Bacteria are the most primitive and oldest kinds of organism on earth. They first appeared in the fossil record some 3.5 Ga ago and they have continued to play a major role in earth surface processes.
Bacterial cells are extremely small cells, generally less than 1 |im in diameter. They may be single or colonial; the latter enclosed within a mucilaginous sheath called a capsule. Many bacterial cells bear a whip-like thread (flagellum) and some contain chlorophyll pigments for photosynthesis.
Bacteria are important in the formation of microbial sediments, such as bacterial mats and stromatolites, iron and manganese ores, carbonate concretions, sulphide and sulphate minerals. They also yield important information about early evolution of the cell, and the history of photosynthesis and biogeo-chemical cycles.
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