Photosynthesis WORLD OF MICROBIOLOGY AND IMMUNOLOGY436•
PPhotosynthesisHOTOSYNTHESIS
Photosynthesis is the biological conversion of light energy
into chemical energy. This occurs in green plants, algae, and
photosynthetic bacteria.
Much of the early knowledge of bacterial photosynthe-
sis came from the work of Dutch-born microbiologist
Cornelius van Neil(1897–1985). During his career at the
Marine Research Station in Monterey, California, van Neil
studied photosynthesis in anaerobic bacteria. Like higher
plants, these bacteria manufacture carbohydrates during pho-
tosynthesis. But, unlike plants, they do not produce oxygen
during the photosynthetic process. Furthermore, the bacteria
use a compound called bacteriochlorophyll rather than chloro-
phyllas a photosynthetic pigment. Van Neil found that all
species of photosynthetic bacteria require a compound that the
bacteria can oxidize (i.e., remove an electron from). For exam-
ple, the purple sulfur bacteria use hydrogen sulfide.
Since van Neil’s time, the structure of the photosyn-
thetic apparatus has been deduced. The study of photosynthe-
sis is currently an active area of research in biology. Crystals
of the photosynthetic reaction center from the anaerobic pho-
tosynthetic bacterium Rhodopseudomonas viridiswere cre-
ated in the 1980s by Hartmut Michel and Johann Deisenhofer,
who then used x-ray crystallography to determine the three-
dimensional structure of the photosynthetic protein. In 1988,
the two scientists shared the Nobel Prize in Chemistry with
Robert Huber for this research.
Photosynthesis consists of two series of biochemical
reactions, called the light reactions and the dark reactions. The
light reactions use the light energy absorbed by chlorophyll to
synthesize structurally unstable high-energy molecules. The
dark reactions use these high-energy molecules to manufacture
carbohydrates. The carbohydrates are stable structures that can
be stored by plants and by bacteria. Although the dark reactions
do not require light, they often occur in the light because they
are dependent upon the light reactions. In higher plants and
algae, the light and dark reactions of photosynthesis occur in
chloroplasts, specialized chlorophyll-containing intracellular
structures that are enclosed by double membranes.
In the light reactions of photosynthesis, light energy
excites photosynthetic pigments to higher energy levels and
this energy is used to make two high energy compounds,
ATP (adenosine triphosphate) and NADPH ( nicotinamide
adenine dinucleotide phosphate). ATP and NADPH are con-
sumed during the subsequent dark reactions in the synthesis
of carbohydrates.
In algae, the light reactions occur on the so-called thy-
lakoid membranes of the chloroplasts. The thylakoid mem-
branes are inner membranes of the chloroplasts. These
membranes are arranged like flattened sacs. The thylakoids
are often stacked on top of one another, like a roll of coins.
Such a stack is referred to as a granum. ATP can also be made
by a special series of light reactions, referred to as cyclic pho-
tophosphorylation, which occurs in the thylakoid membranes
of the chloroplast.
Algae are capable of photosynthetic generation of
energy. There are many different groups of photosyntheticalgae. Like higher plants, they all have chlorophyll-a as a pho-
tosynthetic pigment, two photosystems (PS-I and PS-II), and
the same overall chemical reactions for photosynthesis. Algae
differ from higher plants in having different complements of
additional chlorophylls. Chlorophytaand Euglenophytahave
chlorophyll-a and chlorophyll-b. Chrysophyta, Pyrrophyta,
and Phaeophyta have chlorophyll-a and chlorophyll-c.
Rhodophytahave chlorophyll-a and chlorophyll-d. The differ-
ent chlorophylls and other photosynthetic pigments allow
algae to utilize different regions of the solar spectrum to drive
photosynthesis.
A number of photosynthetic bacteria are known. One
example are the bacteria of the genus Cyanobacteria. These
bacteria were formerly called the blue-green algaeand were
once considered members of the plant kingdom. However,
unlike the true algae, cyanobacteria are prokaryotes, in that their
DNAis not sequestered within a nucleus. Like higher plants,
they have chlorophyll-a as a photosynthetic pigment, two pho-
tosystems (PS-I and PS-II), and the same overall equation for
photosynthesis (equation 1). Cyanobacteria differ from higher
plants in that they have additional photosynthetic pigments,
referred to as phycobilins. Phycobilins absorb different wave-
lengths of light than chlorophyll and thus increase the wave-
length range, which can drive photosynthesis. Phycobilins are
also present in the Rhodophyte algae, suggesting a possible evo-
lutionary relationship between these two groups.
Cyanobacteria are the predominant photosynthetic
organism in anaerobic fresh and marine water.
Another photosynthetic bacterial group is called clorox-
ybacteria. This group is represented by a single genus called
Prochloron. Like higher plants, Prochloronhas chlorophyll-a,
chlorophyll-b, and carotenoids as photosynthetic pigments,
two photosystems (PS-I and PS-II), and the same overall equa-
tion for photosynthesis. Prochloronis rather like a free-living
chloroplast from a higher plant.
Another group of photosynthetic bacteria are known as
the purple non-sulfur bacteria (e.g., Rhodospirillum rubrum.
The bacteria contain bacteriochlorophyll a or b positioned on
specialized membranes that are extensions of the cytoplasmic
membrane.
Anaerobic photosynthetic bacteria is a group of bacteria
that do not produce oxygen during photosynthesis and only
photosynthesize in environments that are devoid of oxygen.
These bacteria use carbon dioxide and a substrate such as
hydrogen sulfide to make carbohydrates. They have bacteri-
ochlorophylls and other photosynthetic pigments that are sim-
ilar to the chlorophylls used by higher plants. But, in contrast
to higher plants, algae and cyanobacteria, the anaerobic pho-
tosynthetic bacteria have just one photosystem that is similar
to PS-I. These bacteria likely represent a very ancient photo-
synthetic microbe.
The final photosynthetic bacteria are in the genus
Halobacterium. Halobacteria thrive in very salty environ-
ments, such as the Dead Sea and the Great Salt Lake.
Halobacteria are unique in that they perform photosynthesis
without chlorophyll. Instead, their photosynthetic pigments are
bacteriorhodopsin and halorhodopsin. These pigments are sim-
ilar to sensory rhodopsin, the pigment used by humans andwomi_P 5/7/03 11:09 AM Page 436