Calcium Oxalate Formation Sequesters Carbon Dioxide

multiple tall saguaro cactus plants seen flowering in an Arizona landscape

Biological Strategy

Saguaro

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Living systems often require chemical elements and chemical compounds, including complex sugars, proteins, and odor-making compounds, to perform critical activities. These compounds exist in various states–solid, liquid, and gas–and are ubiquitous in soil, water, and air. This requires that living systems not only have ways to capture, absorb, or filter them, but also ways to differentiate among them, selecting those that are valuable or harmful. For example, mangrove trees live with their roots in salty water and sediments. Various mangrove species have different strategies for removing salt from the water they take in so that their tissues can use the fresh water.

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Gases of particular importance to living systems are oxygen, carbon dioxide, and nitrogen. Oxygen and carbon dioxide are involved in respiration, so distributing these gases efficiently and effectively is important for a living system’s survival. However, gases are difficult to contain because they disperse easily. To accommodate this, living systems have strategies for confining gases and using gases’ properties to their advantage. For example, prairie dogs and mound-building termites build systems of tunnels and mounds that take advantage of wind to ventilate their underground homes.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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Cacti sequester atmospheric carbon dioxide by converting it to oxalate and combining it with soil-derived calcium ions which ultimately lead to the formation of solid calcium carbonate.

Through the process of , plants remove carbon dioxide from the atmosphere and use it to build all the carbon-based compounds it needs for structure and function. When most plants die, these carbon-based compounds break down into their constituent components with a re-release of carbon dioxide back into the atmosphere. Saguaro cactus uses some of the carbon dioxide it removes from the atmosphere to make compounds called oxalates which combine with calcium ions taken up from the soil by the plants roots. The resulting calcium oxalate takes a different path following the death of the cactus. Rather than degrade to its constituent components, calcium oxalate slowly transforms into solid calcium carbonate (calcite), thus essentially sequestering atmospheric carbon dioxide into the soil.

Last Updated July 20, 2017

References

“Cacti contain large quantities of Ca Oxalate biominerals, with C derived from atmospheric CO₂. Their death releases these biominerals into the environment, which subsequently transforms to calcite via a monohydrocalcite intermediate…Calcium oxalates form in plants from soil-derived Ca and biologically synthesized oxalate. The C in the oxalates forms from atmospheric CO₂ (C_atm) via a series of complex biochemical pathways…A large [Carnegiea] gigantea contains on the order of 1×10⁵ g of the Ca oxalate weddellite—CaC₂O₄·2H₂O. In areas with high C. gigantea density, there is an estimated 40 g C_atm m⁻² sequestered in Ca oxalates. Following the death of the plant, the weddellite transforms to calcite on the order to 10–20 years. In areas with high saguaro density, there is an estimated release of up to 2.4 g calcite m⁻² year⁻¹ onto the desert soil. Similar transformation mechanisms occur with the Ca oxalates that are abundant in the majority of cacti. Thus, the total atmospheric C returned to the soil of areas with a high number density of cacti is large, suggesting that there may be a significant long-term accumulation of atmospheric C in these soils derived from Ca oxalate biominerals. These findings demonstrate that plant decay in arid environments may have locally significant impacts on the Ca and inorganic C cycles.” (Garvie 2006:114)

“The source of the Ca in the saguaro is from water uptake by the roots of the rhizosphere. The Ca enters the soil solution as a result of abiotic and biotic dissolution of Ca-bearing minerals such as calcite, feldspars, and micas. While calcite is undoubtedly a significant source of Ca for the saguaro, the importance of other Ca-bearing minerals cannot be ignored…Organic acids produced by soil microorganisms and plant roots greatly enhance the dissolution of soil minerals, making essential plant nutrients available for transport by the xylem.” (Garvie 2006:117)

Journal article

Decay of cacti and carbon cycling

Naturwissenschaften | 01/02/2006 | Laurence A. J. Garvie

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