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7.3: The Carbon Cycle

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    Carbon, just like all other elements, cycles through the environment and is constantly in the process of changing forms and locations. In this section, as in many other pieces of scientific literature, we will periodically refer to carbon by its chemical symbol, C. There is no new carbon in the world, rather, all carbon is continuously recycled from one form to another. All plants, animals (including humans!), fungi, bacteria, and archaea are made of mostly carbon-based molecules such as lipids, carbohydrates, proteins, and nucleic acids. Carbon is also prevalent in soils, rocks and sediments, water bodies (dissolved), and the atmosphere. These locations where carbon resides are known as pools or reservoirs, and the processes that move carbon from one location to another are called fluxes. Figure 7.1.1 shows a simplified version of the global carbon cycle.

    Screenshot (85).pngFigure \(\PageIndex{1}\): A simplified carbon cycle. Diagram adapted from U.S. DOE, Biological and Environmental Research Information System.

    Some reservoirs hold on to carbon for only a short time. Aerobic (oxygen-using) organisms convert carbohydrates created by other organisms into carbon dioxide (CO2) almost instantaneously, which they exhale into the atmosphere. When considering the flux of respiration, living organisms are the source of carbon, and the atmosphere is the sink. The carbon stays in the reservoir of living organisms for a relatively short time, depending on their life span, from hours and days to years and decades. In contrast, the residence time of carbon in the fossil pool is dramatically different. Fossil fuels form over a course of 300-400 million years, forming from ancient plants and animals that decomposed slowly under very specific, anaerobic (without oxygen) conditions in wetland environments. Their bodies were gradually transformed by the heat and pressure of the Earth’s crust into the fossil fuels that we mine today to provide petroleum oil, natural gas, and coal (see more on this in chapter 4).

    Reservoirs and fluxes of importance

    The two largest reservoirs of carbon on Earth are the oceans, which cover the majority of Earth’s surface, and the lithosphere (the mineral fraction of Earth: soils, rocks, and sediments). Each of these reservoirs holds more carbon than all of the other reservoirs combined. Much of the carbon stored in these reservoirs, especially deep in the lithosphere or in deep ocean environments, has an extremely long residence time, and does not actively participate in rapid fluxes. The notable exceptions here, of course, are fossil fuels, which are mined by humans and converted into gaseous forms of carbon through combustion.

    Biomass, which is biological material derived from living, or recently living organisms, is a much smaller reservoir of carbon. The amount of carbon stored in all of the terrestrial vegetation (550 Gt C) (Gt = gigatonne = 109 metric tons = 1015 g) is just a fraction of that stored in the oceans (38,000 Gt C) and lithosphere (18,000 Gt C). All of the carbon that is currently stored in all of the vegetation on Earth got there through the process of photosynthesis. Plants and other photosynthetic organisms are called primary producers, because they “fix” atmospheric CO2 into organic carbon, such as sugar, a form that is usable by animals and other organisms that need to consume their carbon molecules.

    Photosynthetic organisms, such as plants, algae, and cyanobacteria, bring in CO2 from the atmosphere and, using energy from the sun, convert CO2 and water into glucose molecules (organic carbon). The products of photosynthesis are oxygen and glucose (Equation \(\PageIndex{1}\)). These glucose molecules are simple sugars that autotrophs (“self-feeders”) can “burn” for energy, or transform into other usable carbon molecules through the process of cellular respiration (described in the next paragraph), or to build plant biomass. Photosynthesis takes place in organelles called chloroplasts, shown in Figure \(\PageIndex{2}\). Photosynthesis accounts for 123 Gt of C per year that is removed from the atmosphere and stored in plant biomass. Such a massive amount of photosynthesis occurs on Earth that no other single flux moves as much carbon in the same timeframe.

    \[ 6 CO_{2} + 6H_{2}O + \text {solar energy} \rightarrow C_{6}H_{12}O_{6} + 6O_{2}\]

    Screenshot (86).pngFigure \(\PageIndex{2}\): Chloroplasts visible in freshwater algae. Chloroplasts are green in color due to the chlorophyll a they contain, and are the site of photosynthesis. Chlorophyll a is the green pigment that allows plants, algae, and cyanobacteria to absorb the energy they need for photosynthesis from sunlight. a) Closterium moniliferum Ralfs, (Chlorophyta) green coccoid algae; b) Botryococcus braunii Kützing, (Chlorophyta) green coccoid algae with discoid chloroplasts. Image credit: K. Manoylov, Lake Sinclair, GA

    Biomass in the carbon cycle, including plants and animals, is the reservoir of carbon that we are most likely most familiar with, and also the reservoir that is most readily available to us. We all participate in the flux of consumption of carbon when we eat food. All of our food is simply plant and/or animal biomass. Our body takes the carbon molecules contained in this biomass, and uses them, along with the oxygen we breathe in, for cellular respiration to create the adenosine triphosphate (ATP) we need for energy. The products of cellular respiration include the CO2 we exhale, water, and energy that is stored in ATP (Equation \(\PageIndex{2}\)). Our bodies also builds additional biomass out of the carbon molecules in this food, allowing us to create new cells for growth or replenishment. This is the only way we, and all other heterotrophs (“other-eaters”), can bring in the carbon we need to build and maintain our bodies. Remember, you are what you eat!

    \[ C_{6}H_{12}O_{6} + CO_{2} \rightarrow 6CO_{2} + 6H_{2}O + \text {energy} \]

    Cellular respiration is an important flux in the carbon cycle, and one that contributes carbon to the atmosphere. Remember that animals and other heterotrophs complete cellular respiration using the carbon molecules that they bring in through their food. Plants and other photosynthetic autotrophs complete cellular respiration using the carbon molecules they formed from CO2 through photosynthesis. Any carbon molecules that are left over after the organism has acquired sufficient energy through cellular respiration make up the biomass of the plant. As plants and animals die and decompose, their bodies are consumed by decomposer organisms such as fungi and bacteria. Through the flux of decomposition, some decaying biomass is converted into atmospheric carbon by the decomposers, while most of the biomass is buried into the soil, contributing to soil carbon. In oxygen-rich environments, decomposers rapidly consume dead and decaying biomass using the same process of aerobic cellular respiration described above. In oxygen-deficient environments, decomposers complete other metabolic pathways, and very slowly consume the organic matter. Some of the gases produced from anaerobic decomposition include methane (CH4), nitrous oxide (N2O), and the foul-smelling hydrogen sulfide (H2S).

    The biomass reservoir of the carbon cycle is also important to us as a source of energy. Through the flux of combustion, we convert the potential energy held in biomass into heat energy that we can use, and release carbon dioxide in the process. If you have ever burned logs on a campfire, or even burned food on the stove, you have completed this flux of biomass combustion. Of course, this happens naturally as well, the best example being natural forest fires caused by lightning strikes. The chemical reaction for combustion is identical to the chemical reaction for cellular respiration. The difference is that in cellular respiration, energy is released in a controlled fashion, and captured in ATP molecules. In combustion, all of this energy is released rapidly in the form of light and heat.

    As all of the fluxes we’ve discussed so far involve the atmosphere, we have not yet discussed the flux that connects the atmosphere to the oceans. Carbon can enter the oceans through two primary fluxes: first through photosynthesis by algae or cyanobacteria (also called phytoplankton in Figure \(\PageIndex{1}\)), and second through the chemical reaction of ocean-atmosphere exchange. The ocean, as with all surface water bodies, always contains some dissolved CO2. This CO2 is in equilibrium with the CO2 in the air. Some atmospheric CO2 is constantly dissolving into the ocean, while some dissolved CO2 is constantly diffusing into the atmosphere. Under normal conditions, these two fluxes will be happening at equal rates. As you can see in Figure \(\PageIndex{1}\), however, this is no longer the case. In the section Human impacts on the carbon cycle, we will discuss why this is the case.

    Activity: Better understanding the carbon cycle

    To further review the carbon cycle, and better understand the human impacts on it, use this interactive graphic from Woods Hole laboratories: As you will see, the information described in this text is only a small portion of the total carbon cycle on Earth. Finally, complete Table \(\PageIndex{1}\) as a way to review the sink/source relationship within this cycle. See if you can correctly identify the source and sink of carbon for each of these important fluxes in the carbon cycle.

    Table \(\PageIndex{1}\): Practice understanding the sink/source relationship with cycles
    Carbon flux Carbon source Carbon sink
    Cellular respiration Carbohydrates in living organisms CO2 in the atmosphere
    Ocean/atmosphere exchange    
    Fossil fuel formation