..
➢ This accounts for the different absorption maxima (wavelength of
light), by different Bchl molecules.

6. 2. Carotenoids :
Although chlorophyll or bacteriochlorophyll is required for
photosynthesis, phototrophic organisms contain many accessory
pigments like carotenoids and phycobiliproteins.
Carotenoids are typically yellow, red, brown or green in color and
absorb light in the blue region of the spectrum. Carotenoids are closely
associated with bacteriochlorophyll in photosynthetic complexes, and
energy absorbed by carotenoids can be transferred to the reaction
center. Absorption spectrum is 450nm.
Carotenoids play a phyto-protective role in both oxygenic and
anoxygenic photosynthesis. They are present in all photosynthetic
tissues and they are important pigments in photosynthesis. Cells that
are rich in carotenoids – if they do not contain chlorophyll, do not
photosynthesize.
Bright light can be harmful to cells. Carotenoids inhibit toxic
oxygen species by absorbing much of this harmful light and prevent
these dangerous photooxidations.

7. 3. Phycobilins-Phycobiliproteins and
Phycobilisomes
❖ Cyanobacteria and the chloroplasts of red algae
contain phycobiliproteins, which are the main
light-harvesting systems in these phototrophs.
❖ The red phycobiliprotein, called phycoerythrin,
absorbs most strongly at wavelengths around
550 nm, whereas the blue phycobiliprotein -
phycocyanin, absorbs most strongly at 620 nm.
❖ Phycobiliproteins assemble into aggregates
called phycobilisomes.
❖ Phycobilisomes facilitate energy transfer to allow
cyanobacteria to grow at low light intensities.

8. 4. Bacteriorhodopsin:
❖ Rhodopsin is purple in colour, but under high
aeration conditions, change colour, to limit O2.
❖ They are present in halophiles, where aeration is
low and Bchl is not involved.
❖ They absorb light at 570nm (green light).
❖ Under low O2 conditions, a proton (H+) is inserted
into the membrane.
❖ The bacteriorhodopsin, absorbs light and
transfers the H+ across the membrane. Thus, it
acts as a proton pump. The resulting proton
gradient is converted into chemical energy (ATP).