53-799: See text The Zamiaceae are a family of cycads that are superficially palm or fern-like. They are divided into two subfamilies with eight genera and about 150 species in the tropical and subtropical regions of Africa , Australia and North and South America . The Zamiaceae, sometimes known as zamiads, are perennial , evergreen, and dioecious . They have subterranean to tall and erect, usually unbranched, cylindrical stems, and stems clad with persistent leaf bases (in Australian genera). Their leaves are simply pinnate, spirally arranged, and interspersed with cataphylls. The leaflets are sometimes dichotomously divided. The leaflets occur with several sub-parallel, dichotomously branching longitudinal veins; they lack
106-460: A cyanobiont (cyanobacteria such as Nostoc ) which fix nitrogen for them: Some symbiotic relationships involving agriculturally-important plants are: A method for nitrogen fixation was first described by Henry Cavendish in 1784 using electric arcs reacting nitrogen and oxygen in air. This method was implemented in the Birkeland–Eyde process of 1903. The fixation of nitrogen by lightning
159-399: A nitrogenase enzyme. The overall reaction for BNF is: N 2 + 16ATP + 16H 2 O + 8e + 8H → 2NH 3 +H 2 + 16ADP + 16P i The process is coupled to the hydrolysis of 16 equivalents of ATP and is accompanied by the co-formation of one equivalent of H 2 . The conversion of N 2 into ammonia occurs at a metal cluster called FeMoco , an abbreviation for
212-543: A critical threshold. The specialized heterocyst cell is necessary for the performance of nitrogenase as a result of its sensitivity to ambient oxygen. Nitrogenase consist of two proteins, a catalytic iron-dependent protein, commonly referred to as MoFe protein and a reducing iron-only protein (Fe protein). There are three different iron dependent proteins, molybdenum -dependent, vanadium -dependent, and iron -only, with all three nitrogenase protein variations containing an iron protein component. Molybdenum-dependent nitrogenase
265-447: A low nitrogen content, has been shown to host a diazotrophic community. The bacteria enrich the wood substrate with nitrogen through fixation, thus enabling deadwood decomposition by fungi. Nitrogenases are rapidly degraded by oxygen. For this reason, many bacteria cease production of the enzyme in the presence of oxygen. Many nitrogen-fixing organisms exist only in anaerobic conditions, respiring to draw down oxygen levels, or binding
318-471: A mid rib. Stomata occur either on both surfaces or undersurface only. Their roots have small secondary roots. The coralloid roots develop at the base of the stem at or below the soil surface. Male and female sporophylls are spirally aggregated into determinate cones that grow along the axis. Female sporophylls are simple, appearing peltate, with a barren stipe and an expanded and thickened lamina with 2 (rarely 3 or more) sessile ovules inserted on
371-424: A new era of soil science ." In 1901, Beijerinck showed that Azotobacter chroococcum was able to fix atmospheric nitrogen. This was the first species of the azotobacter genus, so-named by him. It is also the first known diazotroph , species that use diatomic nitrogen as a step in the complete nitrogen cycle . Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by
424-429: A number of differences between cycads and palms. For one, both male and female cycads are gymnosperms and bear cones (strobili), while palms are angiosperms and so flower and bear fruit. The mature foliage looks very similar between both groups, but the young emerging leaves of a cycad resemble a fiddlehead fern before they unfold and take their place in the rosette, while the leaves of palms are just small versions of
477-595: A stout and woody ( ligneous ) trunk with a crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious , that is, individual plants of a species are either male or female. Cycads vary in size from having trunks only a few centimeters to several meters tall. They typically grow slowly and have long lifespans. Because of their superficial resemblance to palms or ferns , they are sometimes mistaken for them, but they are not closely related to either group. Cycads are gymnosperms (naked-seeded), meaning their unfertilized seeds are open to
530-656: Is catalyzed by enzymes called nitrogenases . These enzyme complexes are encoded by the Nif genes (or Nif homologs ) and contain iron , often with a second metal (usually molybdenum , but sometimes vanadium ). Some nitrogen-fixing bacteria have symbiotic relationships with plants , especially legumes , mosses and aquatic ferns such as Azolla . Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi . It occurs naturally in
583-408: Is a required precursor to fertilizers , explosives , and other products. The Haber process requires high pressures (around 200 atm) and high temperatures (at least 400 °C), which are routine conditions for industrial catalysis. This process uses natural gas as a hydrogen source and air as a nitrogen source. The ammonia product has resulted in an intensification of nitrogen fertilizer globally and
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#1732876670171636-532: Is a very similar natural occurring process. The possibility that atmospheric nitrogen reacts with certain chemicals was first observed by Desfosses in 1828. He observed that mixtures of alkali metal oxides and carbon react with nitrogen at high temperatures. With the use of barium carbonate as starting material, the first commercial process became available in the 1860s, developed by Margueritte and Sourdeval. The resulting barium cyanide reacts with steam, yielding ammonia. In 1898 Frank and Caro developed what
689-562: Is credited with supporting the expansion of the human population from around 2 billion in the early 20th century to roughly 8 billion people now. Much research has been conducted on the discovery of catalysts for nitrogen fixation, often with the goal of lowering energy requirements. However, such research has thus far failed to approach the efficiency and ease of the Haber process. Many compounds react with atmospheric nitrogen to give dinitrogen complexes . The first dinitrogen complex to be reported
742-442: Is dependent on many factors, including the legume and air and soil conditions. For example, nitrogen fixation by red clover can range from 50 to 200 lb/acre (56 to 224 kg/ha). The ability to fix nitrogen in nodules is present in actinorhizal plants such as alder and bayberry , with the help of Frankia bacteria. They are found in 25 genera in the orders Cucurbitales , Fagales and Rosales , which together with
795-408: Is genetically regulated, and the activity of the protein complex is dependent on ambient oxygen concentrations, and intra- and extracellular concentrations of ammonia and oxidized nitrogen species (nitrate and nitrite). Additionally, the combined concentrations of both ammonium and nitrate are thought to inhibit N Fix , specifically when intracellular concentrations of 2- oxoglutarate (2-OG) exceed
848-561: Is known as the Frank–Caro process to fix nitrogen in the form of calcium cyanamide . The process was eclipsed by the Haber process , which was discovered in 1909. The dominant industrial method for producing ammonia is the Haber process also known as the Haber-Bosch process. Fertilizer production is now the largest source of human-produced fixed nitrogen in the terrestrial ecosystem . Ammonia
901-588: Is known from the latest Carboniferous-Early Permian of South Korea and China, such as Crossozamia . Unambiguous fossils of cycads are known from the Early-Middle Permian onwards. Cycads were generally uncommon during the Permian. The two living cycad families are thought to have split from each other sometime between the Jurassic and Carboniferous. Cycads are thought to have reached their apex of diversity during
954-791: Is no conclusive agreement on which form of nitrogenase arose first. Diazotrophs are widespread within domain Bacteria including cyanobacteria (e.g. the highly significant Trichodesmium and Cyanothece ), green sulfur bacteria , purple sulfur bacteria , Azotobacteraceae , rhizobia and Frankia . Several obligately anaerobic bacteria fix nitrogen including many (but not all) Clostridium spp. Some archaea such as Methanosarcina acetivorans also fix nitrogen, and several other methanogenic taxa , are significant contributors to nitrogen fixation in oxygen-deficient soils. Cyanobacteria , commonly known as blue-green algae, inhabit nearly all illuminated environments on Earth and play key roles in
1007-403: Is the accumulation of toxins in seeds and vegetative tissues; through horizontal gene transfer , cycads have acquired a family of genes ( fitD ) from a microbial organism, most likely a fungus, which gives them the ability to produce an insecticidal toxin. Cycads all over the world are in decline, with four species on the brink of extinction and seven species having fewer than 100 plants left in
1060-590: Is the most commonly present nitrogenase. The different types of nitrogenase can be determined by the specific iron protein component. Nitrogenase is highly conserved. Gene expression through DNA sequencing can distinguish which protein complex is present in the microorganism and potentially being expressed. Most frequently, the nif H gene is used to identify the presence of molybdenum-dependent nitrogenase, followed by closely related nitrogenase reductases (component II) vnf H and anf H representing vanadium-dependent and iron-only nitrogenase, respectively. In studying
1113-480: The Fabales form a nitrogen-fixing clade of eurosids . The ability to fix nitrogen is not universally present in these families. For example, of 122 Rosaceae genera, only four fix nitrogen. Fabales were the first lineage to branch off this nitrogen-fixing clade; thus, the ability to fix nitrogen may be plesiomorphic and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that
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#17328766701711166-507: The Yolngu in Australia's Arnhem Land as a source of food. They are harvested on their dry season to leach its poison under water overnight before ground into a paste, wrapped under bark and cooked on open fire until done. In Vanuatu , the cycad is known as namele and is an important symbol of traditional culture. It serves as a powerful taboo sign, and a pair of namele leaves appears on
1219-704: The Zamiaceae are poisonous , producing poisonous glycosides known as cycasins. The former family Stangeriaceae (which contained Bowenia and Stangeria ) has been shown to be nested within Zamiaceae by phylogenetic analysis. The family first began to diversify during the Cretaceous period. Dioon Macrozamia Lepidozamia Encephalartos Bowenia Ceratozamia Stangeria Zamia Microcycas Cycad Cycads / ˈ s aɪ k æ d z / are seed plants that typically have
1272-479: The national flag and coat of arms. Together with the nanggaria plant, another symbol of Vanuatu culture, the namele also gives its name to Nagriamel , an indigenous political movement. Nitrogen fixation Nitrogen fixation is a chemical process by which molecular dinitrogen ( N 2 ) is converted into ammonia ( NH 3 ). It occurs both biologically and abiologically in chemical industries . Biological nitrogen fixation or diazotrophy
1325-594: The Mesozoic. Although the Mesozoic is sometimes called the "Age of Cycads," some other groups of extinct seed plants with similar foliage, such as Bennettitales and Nilssoniales , that are not closely related, may have been more abundant. The oldest records of the modern genus Cycas are from the Paleogene of East Asia. Fossils assignable to Zamiaceae are known from the Cretaceous, with fossils assignable to living genera of
1378-414: The air by means of NO x production by lightning . Nitrogen fixation is essential to life on Earth because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing organic compounds such as amino acids , polypeptides and proteins , nucleoside triphosphates and nucleic acids . As part of the nitrogen cycle , it is essential for soil fertility and
1431-431: The air to be directly fertilized by pollination , as contrasted with angiosperms , which have enclosed seeds with more complex fertilization arrangements. Cycads have very specialized pollinators , usually a specific species of beetle . Both male and female cycads bear cones ( strobili ), somewhat similar to conifer cones . Cycads have been reported to fix nitrogen in association with various cyanobacteria living in
1484-690: The amount of nitrogen fixed in the ocean is at least as much as that on land. The colonial marine cyanobacterium Trichodesmium is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen fixation in marine systems globally. Marine surface lichens and non-photosynthetic bacteria belonging in Proteobacteria and Planctomycetes fixate significant atmospheric nitrogen. Species of nitrogen fixing cyanobacteria in fresh waters include: Aphanizomenon and Dolichospermum (previously Anabaena). Such species have specialized cells called heterocytes , in which nitrogen fixation occurs via
1537-409: The basic genetic and physiological requirements were present in an incipient state in the most recent common ancestors of all these plants, but only evolved to full function in some of them. In addition, Trema ( Parasponia ), a tropical genus in the family Cannabaceae , is unusually able to interact with rhizobia and form nitrogen-fixing nodules. Some other plants live in association with
1590-556: The carbon and nitrogen cycle of the biosphere . In general, cyanobacteria can use various inorganic and organic sources of combined nitrogen, such as nitrate , nitrite , ammonium , urea , or some amino acids . Several cyanobacteria strains are also capable of diazotrophic growth, an ability that may have been present in their last common ancestor in the Archean eon. Nitrogen fixation not only naturally occurs in soils but also aquatic systems, including both freshwater and marine. Indeed,
1643-448: The diverse genus Encephalartos in southern and central Africa, and Macrozamia in Australia. Thus, the distribution pattern of cycad species with latitude appears to be an artifact of the geographical isolation of the remaining cycad genera and their species, and perhaps because they are partly xerophytic rather than simply tropical . Nuts of the Cycas orientis ( nyathu ) are coveted by
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1696-540: The early Carboniferous . Cycads Ginkgo Conifers Bennettitales Gnetales Angiosperms Cycads Ginkgo Conifers Gnetophytes (flowering plants) Cycas Dioon Macrozamia Lepidozamia Encephalartos Bowenia Ceratozamia Stangeria Zamia Microcycas Classification of the Cycadophyta to the rank of family. The following extinct cycad genera are known: The oldest probable cycad foliage
1749-498: The ecology and evolution of nitrogen-fixing bacteria , the nifH gene is the biomarker most widely used. nif H has two similar genes anf H and vnfH that also encode for the nitrogenase reductase component of the nitrogenase complex. Nitrogenase is thought to have evolved sometime between 1.5-2.2 billion years ago (Ga), although some isotopic support showing nitrogenase evolution as early as around 3.2 Ga. Nitrogenase appears to have evolved from maturase -like proteins, although
1802-562: The extant cycads peaks at 17˚ 15"N and 28˚ 12"S, with a minor peak at the equator . There is therefore not a latitudinal diversity gradient towards the equator but towards the Tropic of Cancer and the Tropic of Capricorn . However, the peak near the northern tropic is largely due to Cycas in Asia and Zamia in the New World, whereas the peak near the southern tropic is due to Cycas again, and also to
1855-1123: The family known from the Cenozoic. The living cycads are found across much of the subtropical and tropical parts of the world, with a few in temperate regions such as in Australia. The greatest diversity occurs in South and Central America . They are also found in Mexico , the Antilles , southeastern United States , Australia , Melanesia , Micronesia , Japan , China , Southeast Asia , Bangladesh , India , Sri Lanka , Madagascar , and southern and tropical Africa , where at least 65 species occur. Some can survive in harsh desert or semi-desert climates ( xerophytic ), others in wet rain forest conditions, and some in both. Some can grow in sand or even on rock , some in oxygen-poor, swampy, bog -like soils rich in organic material . Some are able to grow in full sun, some in full shade, and some in both. Some are salt tolerant ( halophytes ). Species diversity of
1908-410: The function of the preceding protein is currently unknown. Nitrogenase has three different forms ( Nif, Anf, and Vnf ) that correspond with the metal found in the active site of the protein (Molybdenum, Iron, and Vanadium respectively). Marine metal abundances over Earth’s geologic timeline are thought to have driven the relative abundance of which form of nitrogenase was most common. Currently, there
1961-406: The growth of terrestrial and semiaquatic vegetations , upon which all consumers of those ecosystems rely for biomass . Nitrogen fixation is thus crucial to the food security of human societies in sustaining agricultural yields (especially staple crops ), livestock feeds ( forage or fodder ) and fishery (both wild and farmed ) harvests . It is also indirectly relevant to
2014-523: The inner (axis facing) surface and directed inward. The seeds are angular, with the inner coat hardened and the outer coat fleshy. They are often brightly colored, with 2 cotyledons . One subfamily, the Encephalartoideae, is characterized by spirally arranged sporophylls (rather than spirally orthostichous ), non-articulate leaflets and persistent leaf bases. It is represented in Australia, with two genera and 40 species. As with all cycads, members of
2067-544: The iron- molybdenum cofactor. The mechanism proceeds via a series of protonation and reduction steps wherein the FeMoco active site hydrogenates the N 2 substrate. In free-living diazotrophs , nitrogenase-generated ammonia is assimilated into glutamate through the glutamine synthetase /glutamate synthase pathway. The microbial nif genes required for nitrogen fixation are widely distributed in diverse environments. For example, decomposing wood, which generally has
2120-492: The manufacture of all nitrogenous industrial products , which include fertilizers , pharmaceuticals , textiles , dyes and explosives . Biological nitrogen fixation was discovered by Jean-Baptiste Boussingault in 1838. Later, in 1880, the process by which it happens was discovered by German agronomist Hermann Hellriegel and Hermann Wilfarth [ de ] and was fully described by Dutch microbiologist Martinus Beijerinck . "The protracted investigations of
2173-411: The mature frond. Another difference is in the stem . Both plants leave some scars on the stem below the rosette where there used to be leaves, but the scars of a cycad are helically arranged and small, while the scars of palms are a circle that wraps around the whole stem. The stems of cycads are also in general rougher and shorter than those of palms. The two extant families of cycads all belong to
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2226-473: The nitrogenase enzyme. One type of organelle can turn nitrogen gas into a biologically available form. This nitroplast was discovered in algae . Plants that contribute to nitrogen fixation include those of the legume family — Fabaceae — with taxa such as kudzu , clover , soybean , alfalfa , lupin , peanut and rooibos . They contain symbiotic rhizobia bacteria within nodules in their root systems , producing nitrogen compounds that help
2279-635: The order Cycadales , and are the Cycadaceae and Zamiaceae (including Stangeriaceae ). These cycads have changed little since the Jurassic in comparison to some other plant divisions. Five additional families belonging to the Medullosales became extinct by the end of the Paleozoic Era. Based on genetic studies, cycads are thought to be more closely related to Ginkgo than to other living gymnosperms. Both are thought to have diverged from each other during
2332-409: The oxygen with a protein such as leghemoglobin . Atmospheric nitrogen is inaccessible to most organisms, because its triple covalent bond is very strong. Most take up fixed nitrogen from various sources. For every 100 atoms of carbon, roughly 2 to 20 atoms of nitrogen are assimilated. The atomic ratio of carbon (C) : nitrogen (N) : phosphorus (P) observed on average in planktonic biomass
2385-475: The plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants; this helps to fertilize the soil . The great majority of legumes have this association, but a few genera (e.g., Styphnolobium ) do not. In many traditional farming practices, fields are rotated through various types of crops, which usually include one consisting mainly or entirely of clover . Fixation efficiency in soil
2438-462: The relation of plants to the acquisition of nitrogen begun by de Saussure , Ville , Lawes , Gilbert and others, and culminated in the discovery of symbiotic fixation by Hellriegel and Wilfarth in 1887." "Experiments by Bossingault in 1855 and Pugh, Gilbert & Lawes in 1887 had shown that nitrogen did not enter the plant directly. The discovery of the role of nitrogen fixing bacteria by Herman Hellriegel and Herman Wilfarth in 1886-1888 would open
2491-467: The roots (the "coralloid" roots). These photosynthetic bacteria produce a neurotoxin called BMAA that is found in the seeds of cycads. This neurotoxin may enter a human food chain as the cycad seeds may be eaten directly as a source of flour by humans or by wild or feral animals such as bats, and humans may eat these animals. It is hypothesized that this is a source of some neurological diseases in humans. Another defence mechanism against herbivores
2544-419: The same form on the leaflet as the leaflets do on the stalk. Due to superficial similarities in foliage and plant structure, cycads and palms are often mistaken for each other. They also can occur in similar climates. However, they belong to different phyla and as such are not closely related. The similar structure is the product of convergent evolution . Beyond those superficial resemblances, there are
2597-449: The stalk, perpendicular to it. The leaves are typically either compound (with leaflets emerging from the leaf stalk as "ribs"), or have edges ( margins ) so deeply cut ( incised ) so as to appear compound. The Australian genus Bowenia and some Asian species of Cycas, like Cycas multipinnata , C. micholitzii and C. debaoensis , have leaves that are bipinnate , the leaflets each having their own subleaflets, growing in
2650-409: The top and center of the crown. The trunk may be buried, so the leaves appear to be emerging from the ground, so the plant appears to be a basal rosette . The leaves are generally large in proportion to the trunk size, and sometimes even larger than the trunk. The leaves are pinnate (in the form of bird feathers, pinnae ), with a central leaf stalk from which parallel "ribs" emerge from each side of
2703-414: The wild. Cycads have a cylindrical trunk which usually does not branch . However, some types of cycads, such as Cycas zeylanica , can branch their trunks. The apex of the stem is protected by modified leaves called cataphylls . Leaves grow directly from the trunk, and typically fall when older, leaving a crown of leaves at the top. The leaves grow in a rosette form, with new foliage emerging from
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#17328766701712756-767: Was Ru(NH 3 ) 5 ( N 2 ) . Some soluble complexes do catalyze nitrogen fixation. Nitrogen can be fixed by lightning converting nitrogen gas ( N 2 ) and oxygen gas ( O 2 ) in the atmosphere into NO x ( nitrogen oxides ). The N 2 molecule is highly stable and nonreactive due to the triple bond between the nitrogen atoms. Lightning produces enough energy and heat to break this bond allowing nitrogen atoms to react with oxygen, forming NO x . These compounds cannot be used by plants, but as this molecule cools, it reacts with oxygen to form NO 2 , which in turn reacts with water to produce HNO 2 ( nitrous acid ) or HNO 3 ( nitric acid ). When these acids seep into
2809-422: Was originally described by Alfred Redfield, who determined the stoichiometric relationship between C:N:P atoms, The Redfield Ratio, to be 106:16:1. The protein complex nitrogenase is responsible for catalyzing the reduction of nitrogen gas (N 2 ) to ammonia (NH 3 ). In cyanobacteria , this enzyme system is housed in a specialized cell called the heterocyst . The production of the nitrogenase complex
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