10. Microbial Ecology and Applied Microbiology
10.6 Biogeochemical Cycles
Learning Objectives
- Define and describe the importance of microorganisms in the biogeochemical cycles of carbon, nitrogen, and sulphur
- Distinguish between the oxidation reactions and the reduction reactions in the carbon, nitrogen and sulphur cycles
- Explain why the microbes are recognized as the “biological infrastructure” of the planet
Energy flows directionally through ecosystems, entering as sunlight for phototrophs or as inorganic molecules for chemoautotrophs. The six most common elements associated with organic molecules—carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulphur—take a variety of chemical forms and may exist for long periods in the atmosphere, on land, in water, or beneath earth’s surface. Geologic processes, such as erosion, water drainage, the movement of the continental plates, and weathering, all are involved in the cycling of elements on earth. Because geology and chemistry have major roles in the study of this process, the recycling of chemical matter between living organisms and their nonliving environment is called a biogeochemical cycle. Here, we will focus on the function of microorganisms in these cycles, which play roles at each step. The reactions in these cycles are all redox reactions: in one direction, a compound is oxidized, and in the reverse direction, it is reduced. Because some of those reactions are primarily, or exclusively, performed by the microbes, specifically the bacteria and archaea, the microbes are critical to the continued function of the biosphere [1] and have been described as the “biological infrastructure” of the planet.
Carbon Cycle
Carbon is one of the most important elements to living organisms, as shown by its abundance and presence in all organic molecules. The carbon cycle exemplifies the connection between organisms in various ecosystems. Carbon is exchanged between heterotrophs and autotrophs within and between ecosystems, primarily by way of atmospheric CO2, a fully oxidized version of carbon that serves as the basic building block that autotrophs use to build multi-carbon, high-energy organic molecules such as glucose (see Biosynthesis). Photoautotrophs and chemoautotrophs (i.e. the lithotrophs) harness energy from the sun and from inorganic chemical compounds, respectively, to covalently bond carbon atoms together into reduced organic compounds. The potential energy in these molecules can then be accessed through the processes of respiration and fermentation (Figure 10.32). Overall, there is a constant exchange of CO2 between the heterotrophs (which produce CO2 as a result of respiration or fermentation) and the autotrophs (which use the CO2 for fixation).
Reduced one-carbon compounds like methane accumulate in certain anaerobic environments when methanogens use CO2 as a terminal electron acceptor in a complex form of anaerobic respiration (see Methanogens and Syntrophy). These organisms are presen