Microbiology, 2021

8.7 • Biogeochemical Cycles 315
Light-Independent Reactions
After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH
molecules (having lifespans of millionths of a second), photoautotrophs have the fuel needed to build
multicarbon carbohydrate molecules, which can survive for hundreds of millions of years, for long-term
energy storage. The carbon comes from CO 2 , the gas that is a waste product of cellular respiration.
The Calvin-Benson cycle (named for Melvin Calvin [1911–1997] and Andrew Benson [1917–2015]), the
biochemical pathway used for fixation of CO 2 , is located within the cytoplasm of photosynthetic bacteria and in
the stroma of eukaryotic chloroplasts. The light-independent reactions of the Calvin cycle can be organized
into three basic stages: fixation, reduction, and regeneration (see Appendix C for a detailed illustration of the
Calvin cycle).
• Fixation: The enzyme ribulose bisphosphate carboxylase (RuBisCO) catalyzes the addition of a CO 2 to
ribulose bisphosphate (RuBP). This results in the production of 3-phosphoglycerate (3-PGA).
• Reduction: Six molecules of both ATP and NADPH (from the light-dependent reactions) are used to
convert 3-PGA into glyceraldehyde 3-phosphate (G3P). Some G3P is then used to build glucose.
• Regeneration: The remaining G3P not used to synthesize glucose is used to regenerate RuBP, enabling the
system to continue CO 2 fixation. Three more molecules of ATP are used in these regeneration reactions.
The Calvin cycle is used extensively by plants and photoautotrophic bacteria, and the enzyme RuBisCO is said
to be the most plentiful enzyme on earth, composing 30%–50% of the total soluble protein in plant
chloroplasts. 1 However, besides its prevalent use in photoautotrophs, the Calvin cycle is also used by many
nonphotosynthetic chemoautotrophs to fix CO 2 . Additionally, other bacteria and archaea use alternative
systems for CO 2 fixation. Although most bacteria using Calvin cycle alternatives are chemoautotrophic, certain
green sulfur photoautotrophic bacteria have been also shown to use an alternative CO 2 fixation pathway.
CHECK YOUR UNDERSTANDING
• Describe the three stages of the Calvin cycle.
8.7 Biogeochemical Cycles
Learning Objectives
By the end of this section, you will be able to:
• 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 phototrophs or as inorganic molecules
for chemoautotrophs. 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.
1 A. Dhingra et al. “Enhanced Translation of a Chloroplast-Expressed RbcS Gene Restores Small Subunit Levels and Photosynthesis
in Nuclear RbcS Antisense Plants.” Proceedings of the National Academy of Sciences of the United States of America 101 no. 16
(2004):6315–6320.