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Natural Carbon Cycle and Anthropogenic Carbon Cycle

2.1 Definition and General Process

Carbon is an essential part of all life forms on Earth. There are four main carbon pools in the earth system: atmospheric carbon pool, marine carbon pool, terrestrial ecosystem carbon pool, and lithosphere carbon pool. Among them, the lithospheric carbon pool mainly exists in the earth's rocks and its cycle period is the geological age scale, which is up to millions of years. It can be considered that the lithospheric carbon pool is fixed on the scale of hundreds of years, so the natural carbon cycle mainly refers to the cyclic change of carbon in the three-carbon pools of atmospheric carbon pool, marine carbon pool, and terrestrial ecosystem carbon pool. The size of the atmospheric carbon pool is about 700?GtC, which is the smallest carbon pool among the three-carbon pools, but because the atmosphere directly affects human life, the atmospheric carbon pool was the first to attract people's attention. The carbon in the atmospheric carbon pool mainly exists in the form of CO2 gas. The ocean, which accounts for 71% of the earth's surface, is a huge carbon pool with carbon storage of about 38?000?Gt. It is more than 50 times the atmospheric carbon pool and 20 times the terrestrial carbon pool, the largest of the three-carbon pools. The primary forms of carbon in the ocean are dissolved inorganic carbon, dissolved organic carbon, carbonate, particulate organic carbon, etc., of which more than 97% is dissolved in the form of inorganic carbon. The carbon storage of terrestrial ecosystems is about 2000?Gt, of which the carbon storage of living organisms is 600-1000?Gt and the storage of soil organic carbon such as biological residues is about 1200?Gt. The basic process of the terrestrial ecosystem carbon cycle is: CO2 in the atmosphere is solidified into organic carbon through photosynthesis of plants and stored in plants [1,2]. The chemical expression of photosynthesis is formula (Figure 2.1). Part of the organic carbon in plants releases CO2 into the atmosphere through the plant's own respiration (i.e. autotrophic respiration), the consumption of organic carbon by animals, and the decomposition of organic matter by microorganisms (i.e. heterotrophic respiration), forming a terrestrial ecology system carbon cycle process. In short, the natural carbon cycle includes carbon chemical and biological processes and is a comprehensive and complex system. In this article, we focus on the chemical processes of the carbon cycle.

Figure 2.1 Light and dark reactions of photosynthesis.

Source: Dogutan and Nocera [3]. © 2019, American Chemical Society.

2.2 From Inorganic Carbon to Organic Carbon

In the natural carbon cycle, the energy of the sun is used by vegetation, plankton, algae through photosynthesis, along with the recovery of carbon dioxide from the natural environment. In this process, water serves as a source of hydrogen and green chlorophyll as a catalyst, which ultimately creates new plant life. It will eventually be converted into fossil fuels over millions of years [4].

Photosynthesis occurs in the chloroplasts of plant cells and is divided into two stages: the light reaction stage and the dark reaction stage [3]. Under the catalysis of the photosynthetic system II (Photosystem II), water is broken down into electrons, oxygen, and protons (photoreaction, Figure 2.1). Among them, electrons are transported along the electron transfer chain to Photosystem I (Photosystem I) through a series of electron transport substances and participate in the generation of reduced coenzyme II (triphosphopyridine nucleotide, nicotinamide adenine dinucleotide phosphate [NADPH]), which is used to fix carbon dioxide in the subsequent dark cycle (dark reaction, Figure 2.1). The whole process involves not only the synthesis and decomposition of various substances but also various energy conversions: from light energy to electrical energy, to biological energy, and finally to chemical energy. This series of processes achieves energy conversion and cleverly fixes solar energy into green plants.

The hydrogen evolution reaction (HER) is a simple 2e-/2H+ process, which is kinetically easier than reducing the multi-electron/multi-proton of carbon dioxide to biomass. Therefore, systems that directly reduce carbon dioxide in water face the challenge of suppressing HER with greater kinetic advantages and competitiveness. Photosynthesis does not use H2O but uses H2 equivalent (NADPH/H+) to reduce CO2, thereby avoiding the HER during CO2 reduction. However, photosynthesis is relatively inefficient in converting solar energy in the form of sugar, cellulose, lignin, etc. into chemical energy. In spite of some green plants enable about 8% of solar energy to be converted into biomass, the photosynthetic efficiency of most crops is usually limited to 0.5-2%. Substances produced by photosynthesis can eventually be transformed into fossil fuels under anaerobic conditions through the decay of animals and plants. Therefore, fossil fuels are also often regarded as stored fossil solar energy. However, the natural circulation of regenerated fossil fuels requires certain special conditions to proceed, and this process is prolonged.

Carbon dioxide can be used as the only carbon source for many microorganisms found in the microbial world. There are several ways and solutions for organisms to reduce carbon dioxide to organic carbon. The most important one is Calvin reducing the pentose phosphate pathway. Throughout the evolutionary process, the immobilization, reduction, and reconstruction of CO2 receptor molecules will be involved. Therefore, some autotrophic bacteria, almost all photosynthetic bacteria, eukaryotic algae, and some prokaryotes and green plants absorb carbon dioxide through the Calvin cycle. Reactions unique to this pathway are catalyzed by ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase, and phosphoribulokinase (PRK). In some cases, there is a distinct sedoheptulose 1,7-bisphosphatase, separate from fructose 1,6-bisphosphatase, that may be considered unique to the Calvin cycle [5]. Although the Calvin reductive pentose phosphate pathway is the major assimilatory path used in the biosphere, many autotrophic species fix CO2 by different routes. In particular, the acetogenic bacteria and the methanogens reduce CO2 to acetate (and other short-chain fatty acids) or methane, respectively. The green photosynthetic bacteria appear to be unusual among photosynthetic organisms in not using the Calvin cycle to reduce CO2.

2.3 From Organic Carbon to Inorganic Carbon

Carbon dioxide gas is part of the atmosphere (about 0.03% of the total volume of the atmosphere) and is abundant in nature. The main ways of producing carbon dioxide in nature are: the respiration of plants and animals converts part of the organic carbon taken into the body into carbon dioxide and releases it into the atmosphere, and the other part constitutes the organism's body or is stored in the body. After the death of animals and plants, the organic carbon in the debris is also converted into carbon dioxide by the decomposition of microorganisms and finally discharged into the atmosphere. The cycle of carbon dioxide in the atmosphere takes about 20?years.

2.4 Anthropogenic Carbon Cycle

By imitating the natural carbon cycle, humans have developed an artificial model to utilize CO2 present in the atmosphere and water (Figure 2.2) [6]. The anthropogenic carbon cycle is based on the capture, and where needed, temporary storage of CO2 followed by its chemical/physical recycling (carbon capture and recycling) into products. Compared with the natural carbon cycle, the anthropogenic carbon cycle can be industrially scaled to produce sustainable, renewable, and environmentally friendly carbon sources. At the same time, human-made carbon dioxide recycling can reduce the harmful effects of excess carbon dioxide in the atmosphere in the global environment [7]. Therefore, carbon dioxide should be regarded as a valuable industrial C1 raw material, not just a greenhouse gas that harms the global environment.

Figure 2.2 Sustainable anthropogenic recycling of atmospheric CO2 to fuels and materials. CCR, carbon capture and recycling; CCS, carbon capture and storage; DME, dimethoxyethane.

Source: Goeppert et al. [6]. © 2013, Newslands Press Ltd.

2.4.1 Anthropogenic Carbon Emissions

Anthropogenic carbon emissions refer to various forms of carbon emissions (mainly carbon dioxide) resulted from human activities. Human activities that increase net emissions include but are not limited to the burning of fossil fuels, logging of forests, changes in land-use patterns, livestock, and fertilizers. The concentration of CO2 in the atmosphere has kept steadily increasing in the last 60?years, rising to 416?ppm from 315?ppm [8]. Global CO2 emissions from fossil fuel combustion and industrial activities have increased every 10?years, from an average of 11.4?Gt CO2 per year in the 1960s to an average of 34.7?±?2?Gt CO2 per year from 2009 to 2018. Emissions in 2018 reached a record 36.6?±?2?Gt CO2;...