As they are so minute the mycoplasma at one time were considered to be viruses.
Not all bacteria, however, are that small. Escherichia coli, the most well studied of bacteria have individual cells each about 7 μm long and 2 μm in diameter. A few of bacteria are giants amongst the microbes. Like the Epulopiscium fishelsoni, which lives in the guts of a brown surgeonfish found in the Red Sea, is a giant measuring 80 μm wide and over 600 μm long. That is more than half a millimeter long and visible to the naked eye. But that is not the biggest any more as we now know an even larger bacterium Thiomargarita namibiensis. This bacterium is at least 750 μm long. They often exist as chains of ten or more cells.
The bacteria despite their minute size carry out a vast range of biochemical reactions. Some of them have colored pigments like bacteriochlorophylls and can absorb sunlight. They can photosynthesize. The process very much resembles that in higher organisms like green plants. However, some bacteria photosynthesize but do not produce any oxygen. They learned this function perhaps two billion years ago. Later appeared the bacteria that photosynthesized and did evolve oxygen. http://edmall.gsfc.nasa.gov/aacps/news/P...
Virtually all oxygen in the atmosphere is thought to have come through the process of photosynthesis and bacteria pioneered the most important process for sustenance of life the photosynthesis. Every bit of food each of us eats and every breath each one of us takes has originated from photosynthesis.
Some of the bacteria called the cyanobacteria took up residence inside the cells of plants and as if in returning favor for a favor started making food for the plant. Eventually these cyanobacteria have become chloroplastids, the chlorophyll containing cell organelles of plants. Such a type of mutually beneficial association of two different species of living things is called endosymbiosis.
Take a look at light harvesting complex in the photosynthetic apparatus of purple bacteria http://www.ks.uiuc.edu/Research/bio_ener...
And perform some on line experiments virtually to get to know how scientists learn more about photosynthesis -
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