8.7 Biogeochemical cycles

Learning objectives

• Define and describe the importance of microorganisms in the biogeochemical cycles of carbon, nitrogen, and sulfur
• Define and give an example of bioremediation

Energy flows directionally through ecosystems, entering as sunlight for phototroph s or as inorganic molecules for chemoautotroph s. The six most common elements associated with organic molecules—carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—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 inorganic 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, most frequently interconverting oxidized versions of molecules with reduced ones.

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 heterotroph s and autotroph s within and between ecosystems primarily by way of atmospheric CO ₂ , a fully oxidized version of carbon that serves as the basic building block that autotrophs use to build multicarbon, high-energy organic molecules such as glucose. Photoautotrophs and chemoautotrophs harness energy from the sun and from inorganic chemical compounds, respectively, to covalently bond carbon atoms together into reduced organic compounds whose energy can be later accessed through the processes of respiration and fermentation ( [link] ).

Overall, there is a constant exchange of CO ₂ between the heterotrophs (which produce CO ₂ as a result of respiration or fermentation) and the autotrophs (which use the CO ₂ for fixation). Autotrophs also respire or ferment, consuming the organic molecules they form; they do not fix carbon for heterotrophs, but rather use it for their own metabolic needs.

Bacteria and archaea that use methane as their carbon source are called methanotroph s. Reduced one-carbon compounds like methane accumulate in certain anaerobic environments when CO ₂ is used as a terminal electron acceptor in anaerobic respiration by archaea called methanogen s. Some methanogens also ferment acetate (two carbons) to produce methane and CO ₂ . Methane accumulation due to methanogenesis occurs in both natural anaerobic soil and aquatic environments; methane accumulation also occurs as a result of animal husbandry because methanogens are members of the normal microbiota of ruminants. Environmental methane accumulation due to methanogenesis is of consequence because it is a strong greenhouse gas, and methanotrophs help to reduce atmospheric methane levels.