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Chapter 9 : Microbes and the Carbon Cycle
Microbes are prominent agents in the recycling of several major chemical elements on Earth, notably oxygen, carbon, nitrogen, and sulfur. The microbes decompose organic matter to obtain energy and/or nutrients for their own multiplication in several ways including fermentation. Numerous species of microbes are engaged in this phase of the carbon cycle, which results in the conversion of organic carbon to CO2. This chapter examines how carbon moves through the cycle. In contrast to animals, which require organic compounds of carbon, plants grow on CO2, the major form of inorganic carbon. The utilization of CO2 by green plants through photosynthesis is the largest chemical process on Earth. Microbes are important agents of much of this carbon atom traffic, which is illustrated in a diagram that indicates only the general outline of the carbon cycle and introduces two new terms: autotroph and heterotroph. Shut off from sunlight, the plants died, and their decomposition was greatly slowed due to shortage of O2 in the muds. The organic matter was consequently converted to peat (partly decayed vegetation), some of which was further transformed to coal. In other words, coal deposits represent huge amounts of modified organic plant materials that have escaped the dynamic carbon cycle. When we burn coal and thereby convert it to CO2 we are accelerating return of carbon to the active cycle. Combustion of oil also restores CO2 to the atmosphere, and there is considerable evidence indicating that oil deposits were formed, in part, by ancient microbial processes.