Environmental Microbiology of Aquatic and Waste Systems

68 4 Taxonomy, Physiology, and Ecology of Aquatic Microorganisms

for the production of energy rich carbohydrates
( sugars) in the Calvin cycle (see Fig. 4.1 1 ).
The fixation or reduction of carbon dioxide is a
light­independent process in which carbon dioxide
combines with a five­carbon sugar, ribulose 1,5­bis­
phosphate (RuBP), to yield two molecules of a three­
carbon compound, glycerate 3­phosphate (GP), also
known as 3­phosphoglycerate (PGA). GP, in the pres­
ence of ATP and NADPH from the light­dependent
stages, is reduced to glyceraldehyde 3­phosphate
(G3P) and enters the citric acid cycle.
The processes of photosynthesis can be represented
by the general formula:

where H 2 A is the source of the reducing power for the
conversion of CO 2 to carbohydrates.
In higher plants, algae and cyanobacteria, where
water is the source of the reducing power photosynthe­
sis, can be represented thus:

In bacteria, other than cyanobacteria, where water
is not the source of reducing power and hence oxygen
is not involved (anoxygenic), for example, the green
sulfur bacteria, where hydrogen sulfide is utilized, the
photosynthetic equation is given thus:

Summary: Differences Between Photosynthesis

in Plants and in the Bacteria

Like green plants, some bacteria are photosynthetic,

using the energy of sunlight to reduce carbon dioxide to

carbohydrate. There are a number of differences between

the two groups which are summarized below:

1. Chlorophyll and bacteriochlorophyll
   Chlorophyll, the photosynthetic pigment in plants,
   is replaced in bacteria by bacteriochlorophyll
   (except in the Cyanobacteria). Both types of
   pigments are similar and differ only in some side
   chains (see Fig. 4.12).


2. Sites for photosynthesis in green plants and bacteria
   In higher plants, photosynthesis takes places in
   membraneous structures known as thylakoids which
   are located in organelles known as chloroplasts. In
   bacteria, the site for photosynthesis varies from one
   group of bacteria to the other. In the cyanobacteria,
   although chloroplasts are absent, photosynthesis
   occurs in thylakoid­like structures; in helicobacte­
   ria, it takes place on the cell membranes; in the
   purple bacteria, it takes place in invaginations of the
   cell membrane; in the green bacteria, it takes place
   on the cell membrane as well as in special membrane
   folding known as chlorosomes.


3. Oxygenic and anoxygenic photosynthesis
   In higher plants, algae and cyanobacteria, the light
   energy excites the molecules of chlorophyll leading
   to release of energy which splits the water molecule
   and to the release of oxygen as a by­product, and
   finally the provision of H for fixing the CO 2. In most

CO 22 +⇒ ++2H A CH O H O 2A,2 2

CO 22 + ⇒ ++2H O CH O H O O .2 2 2

CO 22 + = ++2H S CH O H O 2S.2 2

Fig. 4.13 Wavelengths
of chorophylls and
photosythetic accessory
pigments (Modified from
Photosynthesis: Light
energy transduced to
chemical energy; http://
phototroph.blogspot.
com/2006/11/pigments-
and­absorption­spectra.
html)