Biological carbon fixation

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Biological_carbon_fixation

Cyanobacteria such as these carry out photosynthesis. Their emergence foreshadowed the evolution of many photosynthetic plants and oxygenated Earth's atmosphere.

Biological carbon fixation or сarbon assimilation is the process by which inorganic carbon (particularly in the form of carbon dioxide) is converted to organic compounds by living organisms. The compounds are then used to store energy and as structure for other biomolecules. Carbon is primarily fixed through photosynthesis, but some organisms use a process called chemosynthesis in the absence of sunlight.

Organisms that grow by fixing carbon are called autotrophs, which include photoautotrophs (which use sunlight), and lithoautotrophs (which use inorganic oxidation). Heterotrophs are not themselves capable of carbon fixation but are able to grow by consuming the carbon fixed by autotrophs or other heterotrophs. "Fixed carbon", "reduced carbon", and "organic carbon" may all be used interchangeably to refer to various organic compounds. Chemosynthesis is carbon fixation driven by chemical energy, rather than from sunlight. Sulfur- and hydrogen-oxidizing bacteria often use the Calvin cycle or the reductive citric acid cycle.

Net vs. gross CO₂ fixation

Graphic showing net annual amounts of CO₂ fixation by land and sea-based organisms.

The primary form of inorganic carbon that is fixed is carbon dioxide (CO₂). It is estimated that approximately 250 billion tons of carbon dioxide are converted by photosynthesis annually. The majority of the fixation occurs in terrestrial environments, especially the tropics. The gross amount of carbon dioxide fixed is much larger since approximately 40% is consumed by respiration following photosynthesis. Historically it is estimated that approximately 2×10¹¹ billion tons of carbon has been fixed since the origin of life.

Overview of pathways

Seven autotrophic carbon fixation pathways are known. The Calvin cycle fixes carbon in the chloroplasts of plants and algae, and in the cyanobacteria. It also fixes carbon in the anoxygenic photosynthesis in one type of Pseudomonadota called purple bacteria, and in some non-phototrophic Pseudomonadota.

Of the five other autotrophic pathways, two are known only in bacteria (the reductive citric acid cycle and the 3-hydroxypropionate cycle), two only in archaea (two variants of the 3-hydroxypropionate cycle), and one in both bacteria and archaea (the reductive acetyl CoA pathway).

List of pathways

Calvin cycle

The Calvin cycle accounts for 90% of biological carbon fixation. Consuming ATP and NADPH, the Calvin cycle in plants accounts for the preponderance of carbon fixation on land. In algae and cyanobacteria, it accounts for the preponderance of carbon fixation in the oceans. The Calvin cycle converts carbon dioxide into sugar, as triose phosphate (TP), which is glyceraldehyde 3-phosphate (GAP) together with dihydroxyacetone phosphate (DHAP):

3 CO₂ + 12 e⁻ + 12 H⁺ + P_i → TP + 4 H₂O

An alternative perspective accounts for NADPH (source of e⁻) and ATP:

3 CO₂ + 6 NADPH + 6 H⁺ + 9 ATP + 5 H₂O → TP + 6 NADP⁺ + 9 ADP + 8 P_i

The formula for inorganic phosphate (P_i) is HOPO₃²⁻ + 2H⁺. Formulas for triose and TP are C₂H₃O₂-CH₂OH and C₂H₃O₂-CH₂OPO₃²⁻ + 2H⁺

Reverse Krebs cycle

The reverse Krebs cycle, also known as reverse TCA cycle (rTCA) or reductive citric acid cycle, is an alternative to the standard Calvin-Benson cycle for carbon fixation. It has been found in strict anaerobic or microaerobic bacteria (as Aquificales) and anaerobic archea. It was discovered by Evans, Buchanan and Arnon in 1966 working with the photosynthetic green sulfur bacterium Chlorobium limicola. In particular, it is one of the most used pathways in hydrothermal vents by the Campylobacterota. This feature is very important in oceans. Without it, there would be no primary production in aphotic environments, which would lead to habitats without life. So this kind of primary production is called "dark primary production".

The cycle involves the biosynthesis of acetyl-CoA from two molecules of CO₂. The key steps of the reverse Krebs cycle are:

• Oxaloacetate to malate, using NADH + H⁺

  ${\displaystyle \text{Oxaloacetate}+\text{NADH}/{\text{H}}^{+}⟶\text{Malate}+{\text{NAD}}^{+}}$

• Fumarate to succinate, catalyzed by an oxidoreductase, Fumarate reductase

  ${\displaystyle \text{Fumarate}+{\text{FADH}}_2^{}\stackrel{-\rightharpoonup }{\leftharpoondown -}\text{Succinate}+\text{FAD}}$

• Succinate to succinyl-CoA, an ATP dependent step

  ${\displaystyle \text{Succinate}+\text{ATP}+\text{CoA}⟶\text{Succinyl}-\text{CoA}+\text{ADP}+\text{Pi}}$

• Succinyl-CoA to alpha-ketoglutarate, using one molecule of CO₂

  ${\displaystyle \text{Succinyl}-\text{CoA}+{\text{CO}}_2^{}+\text{Fd}(\text{red})⟶\text{alpha}-\text{ketoglutarate}+\text{Fd}(\text{ox})}$

• Alpha-ketoglutarate to isocitrate, using NADPH + H⁺ and another molecule of CO₂

  ${\displaystyle \text{Alpha}-\text{ketoglutarate}+{\text{CO}}_2^{}+\text{NAD}(\text{P})\text{H}/{\text{H}}^{+}⟶\text{Isocitrate}+\text{NAD}{(\text{P})}^{+}}$

• Citrate converted into oxaloacetate and acetyl-CoA, this is an ATP dependent step and the key enzyme is the ATP citrate lyase

  ${\displaystyle \text{Citrate}+\text{ATP}+\text{CoA}⟶\text{Oxaloacetate}+\text{Acetyl}-\text{CoA}+\text{ADP}+\text{Pi}}$

This pathway is cyclic due to the regeneration of the oxaloacetate.

The bacteria Gammaproteobacteria and Riftia pachyptila switch from the Calvin-Benson cycle to the rTCA cycle in response to concentrations of H₂S.

Reductive acetyl CoA pathway

The reductive acetyl CoA pathway (CoA) pathway, also known as the Wood-Ljungdahl pathway uses CO₂ as electron acceptor and carbon source, and H₂ as an electron donor to form acetic acid. This metabolism is wide spread within the phylum Bacillota, especially in the Clostridia.

The pathway is also used by methanogens, which are mainly Euryarchaeota, and several anaerobic chemolithoautotrophs, such as sulfate-reducing bacteria and archaea. It is probably performed also by the Brocadiales, an order of Planctomycetota that oxidize ammonia in anaerobic condition. Hydrogenotrophic methanogenesis, which is only found in certain archaea and accounts for 80% of global methanogenesis, is also based on the reductive acetyl CoA pathway.

The Carbon Monoxide Dehydrogenase/Acetyl-CoA Synthase is the oxygen-sensitive enzyme that permits the reduction of CO₂ to CO and the synthesis of acetyl-CoA in several reactions.

One branch of this pathway, the methyl branch, is similar but non-homologous between bacteria and archaea. In this branch happens the reduction of CO₂ to a methyl residue bound to a cofactor. The intermediates are formate for bacteria and formyl-methanofuran for archaea, and also the carriers, tetrahydrofolate and tetrahydropterins respectively in bacteria and archaea, are different, such as the enzymes forming the cofactor-bound methyl group.

Otherwise, the carbonyl branch is homologous between the two domains and consists of the reduction of another molecule of CO₂ to a carbonyl residue bound to an enzyme, catalyzed by the CO dehydrogenase/acetyl-CoA synthase. This key enzyme is also the catalyst for the formation of acetyl-CoA starting from the products of the previous reactions, the methyl and the carbonyl residues.

This carbon fixation pathway requires only one molecule of ATP for the production of one molecule of pyruvate, which makes this process one of the main choice for chemolithoautotrophs limited in energy and living in anaerobic conditions.

3-Hydroxypropionate bicycle

The 3-Hydroxypropionate bicycle, also known as 3-HP/malyl-CoA cycle, discovered only in 1989, is utilized by green non-sulfur phototrophs of Chloroflexaceae family, including the maximum exponent of this family Chloroflexus auranticus by which this way was discovered and demonstrated. The 3-Hydroxipropionate bicycle is composed of two cycles and the name of this way comes from the 3-Hydroxyporopionate which corresponds to an intermediate characteristic of it.

The first cycle is a way of synthesis of glyoxylate. During this cycle, two equivalents of bicarbonate are fixed by the action of two enzymes: the Acetyl-CoA carboxylase catalyzes the carboxylation of the Acetyl-CoA to Malonyl-CoA and Propionyl-CoA carboxylase catalyses the carboxylation of propionyl-CoA to methylamalonyl-CoA. From this point a series of reactions lead to the formation of glyoxylate which will thus become part of the second cycle.

In the second cycle, glyoxylate is approximately one equivalent of propionyl-CoA forming methylamalonyl-CoA. This, in turn, is then converted through a series of reactions into citramalyl-CoA. The citramalyl-CoA is split into pyruvate and Acetyl-CoA thanks to the enzyme MMC lyase. At this point the pyruvate is released, while the Acetyl-CoA is reused and carboxylated again at Malonyl-CoA thus reconstituting the cycle.

A total of 19 reactions are involved in 3-hydroxypropionate bicycle and 13 multifunctional enzymes are used. The multifunctionality of these enzymes is an important feature of this pathway which thus allows the fixation of three bicarbonate molecules.

It is a very expensive pathway: 7 ATP molecules are used for the synthesis of the new pyruvate and 3 ATP for the phosphate triose.

An important characteristic of this cycle is that it allows the co-assimilation of numerous compounds making it suitable for the mixotrophic organisms.

Cycles related to the 3-hydroxypropionate cycle

A variant of the 3-hydroxypropionate cycle was found to operate in the aerobic extreme thermoacidophile archaeon Metallosphaera sedula. This pathway is called the 3-hydroxypropionate/4-hydroxybutyrate cycle.

Yet another variant of the 3-hydroxypropionate cycle is the dicarboxylate/4-hydroxybutyrate cycle. It was discovered in anaerobic archaea. It was proposed in 2008 for the hyperthermophile archeon Ignicoccus hospitalis.

enoyl-CoA carboxylases/reductases

CO₂ fixation is catalyzed by enoyl-CoA carboxylases/reductases.^[22]

Non-autotrophic pathways

Although no heterotrophs use carbon dioxide in biosynthesis, some carbon dioxide is incorporated in their metabolism. Notably pyruvate carboxylase consumes carbon dioxide (as bicarbonate ions) as part of gluconeogenesis, and carbon dioxide is consumed in various anaplerotic reactions.

6-phosphogluconate dehydrogenase catalyzes the reductive carboxylation of ribulose 5-phosphate to 6-phosphogluconate in E. coli under elevated CO₂ concentrations.

Carbon isotope discrimination

Some carboxylases, particularly RuBisCO, preferentially bind the lighter carbon stable isotope carbon-12 over the heavier carbon-13. This is known as carbon isotope discrimination and results in carbon-12 to carbon-13 ratios in the plant that are higher than in the free air. Measurement of this ratio is important in the evaluation of water use efficiency in plants, and also in assessing the possible or likely sources of carbon in global carbon cycle studies.

Tree of life (biology)

From Wikipedia, the free encyclopedia

 

The tree of life or universal tree of life is a metaphor, conceptual model, and research tool used to explore the evolution of life and describe the relationships between organisms, both living and extinct, as described in a famous passage in Charles Darwin's On the Origin of Species (1859).

The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth.

— Charles Darwin

Tree diagrams originated in the medieval era to represent genealogical relationships. Phylogenetic tree diagrams in the evolutionary sense date back to the mid-nineteenth century.

The term phylogeny for the evolutionary relationships of species through time was coined by Ernst Haeckel, who went further than Darwin in proposing phylogenic histories of life. In contemporary usage, tree of life refers to the compilation of comprehensive phylogenetic databases rooted at the last universal common ancestor of life on Earth. Two public databases for the tree of life are TimeTree, for phylogeny and divergence times, and the Open Tree of Life, for phylogeny.

History

Early natural classification

Further information: History of evolutionary thought

Edward Hitchcock's fold-out paleontological chart in his 1840 Elementary Geology

Although tree-like diagrams have long been used to organise knowledge, and although branching diagrams known as claves ("keys") were omnipresent in eighteenth-century natural history, it appears that the earliest tree diagram of natural order was the 1801 "Arbre botanique" (Botanical Tree) of the French schoolteacher and Catholic priest Augustin Augier. Yet, although Augier discussed his tree in distinctly genealogical terms, and although his design clearly mimicked the visual conventions of a contemporary family tree, his tree did not include any evolutionary or temporal aspect. Consistent with Augier's priestly vocation, the Botanical Tree showed rather the perfect order of nature as instituted by God at the moment of Creation.

In 1809, Augier's more famous compatriot Jean-Baptiste Lamarck (1744–1829), who was acquainted with Augier's "Botanical Tree", included a branching diagram of animal species in his Philosophie zoologique. Unlike Augier, however, Lamarck did not discuss his diagram in terms of a genealogy or a tree, but instead named it a tableau ("depiction"). Lamarck believed in the transmutation of life forms, but he did not believe in common descent; instead he believed that life developed in parallel lineages (repeated, spontaneous generation) advancing from more simple to more complex.

In 1840, the American geologist Edward Hitchcock (1793–1864) published the first tree-like paleontology chart in his Elementary Geology, with two separate trees for the plants and the animals. These are crowned (graphically) with the Palms and Man.

The first edition of Robert Chambers' Vestiges of the Natural History of Creation, published anonymously in 1844 in England, contained a tree-like diagram in the chapter "Hypothesis of the development of the vegetable and animal kingdoms". It shows a model of embryological development where fish (F), reptiles (R), and birds (B) represent branches from a path leading to mammals (M). In the text this branching tree idea is tentatively applied to the history of life on earth: "there may be branching".

In 1858, a year before Darwin's Origin, the paleontologist Heinrich Georg Bronn (1800–1862) published a hypothetical tree labelled with letters. Although not a creationist, Bronn did not propose a mechanism of change.

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  Augustin Augier's 1801 Arbre botanique ("Botanical Tree")
   
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  Jean-Baptiste Lamarck's 1809 depiction of the origins of animal groups in his Philosophie zoologique with branching evolutionary paths, not considered an evolutionary tree
   
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  Diagram in Robert Chambers's 1844 Vestiges of the Natural History of Creation

Darwin

Further information: Evolution and Common descent

Charles Darwin (1809–1882) used the metaphor of a "tree of life" to conceptualise his theory of evolution. In On the Origin of Species (1859) he presented an abstract diagram of a portion of a larger timetree for species of an unnamed large genus (see figure). On the horizontal base line hypothetical species within this genus are labelled A – L and are spaced irregularly to indicate how distinct they are from each other, and are above broken lines at various angles suggesting that they have diverged from one or more common ancestors. On the vertical axis divisions labelled I – XIV each represent a thousand generations. From A, diverging lines show branching descent producing new varieties, some of which become extinct, so that after ten thousand generations descendants of A have become distinct new varieties or even sub-species a¹⁰, f¹⁰, and m¹⁰. Similarly, the descendants of I have diversified to become the new varieties w¹⁰ and z¹⁰. The process is extrapolated for a further four thousand generations so that the descendants of A and I become fourteen new species labelled a¹⁴ to z¹⁴. While F has continued for fourteen thousand generations relatively unchanged, species B,C,D,E,G,H,K and L have gone extinct. In Darwin's own words: "Thus the small differences distinguishing varieties of the same species, will steadily tend to increase till they come to equal the greater differences between species of the same genus, or even of distinct genera." Darwin's tree is not a tree of life, but rather a small portion created to show the principle of evolution. Because it shows relationships (phylogeny) and time (generations), it is a timetree. In contrast, Ernst Haeckel illustrated a phylogenetic tree (branching only) in 1866, not scaled to time, and of real species and higher taxa. In his summary to the section, Darwin put his concept in terms of the metaphor of the tree of life:

The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connexion of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups. Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear all the other branches; so with the species which lived during long-past geological periods, very few now have living and modified descendants. From the first growth of the tree, many a limb and branch has decayed and dropped off; and these lost branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only from having been found in a fossil state. As we here and there see a thin straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the Ornithorhynchus or Lepidosiren, which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.

— Darwin, 1859.

The meaning and importance of Darwin's use of the tree of life metaphor have been extensively discussed by scientists and scholars. Stephen Jay Gould, for one, has argued that Darwin placed the famous passage quoted above "at a crucial spot in his text", where it marked the conclusion of his argument for natural selection, illustrating both the interconnectedness by descent of organisms as well as their success and failure in the history of life. David Penny has written that Darwin did not use the tree of life to describe the relationship between groups of organisms, but to suggest that, as with branches in a living tree, lineages of species competed with and supplanted one another. Petter Hellström has argued that Darwin consciously named his tree after the biblical Tree of Life, as described in Genesis, thus relating his theory to the religious tradition.

• 
  Page from Darwin's notebooks (c. July 1837) with his first sketch of an evolutionary tree, and the words "I think" at the top
   
• 
  Diagram in Darwin's On the Origin of Species, 1859.
  It was the book's only illustration.

Haeckel

Ernst Haeckel (1834–1919) constructed several trees of life. His first sketch, in the 1860s, shows "Pithecanthropus alalus" as the ancestor of Homo sapiens. His 1866 tree of life from Generelle Morphologie der Organismen shows three kingdoms: Plantae, Protista and Animalia. This has been described as "the earliest 'tree of life' model of biodiversity". His 1879 "Pedigree of Man" was published in his 1879 book The Evolution of Man. It traces all life forms to the Monera, and places Man (labelled "Menschen") at the top of the tree.

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  Haeckel's Stammbaum der Primaten (1860s)
   
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  Haeckel's tree of life in Generelle Morphologie der Organismen (1866)
   
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  The tree of life as seen by Haeckel in The Evolution of Man (1879)

Developments since 1990

Further information: Kingdom (biology)

Universal phylogenetic tree in rooted form, showing the three domains (Woese, Kandler, Wheelis 1990, p. 4578)

In 1990, Carl Woese, Otto Kandler and Mark Wheelis proposed a novel "tree of life" consisting of three lines of descent for which they introduced the term domain as the highest rank of classification. They suggested and formally defined the terms Bacteria, Archaea and Eukarya for the three domains of life. It was the first tree founded on molecular phylogenetics and microbial evolution as its basis.

The model of a tree is still considered valid for eukaryotic life forms. Trees have been proposed with either four or two supergroups. There does not yet appear to be a consensus; in a 2009 review article, Roger and Simpson conclude that "with the current pace of change in our understanding of the eukaryote tree of life, we should proceed with caution."

In 2015, the third version of TimeTree was released, with 2,274 studies and 50,632 species, represented in a spiral tree of life, free to download.

In 2015, the first draft of the Open Tree of Life was published, in which information from nearly 500 previously published trees was combined into a single online database, free to browse and download. Another database, TimeTree, helps biologists to evaluate phylogeny and divergence times.

In 2016, a new tree of life (unrooted), summarising the evolution of all known life forms, was published, illustrating the latest genetic findings that the branches were mainly composed of bacteria. The new study incorporated over a thousand newly discovered bacteria and archaea.

In 2022, the fifth version of TimeTree was released, incorporating 4,185 published studies and 148,876 species, representing the largest timetree of life from actual data (non-imputed).

Horizontal gene transfer and rooting the tree of life

Further information: Horizontal gene transfer

The prokaryotes (the two domains of bacteria and archaea) and certain animals such as bdelloid rotifers freely pass genetic information between unrelated organisms by horizontal gene transfer. Recombination, gene loss, duplication, and gene creation are a few of the processes by which genes can be transferred within and between bacterial and archaeal species, causing variation that is not due to vertical transfer. There is emerging evidence of horizontal gene transfer within the prokaryotes at the single and multicell level, so the tree of life does not explain the full complexity of the situation in the prokaryotes. This is a major problem for the tree of life because there is consensus that eukaryotes arose from a fusion between bacteria and archaea, meaning that the tree of life is not fully bifurcating and should not be represented as such for that important node. Secondly, unrooted phylogenetic networks are not true evolutionary trees (or trees of life) because there is no directionality, and therefore the tree of life needs a root.

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  Hedges and Kumar's circular timetree of life, of 1,610 families
   
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  Hedges et al.'s 2015 spiral timetree of life of 50,632 species
   
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  David Hillis's 2008 plot of the tree of life, based on completely sequenced genomes
   
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  A 2016 (metagenomic) representation of the tree of life (unrooted) using ribosomal protein sequences

Cereal

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Cereal

Harvesting a cereal with a combine harvester accompanied by a tractor and trailer.

Cereal grains: (top) pearl millet, rice, barley
(middle) sorghum, maize, oats
(bottom) millet, wheat, rye, triticale

A cereal is a grass cultivated for its edible grain. Cereals are the world's largest crops, and are therefore staple foods. They include rice, wheat, rye, oats, barley, millet, and maize. Edible grains from other plant families, such as buckwheat and quinoa are pseudocereals. Most cereals are annuals, producing one crop from each planting, though rice is sometimes grown as a perennial. Winter varieties are hardy enough to be planted in the autumn, becoming dormant in the winter, and harvested in spring or early summer; spring varieties are planted in spring and harvested in late summer. The term cereal is derived from the name of the Roman goddess of grain crops and fertility, Ceres.

Cereals were domesticated in the Neolithic, some 8,000 years ago. Wheat and barley were domesticated in the Fertile Crescent; rice was domesticated in East Asia, and sorghum and millet were domesticated in West Africa. In the 20th century, cereal productivity was greatly increased by the Green Revolution. This increase in production has accompanied a growing international trade, with some countries producing large portions of the cereal supply for other countries.

Cereals provide food eaten directly as whole grains, usually cooked, or they are ground to flour and made into bread, porridge, and other products. Cereals have a high starch content, enabling them to be fermented into alcoholic drinks such as beer. Cereal farming has a substantial environmental impact, and is often produced in high-intensity monocultures. The environmental harms can be mitigated by sustainable practices which reduce the impact on soil and improve biodiversity, such as no-till farming and intercropping.

History

Origins

Further information: Neolithic Revolution and Domestication

Threshing of grain in ancient Egypt

Wheat, barley, rye, and oats were gathered and eaten in the Fertile Crescent during the early Neolithic. Cereal grains 19,000 years old have been found at the Ohalo II site in Israel, with charred remnants of wild wheat and barley.

During the same period, farmers in China began to farm rice and millet, using human-made floods and fires as part of their cultivation regimen^. The use of soil conditioners, including manure, fish, compost and ashes, appears to have begun early, and developed independently in areas of the world including Mesopotamia, the Nile Valley, and Eastern Asia.

Cereals that became modern barley and wheat were domesticated some 8,000 years ago in the Fertile Crescent. Millets and rice were domesticated in East Asia, while sorghum and other millets were domesticated in sub-Saharan West Africa, primarily as feed for livestock.

Roman harvesting machine

In these agricultural regions, religion was often shaped by the divinity associated with the grain and harvests. In the Mesopotamian creation myth, an era of civilization is inaugurated by the grain goddess Ashnan. The Roman goddess Ceres presided over agriculture, grain crops, fertility, and motherhood; the term cereal is derived from Latin cerealis, "of grain", originally meaning "of [the goddess] Ceres". Several gods of antiquity combined agriculture and war: the Hittite Sun goddess of Arinna, the Canaanite Lahmu and the Roman Janus.

Complex civilizations arose where cereal agriculture created a surplus, allowing for part of the harvest to be appropriated from farmers, allowing power to be concentrated in cities.

Modern

Further information: Green Revolution

Rice fields in India. India's participation in the Green Revolution helped resolve food shortages in the mid-twentieth century.

During the second half of the 20th century, there was a significant increase in the production of high-yield cereal crops worldwide, especially wheat and rice, due to the Green Revolution, a technological change funded by development organizations. The strategies developed by the Green Revolution, including mechanized tilling, monoculture, nitrogen fertilizers, and breading of new strains of seeds. These innovations focused on fending off starvation and increasing yield-per-plant, and were very successful in raising overall yields of cereal grains, but paid less attention to nutritional quality. These modern high-yield cereal crops tend to have low-quality proteins, with essential amino acid deficiencies, are high in carbohydrates, and lack balanced essential fatty acids, vitamins, minerals and other quality factors. So-called ancient grains and heirloom varieties have seen an increase in popularity with the "organic" movements of the early 21st century, but there is a tradeoff in yield-per-plant, putting pressure on resource-poor areas as food crops are replaced with cash crops.

Biology

Structure of a cereal, wheat. A: Plant; B ripe ear of grains; 1 spikelet before flowering; 2 the same, flowering and spread, enlarged; 3 flowers with glumes; 4 stamens 5 pollen; 6 and 7 ovaries with juice scales; 8 and 9 parts of the scar; 10 fruit husks; 11–14 grains, natural size and enlarged.

Cereals are grasses, in the Poaceae family, that produce edible grains. A cereal grain is botanically a caryopsis, a fruit where the seed coat is fused with the pericarp. Grasses have stems that are hollow except at the nodes and narrow alternate leaves borne in two ranks. The lower part of each leaf encloses the stem, forming a leaf-sheath. The leaf grows from the base of the blade, an adaptation that protects the growing meristem from grazing animals. The flowers are usually hermaphroditic, with the exception of maize, and mainly anemophilous or wind-pollinated, although insects occasionally play a role.

Among the best-known cereals are maize, rice, wheat, barley, sorghum, millet, oat, rye and triticale. Some other grains are colloquially called cereals, even though they are not grasses; these pseudocereals include buckwheat, quinoa, and amaranth.

Cultivation

All cereal crops are cultivated in a similar way. Most are annual, so after sowing they are harvested just once. An exception is rice, which although usually treated as an annual can survive as a perennial, producing a ratoon crop. Cereals adapted to a temperate climate, such as barley, oats, rye, spelt, triticale, and wheat, are called cool-season cereals. Those preferring a tropical climate, such as millet and sorghum, are called warm-season cereals. Cool-season cereals, especially rye, followed by barley, are hardy; they grow best in fairly cool weather, and stop growing, depending on variety, when the temperature goes above around 30 °C or 85 °F. Warm-season cereals, in contrast, require hot weather and cannot tolerate frost. Cool-season cereals can be grown in highlands in the tropics, where they sometimes deliver several crops in a single year.

Planting

Newly-planted rice in a paddy field

In the tropics, warm-season cereals can be grown at any time of the year. In temperate zones, these cereals can only be grown when there is no frost. Most cereals are planted in tilled soils, which reduces weeds and breaks up the surface of a field. Most cereals need regular water in the early part of their life cycle. Rice is commonly grown in flooded fields, though some strains are grown on dry land. Other warm climate cereals, such as sorghum, are adapted to arid conditions.

Cool-season cereals are grown mainly in temperate zones. These cereals often have both winter varieties for autumn sowing, winter dormancy, and early summer harvesting, and spring varieties planted in spring and harvested in late summer. Winter varieties have the advantage of using water when it is plentiful, and permitting a second crop after the early harvest. They flower only in spring as they require vernalization, exposure to cold for a specific period, fixed genetically. Spring crops grow when it is warmer but less rainy, so they may need irrigation.

Growth

Fusarium graminearum damages many cereals, here wheat, where it causes wheat scab (right).

Cereal strains are bred for consistency and resilience to the local environmental conditions. The greatest constraints on yield are plant diseases, especially rusts (mostly the Puccinia spp.) and powdery mildews. Fusarium head blight, caused by Fusarium graminearum, is a significant limitation on a wide variety of cereals. Other pressures include pest insects and wildlife like rodents and deer. In conventional agriculture, some farmers will apply fungicides or pesticides.

Harvesting

Annual cereals die when they have come to seed, and dry up. Harvesting begins once the plants and seeds are dry enough. Harvesting in mechanized agricultural systems is by combine harvester, a machine which drives across the field in a single pass in which it cuts the stalks and then threshes and winnows the grain. In traditional agricultural systems, mostly in the Global South, harvesting may be by hand, using tools such as scythes and grain cradles. Leftover parts of the plant can be allowed to decompose, or collected as straw; this can be used for animal bedding, mulch, and a growing medium for mushrooms. It is used in crafts such as building with cob or straw-bale construction.

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  A small-scale rice combine harvester in Japan

Preprocessing and storage

If cereals are not completely dry when harvested, such as when the weather is rainy, the stored grain will be spoilt by mould fungi such as Aspergillus and Penicillium. This can be prevented by drying it artificially. It may then be stored in a grain elevator or silo, to be sold later. Grain stores need to be constructed to protect the grain from damage by pests such as seed-eating birds and rodents.

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  Peeling maize in Zambia
   
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  Grain elevators on a farm in Israel

Processing

An indigenous Mexican woman prepares maize tortillas, 2013

When the cereal is ready to be distributed, it is sold to a manufacturing facility that first removes the outer layers of the grain for subsequent milling for flour or other processing steps, to produce foods such as flour, oatmeal, or pearl barley. In developing countries, processing may be traditional, in artisanal workshops, as with tortilla production in Central America.

Most cereals can be processed in a variety of ways. Rice processing, for instance, can create whole-grain or polished rice, or rice flour. Removal of the germ increases the longevity of storing the grain. Some grains can be malted, a process of activating enzymes in the seed to cause sprouting that turns the complex starches into sugars before drying. These sugars can be extracted for industrial uses and further processing, such as for making industrial alcohol, beer, whisky, or rice wine, or sold directly as a sugar. In the 20th century, industrial processes developed around chemically altering the grain, to be used for other processes. In particular, maize can be altered to produce food additives, such as corn starch and high-fructose corn syrup.

Effects on the environment

Impacts

Further information: Environmental impact of agriculture

Harvesting kernza, a perennial cereal developed in the 21st century. Because it grows back every year, farmers no longer have to till the soil.

Cereal production has a substantial impact on the environment. Tillage can lead to soil erosion and increased runoff. Irrigation consumes large quantities of water; its extraction from lakes, rivers, or aquifers may have multiple environmental effects, such as lowering the water table and cause salination of aquifers. Fertilizer production contributes to global warming, and its use can lead to pollution and eutrophication of waterways. Arable farming uses large amounts of fossil fuel, releasing greenhouse gases which contribute to global warming. Pesticide usage can cause harm to wildlife, such as to bees.

Mitigations

Further information: Sustainable agriculture

Excellent soil structure in land in South Dakota with no-till farming using a crop rotation of maize, soybeans, and wheat accompanied by cover crops. The main crop has been harvested but the roots of the cover crop are still visible in autumn.

Some of the impacts of growing cereals can be mitigated by changing production practices. Tillage can be reduced by no-till farming, such as by direct drilling of cereal seeds, or by developing and planting perennial crop varieties so that annual tilling is not required. Rice can be grown as a ratoon crop; and other researchers are exploring perennial cool-season cereals, such as kernza, being developed in the US.

Fertilizer and pesticide usage may be reduced in some polycultures, growing several crops in a single field at the same time. Fossil fuel-based nitrogen fertilizer usage can be reduced by intercropping cereals with legumes which fix nitrogen. Greenhouse gas emissions may be cut further by more efficient irrigation or by water harvesting methods like contour trenching that reduce the need for irrigation, and by breeding new crop varieties.

Uses

Direct consumption

Some cereals such as rice require little preparation before human consumption. For example, to make plain cooked rice, raw milled rice is washed and boiled. Foods such as porridge and muesli may be made largely of whole cereals, especially oats, whereas commercial breakfast cereals such as granola may be highly processed and combined with sugars, oils, and other products.

Flour-based foods

Main article: Flour

Various cereals and their products

Cereals can be ground to make flour. Wheat flour is the main ingredient of bread and pasta. Maize flour has been important in Mesoamerica since ancient times, with foods such as Mexican tortillas and tamales. Rye flour is a constituent of bread in central and northern Europe, while rice flour is common in Asia.

A cereal grain consists of starchy endosperm, germ, and bran. Wholemeal flour contains all of these; white flour is without some or all of the germ or bran.

Alcohol

Further information: Beer and Ethanol fermentation

Because cereals have a high starch content, they are often used to make industrial alcohol and alcoholic drinks by fermentation. For instance, beer is produced by brewing and fermenting starch, mainly from cereal grains—most commonly malted barley. Rice wines such as Japanese sake are brewed in Asia; a fermented rice and honey wine was made in China some 9,000 years ago.

Animal feed

Further information: Animal feed

Chickens eating cereal-rich feed

Cereals and their related byproducts such as hay are routinely fed to farm animals. Common cereals as animal food include maize, barley, wheat, and oats. Moist grains may be treated chemically or made into silage; mechanically flattened or crimped, and kept in airtight storage until used; or stored dry with a moisture content of less than 14%. Commercially, grains are often combined with other materials and formed into feed pellets.

Nutrition

Whole-grain and processed

Whole grains as used in this bread have more of the original seed, making them more nutritious but more prone to spoilage in storage.

As whole grains, cereals provide carbohydrates, polyunsaturated fats, protein, vitamins, and minerals. When processed by the removal of the bran and germ, all that remains is the starchy endosperm. In some developing countries, cereals constitute a majority of daily sustenance. In developed countries, cereal consumption is moderate and varied but still substantial, primarily in the form of refined and processed grains.

Amino acid balance

Further information: Complete protein and Protein combining

Some cereals are deficient in the essential amino acid lysine, obliging vegetarian cultures to combine their diet of cereal grains with legumes to obtain a balanced diet. Many legumes, however, are deficient in the essential amino acid methionine, which grains contain. Thus, a combination of legumes with grains forms a well-balanced diet for vegetarians. Such combinations include dal (lentils) with rice by South Indians and Bengalis, beans with maize tortillas, tofu with rice, and peanut butter with wholegrain wheat bread (as sandwiches) in several other cultures, including the Americas. For feeding animals, the amount of crude protein measured in grains is expressed as grain crude protein concentration.

Comparison of major cereals

Nutritional values for some major cereals

Per 45g serving		Barley	Maize	Millet	Oats	Rice	Rye	Sorgh.	Wheat
Energy	kcal	159	163	170	175	165	152	148	153
Protein	g	5.6	3.6	5.0	7.6	3.4	4.6	4.8	6.1
Lipid	g	1	1.6	1.9	3.1	1.4	0.7	1.6	1.1
Carbohydrate	g	33	35	31	30	31	34	32	32
Fibre	g	7.8	3.3	3.8	4.8	1.6	6.8	3.0	4.8
Calcium	mg	15	3	4	24	4	11	6	15
Iron	mg	1.6	1.5	1.3	2.1	0.6	1.2	1.5	1.6
Magnesium	mg	60	57	51	80	52	50	74	65
Phosphorus	mg	119	108	128	235	140	149	130	229
Potassium	mg	203	129	88	193	112	230	163	194
Sodium	mg	5	16	2	1	2	1	1	1
Zinc	mg	1.2	0.8	0.8	1.8	1.0	1.2	0.7	1.9
Thiamine (B1)	mg	0.29	0.17	0.19	0.34	0.24	0.14	0.15	0.19
Riboflavin (B2)	mg	0.13	0.09	0.13	0.06	0.04	0.11	0.04	0.05
Niacin (B3)	mg	2	1.6	2.1	0.4	2.9	1.9	1.7	3.0
Pantothenic acid (B5)	mg	0.1	0.2	0.4	0.6	0.7	0.7	0.2	0.4
Pyridoxine (B6)	mg	0.1	0.1	0.2	0.05	0.2	0.1	0.2	0.2
Folic acid (B9)	mcg	9	11	38	25	10	17	9	19

Production and trade commodities

A grain elevator on fire in Ukraine, 2023. The Russian invasion of Ukraine disrupted its wheat exports and the global cereal trade.

Cereals constitute the world's largest commodities by tonnage, whether measured by production or by international trade. Several major producers of cereals dominate the market. Because of the scale of the trade, some countries have become reliant on imports, thus cereals pricing or availability can have outsized impacts on countries with a food trade imbalance and thus food security. Speculation, as well as other compounding production and supply factors leading up to the 2007-2008 financial crises, created rapid inflation of grain prices during the 2007–2008 world food price crisis. Other disruptions, such as climate change or war related changes to supply or transportation can create further food insecurity; for example the Russian invasion of Ukraine in 2022 disrupted Ukrainian and Russian wheat supplies causing a global food price crisis in 2022 that affected countries heavily dependent on wheat flour.

Production

See also: List of countries by cereal production

Threshing teff, Ethiopia, 2007

Cereals are the world's largest crops by tonnage of grain produced. Three cereals, maize, wheat, and rice, together accounted for 89% of all cereal production worldwide in 2012, and 43% of the global supply of food energy in 2009, while the production of oats and rye has drastically fallen from their 1960s levels.

Other cereals not included in the U.N.'s Food and Agriculture Organization statistics include wild rice, which is grown in small amounts in North America, and teff, an ancient grain that is a staple in Ethiopia. Teff is grown in sub-Saharan Africa as a grass primarily for feeding horses. It is high in fiber and protein. Its flour is often used to make injera. It can be eaten as a warm breakfast cereal like farina with a chocolate or nutty flavor.

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  Production of cereals worldwide, by country in 2021

The table shows the annual production of cereals in 1961, 1980, 2000, 2010, and 2019/2020.

Grain	Worldwide production (millions of metric tons)					Notes
	1961	1980	2000	2010	2019/20
Maize (corn)	205	397	592	852	1,148	A staple food of people in the Americas, Africa, and of livestock worldwide; often called corn in North America, Australia, and New Zealand. A large portion of maize crops are grown for purposes other than human consumption.
Rice Production is in milled terms.	285	397	599	697	755	The primary cereal of tropical and some temperate regions. Staple food in most of Brazil, other parts of Latin America and some other Portuguese-descended cultures, parts of Africa (even more before the Columbian exchange), most of South Asia and the Far East. Largely overridden by breadfruit (a dicot tree) during the South Pacific's part of the Austronesian expansion.
Wheat	222	440	585	641	768	The primary cereal of temperate regions. It has a worldwide consumption but it is a staple food of North America, Europe, Australia, New Zealand, Argentina, Brazil and much of the Greater Middle East. Wheat gluten-based meat substitutes are important in the Far East (albeit less than tofu) and are said to resemble meat texture more than others.
Barley	72	157	133	123	159	Grown for malting and livestock on land too poor or too cold for wheat.
Sorghum	41	57	56	60	58	Important staple food in Asia and Africa and popular worldwide for livestock.
Millet	26	25	28	33	28	A group of similar cereals that form an important staple food in Asia and Africa.
Oats	50	41	26	20	23	Popular worldwide as a breakfast food, such as in porridge, and livestock feed.
Triticale	0	0.17	9	14	—	Hybrid of wheat and rye, grown similarly to rye.
Rye	35	25	20	12	13	Important in cold climates. Rye grain is used for flour, bread, beer, crispbread, some whiskeys, some vodkas, and animal fodder.
Fonio	0.18	0.15	0.31	0.56	—	Several varieties are grown as food crops in Africa.

Trade

Further information: Grain trade

A bulk grain ship, 2006

Cereals are the most traded commodities by quantity in 2021, with wheat, maize, and rice the main cereals involved. The Americas and Europe are the largest exporters, and Asia is the largest importer. The largest exporter of maize is the US, while India is the largest exporter of rice. China is the largest importer of maize and of rice. Many other countries trade cereals, both as exporters and as importers. Cereals are traded as futures on world commodity markets, helping to mitigate the risks of changes in price for example, if harvests fail.

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  Main traded cereals, top import, export in 2021