5: Global Biogeochemical Cycles
- Page ID
- 131138
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In biogeochemical cycles, elements are transformed among multiple oxidation states by biotic and abiotic reactions. In the iron cycle, for example, iron can be transformed back and forth between ferric iron (Fe(III)) and ferrous iron (Fe(II)). Although some reactions in the iron cycle can occur without microbial catalysis, other reactions require it. As such, microbial populations play essential roles in turning the iron cycle, and the same is true for the other cycles we will consider.
Microorganisms mediate reactions in biogeochemical cycles to obtain energy and fix nutrients or for no reason at all. In the latter case, microbes just happen to provide necessary ingredients that allow the reaction to occur. Regardless of why, their activities can drive a wide variety of reactions and thus microorganisms help cycle many elements on a global scale. However, here we focus our efforts on understanding microbial contributions to cycling just four elements – carbon, nitrogen, iron, and sulfur. These elements are among the most abundant redox active elements in Earth’s crust. We discuss the biogeochemical cycling of these elements individually in this chapter, but they are connected in nature. As you read, you might try to find those connections.
This chapter focuses on the microbial contributions to each cycle. In Part 2 of this book (Chapters 6-10), we consider how environments help link those microbial contributions together and turn cycles. In Part 3 (Chapters 11-14), we focus our attention on how these microbial contributions impact their environments.
- 5.1: Carbon cycle
- The different metabolic and catabolic pathways by which microorganisms contribute to the global carbon cycle. Includes discussion of aerobic and anaerobic respiration, fermentation and syntrophy, methanogenesis, acetogenesis, and methanotrophy.
- 5.2: Nitrogen cycle
- Discussion of the microbial nitrogen transformation reactions in the nitrogen cycle: nitrogen fixation, ammonification, nitrification, anaerobic ammonia oxidation (ammox), denitrification, and dissimilatory nitrate reduction to ammonia (DNRA) reactions.
- 5.3: Iron cycle
- Microbial contributions to the global iron cycle. Includes discussion of both ferric and ferrous iron reduction.
- 5.4: Sulfur cycle
- Microbial contributions to the global sulfur cycle. Includes discussion of sulfate reduction, desulfurylation, oxidation of sulfide and intermediate sulfur compounds, reduction of intermediate sulfur compounds, and sulfur disproportionation.