Photosynthesis (World of Microbiology and Immunology)

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 photosynthesis came from the work of Dutch-born microbiologist Cornelius van Neil (1897985). 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 photosynthesis. But, unlike plants, they do not produce oxygen during the photosynthetic process. Furthermore, the bacteria use a compound called bacteriochlorophyll rather than chlorophyll as 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 example, the purple sulfur bacteria use hydrogen sulfide.

Since van Neil's time, the structure of the photosynthetic apparatus has been deduced. The study of photosynthesis is currently an active area of research in biology. Crystals of the photosynthetic reaction center from the anaerobic photosynthetic bacterium Rhodopseudomonas viridis were created 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 consumed during the subsequent dark reactions in the synthesis of carbohydrates.

In algae, the light reactions occur on the so-called thylakoid membranes of the chloroplasts. The thylakoid membranes 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 photophosphorylation, which occurs in the thylakoid membranes of the chloroplast.

Algae are capable of photosynthetic generation of energy. There are many different groups of photosynthetic algae. Like higher plants, they all have chlorophyll-a as a photosynthetic 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. Chlorophyta and Euglenophyta have chlorophyll-a and chlorophyll-b. Chrysophyta, Pyrrophyta, and Phaeophyta have chlorophyll-a and chlorophyll-c. Rhodophyta have chlorophyll-a and chlorophyll-d. The different 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 algae and were once considered members of the plant kingdom. However, unlike the true algae, cyanobacteria are prokaryotes, in that their DNA is not sequestered within a nucleus. Like higher plants, they have chlorophyll-a as a photosynthetic pigment, two photosystems (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 wavelengths of light than chlorophyll and thus increase the wavelength range, which can drive photosynthesis. Phycobilins are also present in the Rhodophyte algae, suggesting a possible evolutionary relationship between these two groups.

Cyanobacteria are the predominant photosynthetic organism in anaerobic fresh and marine water.

Another photosynthetic bacterial group is called cloroxybacteria. This group is represented by a single genus called Prochloron. Like higher plants, Prochloron has chlorophyll-a, chlorophyll-b, and carotenoids as photosynthetic pigments, two photosystems (PS-I and PS-II), and the same overall equation for photosynthesis. Prochloron is 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 bacteriochlorophylls and other photosynthetic pigments that are similar to the chlorophylls used by higher plants. But, in contrast to higher plants, algae and cyanobacteria, the anaerobic photosynthetic bacteria have just one photosystem that is similar to PS-I. These bacteria likely represent a very ancient photosynthetic microbe.

The final photosynthetic bacteria are in the genus Halobacterium. Halobacteria thrive in very salty environments, 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 similar to sensory rhodopsin, the pigment used by humans and other animals for vision. Bacteriorhodopsin and halorhodopsin are embedded in the cell membranes of halobacteria and each pigment consists of retinal, a vitamin-A derivative, bound to a protein. Irradiation of these pigments causes a structural change in their retinal. This is referred to as photoisomerization. Retinal photoisomerization leads to the synthesis of ATP. Halobacteria have two additional rhodopsins, sensory rhodopsin-I and sensory rhodopsin-II. These compounds regulate phototaxis, the directional movement in response to light.

See also Evolutionary origin of bacteria and viruses