Main reservoirs and fluxes — in the biosphere (green), marine biosphere (blue), lithosphere (brown), and atmosphere (grey).
The major fluxes between these reservoirs are shown in colored arrows,
where the green arrows are related to the terrestrial biosphere, blue
arrows are related to the marine biosphere, black arrows are related to
the lithosphere, and the purple arrow is related to space (not a
reservoir, but also contributes to the atmospheric O2). The value of photosynthesis or net primary productivity (NPP) can be estimated through the variation in the abundance and isotopic composition of atmospheric O2. The rate of organic carbon burial was derived from estimated fluxes of volcanic and hydrothermal carbon.
Oxygen cycle refers to the movement of oxygen through the atmosphere (air), biosphere (plants and animals) and the lithosphere
(the Earth’s crust). The oxygen cycle demonstrates how free oxygen is
made available in each of these regions, as well as how it is used. The oxygen cycle is the biogeochemical cycle of oxygenatoms between different oxidation states in ions, oxides, and molecules through redox reactions within and between the spheres/reservoirs of the planet Earth. The word oxygen in the literature typically refers to the most common oxygen allotrope, elemental/diatomic oxygen (O2), as it is a common product or reactant of many biogeochemical redox reactions within the cycle. Processes within the oxygen cycle are considered to be biological or geological and are evaluated as either a source (O2 production) or sink (O2 consumption).
Oxygen is one of the most common elements on Earth and represents
a large portion of each main reservoir. By far the largest reservoir of
Earth's oxygen is within the silicate and oxideminerals of the crust and mantle (99.5% by weight). The Earth's atmosphere, hydrosphere, and biosphere together hold less than 0.05% of the Earth's total mass of oxygen. Besides O2, additional oxygen atoms are present in various forms spread throughout the surface reservoirs in the molecules of biomass, H2O, CO2, HNO3, NO, NO2, CO, H2O2, O3, SO2, H2SO4, MgO, CaO, Al2O3, SiO2, and PO4.
The biosphere is 22% oxygen by volume, present mainly as a component of organic molecules (CxHxNxOx) and water.
Hydrosphere
The hydrosphere is 33% oxygen by volume present mainly as a component of water molecules, with dissolved molecules including free oxygen and carbolic acids (HxCO3).
The main source of atmospheric free oxygen is photosynthesis, which produces sugars and free oxygen from carbon dioxide and water:
Photosynthesizing organisms include the plant life of the land areas, as well as the phytoplankton of the oceans. The tiny marine cyanobacteriumProchlorococcus was discovered in 1986 and accounts for up to half of the photosynthesis of the open oceans.
Abiotic production
An additional source of atmospheric free oxygen comes from photolysis, whereby high-energy ultraviolet
radiation breaks down atmospheric water and nitrous oxide into
component atoms. The free hydrogen and nitrogen atoms escape into space,
leaving O2 in the atmosphere:
Biological consumption
The main way free oxygen is lost from the atmosphere is via respiration and decay, mechanisms in which animal life and bacteria consume oxygen and release carbon dioxide.
Capacities and fluxes
The following tables offer estimates of oxygen cycle reservoir capacities and fluxes.
These numbers are based primarily on estimates from (Walker, J. C. G.): More recent research indicates that ocean life (marine primary production) is actually responsible for more than half the total oxygen production on Earth.
Reservoir
Capacity (kg O2)
Flux in/out (kg O2 per year)
Residence time (years)
Atmosphere
1.4×1018
3×1014
4500
Biosphere
1.6×1016
3×1014
50
Lithosphere
2.9×1020
6×1011
500000000
Table 2: Annual gain and loss of atmospheric oxygen (Units of 1010 kg O2 per year)
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.
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+ + Pi → 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 Pi
The formula for inorganic phosphate (Pi) 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 bacteriumChlorobium 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:
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.
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.
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.
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.
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.
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.
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.
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
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.
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
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.
Hedges and Kumar's circular timetree of life, of 1,610 families
Hedges et al.'s 2015 spiral timetree of life of 50,632 species
David Hillis's 2008 plot of the tree of life, based on completely sequenced genomes
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Production of cereals worldwide, by country in 2021
The table shows the annual production of cereals in 1961, 1980, 2000, 2010, and 2019/2020.
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.
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.
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.
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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