66 4 Taxonomy, Physiology, and Ecology of Aquatic Microorganisms
them. White pigments/lighter colors reflect all or
almost all of the light energy striking them. Pigments
have their own characteristic absorption spectra, the
absorption pattern of a given pigment.
Chlorophyll (chl) found in plants, algae, and
cyanobacteria, is very similar to bacteriochlorophyll
(bchl) found in bacteria, other than cyanobacteria (see
Fig. 4.12). There are several types of chlorophylls and
of bacteriochlorophylls, (named a, b, c, d, e, and g)
differing from each in slight differences in structure.
Bchls “a” and “b” are found in the purple bacteria;
while bchls “c,” “d,” and “e” are found in Green sulfur
bacteria; bchl “g” is found in Heliobacteria. In higher
plants, photosynthesis takes place only in chl “a”; all
other chlorophylls along with carotenoids are acces
sories and gather light which is channeled to chl “a.”
Similarly, in bacteria, bchl “a” is the site of photosyn
thesis; all the other bacteriochlorophylls are accesso
ries and gather light which is channeled to bchl “a.”
Accessory pigments include carotenoids found in
higher plants and cyanobacteria and phycobilins found
in the algae. Pigments have their own characteristic
absorption spectra. Figure 4.1 3 shows the wavelength
of various chlorophylls and accessory pigments.
The Light Reactions
In higher plants, the light dependent reactions, take
place on membranous structures known as thylakoids
found in chloroplasts in complex processes, that are
not yet fully understood. The process is much simpli
fied as described below.
In plants, light is absorbed by complexes formed
between protein and chlorophyll molecules known as
photosystems, Photosystem I (PSI) and Photosystem II
(PSII). PSII absorbs light energy (photons) at a wave
length of 680 nm and is called P680 while PSI it
absorbs photons at 700 nm and is called P700.
When a pigment absorbs light energy, one of three
things will occur: Energy may be dissipated as heat; it
may be reemitted immediately as a longer wavelength,
a phenomenon known as fluorescence; or the energy
may trigger a chemical reaction, as in photosynthesis.
In plant photosynthesis, the action begins at the PSII
chlorophyll–protein complex which becomes excited
and loses an electron; this electron is passed through a
series of enzymes until it is transferred to water,
causing it to lose electrons:
The electron released from the splitting of water is
transferred to PSI, which can itself capture light
energy; this energy is transferred by enzymes used to
reduce NADP+ to NADPH and ATP the other energy
currency of cells, thus ADP + Pi ⇒ ATP.
While the photosynthetic process in cyanobacteria
is similar to that of plants, green bacteria and purple
sulfur bacteria have photosynthetic processes different
from the process in plants.
Cyanobacteria do not have chloroplasts, but have
structures on their cell membranes which are similar
to thylakoids. They have photosystems similar to
PS II and PS I found in the chloroplasts of higher
plants. They can produce NADPH and ATP in the
way as higher plants and they are the only bacteria
which produce O 2 during photosynthesis. However,
instead of carotenoids or chlorophyll “b” which act as
accessory pigments in higher plants, they have
phycobilins.Purple Bacteria: Purple bacteria and green sulfur
bacteria have only one type of photosystem. The single
photosystem in purple bacteria is structurally related
to PS II in cyanobacteria and plant chloroplasts; it,
however, has a P870 molecule, i.e., it absorbs light at
870 nm and can make ATP in the transfer of
electrons.
In order to make NADPH, purple bacteria use an
external electron donor (hydrogen, hydrogen sulfide,
sulfur, sulfite, or organic molecules such as succinate
and lactate) to feed electrons into a reverse electron
transport chain.Green Sulfur Bacteria: These bacteria contain a pho
tosystem that is analogous to PS I (P840) in chloro
plasts. It makes ATP through the transfer of electrons.
Electrons are removed from an excited chlorophyll
molecule and used to reduce NAD+ to NADH. The
electrons removed from P840 must be replaced. This
is accomplished by removing electrons from H 2 S,
which is oxidized to sulfur which appear as globules
in the cells (hence the name “green sulfur bacteria”).The Dark or Light-Independent Reactions
of Photosynthesis
The energy rich ATP and NADPH molecules formed
2HO4H4eO 22 → +++− in the light dependent phase of photosynthesis are used0
H S 2 ⇒+2H S.