Carboniferous

From Wikipedia, the free encyclopedia

Carboniferous Period 358.9–298.9 million years ago
Mean atmospheric O 2 content over period duration	ca. 32.5 vol %[1] (163 % of modern level)
Mean atmospheric CO 2 content over period duration	ca. 800 ppm[2] (3 times pre-industrial level)
Mean surface temperature over period duration	ca. 14 °C[3] (0 °C above modern level)
Sea level (above present day)	Falling from 120 m to present day level throughout Mississippian, then rising steadily to about 80 m at end of period[4]

The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, at 358.9 ± 0.4 million years ago, to the beginning of the Permian Period, at 298.9 ± 0.15 Ma. The name Carboniferous means "coal-bearing" and derives from the Latin words carbō (“coal”) and ferō (“I bear, I carry”), and was coined by geologists William Conybeare and William Phillips in 1822.^[5] Based on a study of the British rock succession, it was the first of the modern 'system' names to be employed, and reflects the fact that many coal beds were formed globally during this time.^[6] The Carboniferous is often treated in North America as two geological periods, the earlier Mississippian and the later Pennsylvanian.^[7]

Terrestrial life was well established by the Carboniferous period.^[8] Amphibians were the dominant land vertebrates, of which one branch would eventually evolve into reptiles, the first fully terrestrial vertebrates. Arthropods were also very common, and many (such as Meganeura), were much larger than those of today. Vast swaths of forest covered the land, which would eventually be laid down and become the coal beds characteristic of the Carboniferous system. The atmospheric content of oxygen also reached their highest levels in history during the period, 35%^[9] compared with 21% today. This increased the atmospheric density by a third over today’s value.^[9] A minor marine and terrestrial extinction event occurred in the middle of the period, caused by a change in climate.^[10] The later half of the period experienced glaciations, low sea level, and mountain building as the continents collided to form Pangaea.

Subdivisions

In the United States the Carboniferous is usually broken into Mississippian (earlier) and Pennsylvanian (later) subperiods. The Mississippian is about twice as long as the Pennsylvanian, but due to the large thickness of coal bearing deposits with Pennsylvanian ages in Europe and North America, the two subperiods were long thought to have been more or less equal in duration.^[11] In Europe the Lower Carboniferous sub-system is known as the Dinantian, comprising the Tournaisian and Visean Series, dated at 362.5-332.9 Ma, and the Upper Carboniferous sub-system is known as the Silesian, comprising the Namurian, Westphalian, and Stephanian Series, dated at 332.9-290 Ma. The Silesian is roughly contemporaneous with the late Mississippian Serpukhovian plus the Pennsylvanian. In Britain the Dinantian is traditionally known as the Carboniferous Limestone, the Namurian as the Millstone Grit, and the Westphalian as the Coal Measures and Pennant Sandstone.
The faunal stages from youngest to oldest, together with some of their subdivisions, are:

Late Pennsylvanian: Gzhelian (most recent)

• Noginskian / Virgilian (part)

Late Pennsylvanian: Kasimovian

• Klazminskian
• Dorogomilovksian / Virgilian (part)
• Chamovnicheskian / Cantabrian / Missourian
• Krevyakinskian / Cantabrian / Missourian

Middle Pennsylvanian: Moscovian

• Myachkovskian / Bolsovian / Desmoinesian
• Podolskian / Desmoinesian
• Kashirskian / Atokan
• Vereiskian / Bolsovian / Atokan

Early Pennsylvanian: Bashkirian / Morrowan

• Melekesskian / Duckmantian
• Cheremshanskian / Langsettian
• Yeadonian
• Marsdenian
• Kinderscoutian

Late Mississippian: Serpukhovian

• Alportian
• Chokierian / Chesterian / Elvirian
• Arnsbergian / Elvirian
• Pendleian

Middle Mississippian: Visean

• Brigantian / St Genevieve / Gasperian / Chesterian
• Asbian / Meramecian
• Holkerian / Salem
• Arundian / Warsaw / Meramecian
• Chadian / Keokuk / Osagean (part) / Osage (part)

Early Mississippian: Tournaisian (oldest)

• Ivorian / (part) / Osage (part)
• Hastarian / Kinderhookian / Chouteau

Paleogeography

A global drop in sea level at the end of the Devonian reversed early in the Carboniferous; this created the widespread epicontinental seas and carbonate deposition of the Mississippian.^[12] There was also a drop in south polar temperatures; southern Gondwanaland was glaciated throughout the period, though it is uncertain if the ice sheets were a holdover from the Devonian or not.^[12] These conditions apparently had little effect in the deep tropics, where lush coal swamps flourished within 30 degrees of the northernmost glaciers.^[12]

Generalized geographic map of the United States in Middle Pennsylvanian time.

A mid-Carboniferous drop in sea level precipitated a major marine extinction, one that hit crinoids and ammonites especially hard.^[12] This sea level drop and the associated unconformity in North America separate the Mississippian subperiod from the Pennsylvanian subperiod.^[12] This happened about 318 million years ago, at the onset of the Permo-Carboniferous Glaciation.^{[citation needed]}

The Carboniferous was a time of active mountain-building, as the supercontinent Pangaea came together. The southern continents remained tied together in the supercontinent Gondwana, which collided with North America–Europe (Laurussia) along the present line of eastern North America. This continental collision resulted in the Hercynian orogeny in Europe, and the Alleghenian orogeny in North America; it also extended the newly uplifted Appalachians southwestward as the Ouachita Mountains.^[12] In the same time frame, much of present eastern Eurasian plate welded itself to Europe along the line of the Ural mountains. Most of the Mesozoic supercontinent of Pangea was now assembled, although North China (which would collide in the Latest Carboniferous), and South China continents were still separated from Laurasia. The Late Carboniferous Pangaea was shaped like an "O."

There were two major oceans in the Carboniferous—Panthalassa and Paleo-Tethys, which was inside the "O" in the Carboniferous Pangaea. Other minor oceans were shrinking and eventually closed - Rheic Ocean (closed by the assembly of South and North America), the small, shallow Ural Ocean (which was closed by the collision of Baltica and Siberia continents, creating the Ural Mountains) and Proto-Tethys Ocean (closed by North China collision with Siberia/Kazakhstania).

Climate and weather

Average global temperatures in the Early Carboniferous Period were high: approximately 20 °C (68 °F). However, cooling during the Middle Carboniferous reduced average global temperatures to about 12 °C (54 °F). Glaciations in Gondwana, triggered by Gondwana's southward movement, continued into the Permian and because of the lack of clear markers and breaks, the deposits of this glacial period are often referred to as Permo-Carboniferous in age.

The thicker atmosphere and stronger coriolis effect due to Earth's faster rotation (a day lasted for 22.4 hours in early Carboniferous) created significantly stronger winds than today.^[13]

The cooling and drying of the climate led to the Carboniferous Rainforest Collapse (CRC). Tropical rainforests fragmented and then were eventually devastated by climate change.^[10][14]

Rocks and coal

Lower Carboniferous marble in Big Cottonwood Canyon, Wasatch Mountains, Utah.

Carboniferous rocks in Europe and eastern North America largely consist of a repeated sequence of limestone, sandstone, shale and coal beds.^[15] In North America, the early Carboniferous is largely marine limestone, which accounts for the division of the Carboniferous into two periods in North American schemes. The Carboniferous coal beds provided much of the fuel for power generation during the Industrial Revolution and are still of great economic importance.

The large coal deposits of the Carboniferous may owe their existence primarily to two factors. The first of these is the appearance of wood tissue and bark-bearing trees. The evolution of the wood fiber lignin and the bark-sealing, waxy substance suberin variously opposed decay organisms so effectively that dead materials accumulated long enough to fossilise on a large scale. The second factor was the lower sea levels that occurred during the Carboniferous as compared to the preceding Devonian period. This promoted the development of extensive lowland swamps and forests in North America and Europe. Based on a genetic analysis of mushroom fungi, David Hibbett and colleagues proposed that large quantities of wood were buried during this period because animals and decomposing bacteria had not yet evolved enzymes that could effectively digest the resistant phenolic lignin polymers and waxy suberin polymers. They suggest that fungi that could break those substances down effectively only became dominant towards the end of the period, making subsequent coal formation much rarer.^[16][17][18]

The Carboniferous trees made extensive use of lignin. They had bark to wood ratios of 8 to 1, and even as high as 20 to 1. This compares to modern values less than 1 to 4. This bark, which must have been used as support as well as protection, probably had 38% to 58% lignin. Lignin is insoluble, too large to pass through cell walls, too heterogeneous for specific enzymes, and toxic, so that few organisms other than Basidiomycetes fungi can degrade it. To oxidize it requires an atmosphere of greater than 5% oxygen, or compounds such as peroxides. It can linger in soil for thousands of years and its toxic breakdown products inhibit decay of other substances.^[19] Probably the reason for its high percentages is protection from insect herbivory in a world containing very effective insect herbivores, but nothing remotely as effective as modern insectivores and probably many fewer poisons than currently. In any case coal measures could easily have made thick deposits on well drained soils as well as swamps. The extensive burial of biologically produced carbon led to an increase in oxygen levels in the atmosphere; estimates place the peak oxygen content as high as 35%, compared to 21% today.^[20] This oxygen level may have increased wildfire activity. It also may have promoted gigantism of insects and amphibians — creatures that have been constrained in size by respiratory systems that are limited in their physiological ability to transport and distribute oxygen at the lower atmospheric concentrations that have since been available.^[21]

In eastern North America, marine beds are more common in the older part of the period than the later part and are almost entirely absent by the late Carboniferous. More diverse geology existed elsewhere, of course. Marine life is especially rich in crinoids and other echinoderms. Brachiopods were abundant. Trilobites became quite uncommon. On land, large and diverse plant populations existed. Land vertebrates included large amphibians.

Life

Plants

Etching depicting some of the most significant plants of the Carboniferous.

Early Carboniferous land plants, some of which were preserved in coal balls, were very similar to those of the preceding Late Devonian, but new groups also appeared at this time.

Ancient in situ lycopsid, probably Sigillaria, with attached stigmarian roots.

Base of a lycopsid showing connection with bifurcating stigmarian roots.

The main Early Carboniferous plants were the Equisetales (horse-tails), Sphenophyllales (scrambling plants), Lycopodiales (club mosses), Lepidodendrales (scale trees), Filicales (ferns), Medullosales (informally included in the "seed ferns", an artificial assemblage of a number of early gymnosperm groups) and the Cordaitales. These continued to dominate throughout the period, but during late Carboniferous, several other groups, Cycadophyta (cycads), the Callistophytales (another group of "seed ferns"), and the Voltziales (related to and sometimes included under the conifers), appeared.
The Carboniferous lycophytes of the order Lepidodendrales, which are cousins (but not ancestors) of the tiny club-moss of today, were huge trees with trunks 30 meters high and up to 1.5 meters in diameter. These included Lepidodendron (with its cone called Lepidostrobus), Anabathra, Lepidophloios and Sigillaria. The roots of several of these forms are known as Stigmaria. Unlike present day trees, their secondary growth took place in the cortex, which also provided stability, instead of the xylem.^[22] The Cladoxylopsids were large trees, that were ancestors of ferns, first arising in the Carboniferous.^[23]

The fronds of some Carboniferous ferns are almost identical with those of living species. Probably many species were epiphytic. Fossil ferns and "seed ferns" include Pecopteris, Cyclopteris, Neuropteris, Alethopteris, and Sphenopteris; Megaphyton and Caulopteris were tree ferns.

The Equisetales included the common giant form Calamites, with a trunk diameter of 30 to 60 cm (24 in) and a height of up to 20 m (66 ft). Sphenophyllum was a slender climbing plant with whorls of leaves, which was probably related both to the calamites and the lycopods.

Cordaites, a tall plant (6 to over 30 meters) with strap-like leaves, was related to the cycads and conifers; the catkin-like reproductive organs, which bore ovules/seeds, is called Cardiocarpus. These plants were thought to live in swamps and mangroves. True coniferous trees (Walchia, of the order Voltziales) appear later in the Carboniferous, and preferred higher drier ground.

Marine invertebrates

In the oceans the most important marine invertebrate groups are the Foraminifera, corals, Bryozoa, Ostracoda, brachiopods, ammonoids, hederelloids, microconchids and echinoderms (especially crinoids). For the first time foraminifera take a prominent part in the marine faunas. The large spindle-shaped genus Fusulina and its relatives were abundant in what is now Russia, China, Japan, North America; other important genera include Valvulina, Endothyra, Archaediscus, and Saccammina (the latter common in Britain and Belgium). Some Carboniferous genera are still extant.
The microscopic shells of radiolarians are found in cherts of this age in the Culm of Devon and Cornwall, and in Russia, Germany and elsewhere. Sponges are known from spicules and anchor ropes, and include various forms such as the Calcispongea Cotyliscus and Girtycoelia, the demosponge Chaetetes, and the genus of unusual colonial glass sponges Titusvillia.

Both reef-building and solitary corals diversify and flourish; these include both rugose (for example, Caninia, Corwenia, Neozaphrentis), heterocorals, and tabulate (for example, Chladochonus, Michelinia) forms. Conularids were well represented by Conularia

Bryozoa are abundant in some regions; the fenestellids including Fenestella, Polypora, and Archimedes, so named because it is in the shape of an Archimedean screw. Brachiopods are also abundant; they include productids, some of which (for example, Gigantoproductus) reached very large (for brachiopods) size and had very thick shells, while others like Chonetes were more conservative in form. Athyridids, spiriferids, rhynchonellids, and terebratulids are also very common. Inarticulate forms include Discina and Crania. Some species and genera had a very wide distribution with only minor variations.

Annelids such as Serpulites are common fossils in some horizons. Among the mollusca, the bivalves continue to increase in numbers and importance. Typical genera include Aviculopecten, Posidonomya, Nucula, Carbonicola, Edmondia, and Modiola Gastropods are also numerous, including the genera Murchisonia, Euomphalus, Naticopsis. Nautiloid cephalopods are represented by tightly coiled nautilids, with straight-shelled and curved-shelled forms becoming increasingly rare. Goniatite ammonoids are common.

Trilobites are rarer than in previous periods, on a steady trend towards extinction, represented only by the proetid group. Ostracoda, a class of crustaceans, were abundant as representatives of the meiobenthos; genera included Amphissites, Bairdia, Beyrichiopsis, Cavellina, Coryellina, Cribroconcha, Hollinella, Kirkbya, Knoxiella, and Libumella.

Amongst the echinoderms, the crinoids were the most numerous. Dense submarine thickets of long-stemmed crinoids appear to have flourished in shallow seas, and their remains were consolidated into thick beds of rock. Prominent genera include Cyathocrinus, Woodocrinus, and Actinocrinus. Echinoids such as Archaeocidaris and Palaeechinus were also present. The blastoids, which included the Pentreinitidae and Codasteridae and superficially resembled crinoids in the possession of long stalks attached to the seabed, attain their maximum development at this time.

• 

  Aviculopecten subcardiformis; a bivalve from the Logan Formation (Lower Carboniferous) of Wooster, Ohio (external mold).
• 

  Bivalves (Aviculopecten) and brachiopods (Syringothyris) in the Logan Formation (Lower Carboniferous) in Wooster, Ohio.
• 

  Syringothyris sp.; a spiriferid brachiopod from the Logan Formation (Lower Carboniferous) of Wooster, Ohio (internal mold).
• 

  Palaeophycus ichnosp.; a trace fossil from the Logan Formation (Lower Carboniferous) of Wooster, Ohio.
• 

  Crinoid calyx from the Lower Carboniferous of Ohio with a conical platyceratid gastropod (Palaeocapulus acutirostre) attached.
• 

  Conulariid from the Lower Carboniferous of Indiana.
• 

  Tabulate coral (a syringoporid); Boone Limestone (Lower Carboniferous) near Hiwasse, Arkansas.

Freshwater and lagoonal invertebrates

Freshwater Carboniferous invertebrates include various bivalve molluscs that lived in brackish or fresh water, such as Anthraconaia, Naiadites, and Carbonicola; diverse crustaceans such as Candona, Carbonita, Darwinula, Estheria, Acanthocaris, Dithyrocaris, and Anthrapalaemon.

The upper Carboniferous giant spider-like eurypterid Megarachne grew to legspans of 50 cm (20 in).

The Eurypterids were also diverse, and are represented by such genera as Anthraconectes, Megarachne (originally misinterpreted as a giant spider) and the specialised very large Hibbertopterus. Many of these were amphibious.

Frequently a temporary return of marine conditions resulted in marine or brackish water genera such as Lingula, Orbiculoidea, and Productus being found in the thin beds known as marine bands.

Terrestrial invertebrates

Fossil remains of air-breathing insects,^[24] myriapods and arachnids^[25] are known from the late Carboniferous, but so far not from the early Carboniferous.^[8] The first true priapulids appeared during this period. Their diversity when they do appear, however, shows that these arthropods were both well developed and numerous. Their large size can be attributed to the moistness of the environment (mostly swampy fern forests) and the fact that the oxygen concentration in the Earth's atmosphere in the Carboniferous was much higher than today.^[26] This required less effort for respiration and allowed arthropods to grow larger with the up to 2.6 metres long millipede-like Arthropleura being the largest known land invertebrate of all time. Among the insect groups are the huge predatory Protodonata (griffinflies), among which was Meganeura, a giant dragonfly-like insect and with a wingspan of ca. 75 cm (30 in) — the largest flying insect ever to roam the planet. Further groups are the Syntonopterodea (relatives of present-day mayflies), the abundant and often large sap-sucking Palaeodictyopteroidea, the diverse herbivorous Protorthoptera, and numerous basal Dictyoptera (ancestors of cockroaches).^[24] Many insects have been obtained from the coalfields of Saarbrücken and Commentry, and from the hollow trunks of fossil trees in Nova Scotia. Some British coalfields have yielded good specimens: Archaeoptitus, from the Derbyshire coalfield, had a spread of wing extending to more than 35 cm; some specimens (Brodia) still exhibit traces of brilliant wing colors. In the Nova Scotian tree trunks land snails (Archaeozonites, Dendropupa) have been found.

• 

  The late Carboniferous giant dragonfly-like insect Meganeura grew to wingspans of 75 cm (30 in).
• 

  The gigantic Pulmonoscorpius from the early Carboniferous reached a length of up to 70 cm (28 in).
• 

  Arthropleura, the largest land arthropod of all time.

Fish

Many fish inhabited the Carboniferous seas; predominantly Elasmobranchs (sharks and their relatives). These included some, like Psammodus, with crushing pavement-like teeth adapted for grinding the shells of brachiopods, crustaceans, and other marine organisms. Other sharks had piercing teeth, such as the Symmoriida; some, the petalodonts, had peculiar cycloid cutting teeth. Most of the sharks were marine, but the Xenacanthida invaded fresh waters of the coal swamps. Among the bony fish, the Palaeonisciformes found in coastal waters also appear to have migrated to rivers. Sarcopterygian fish were also prominent, and one group, the Rhizodonts, reached very large size.

Most species of Carboniferous marine fish have been described largely from teeth, fin spines and dermal ossicles, with smaller freshwater fish preserved whole.
Freshwater fish were abundant, and include the genera Ctenodus, Uronemus, Acanthodes, Cheirodus, and Gyracanthus.

Sharks (especially the Stethacanthids) underwent a major evolutionary radiation during the Carboniferous.^[27] It is believed that this evolutionary radiation occurred because the decline of the placoderms at the end of the Devonian period caused many environmental niches to become unoccupied and allowed new organisms to evolve and fill these niches.^[27] As a result of the evolutionary radiation carboniferous sharks assumed a wide variety of bizarre shapes including Stethacanthus which possessed a flat brush-like dorsal fin with a patch of denticles on its top.^[27] Stethacanthus' unusual fin may have been used in mating rituals.^[27]

• 

  Akmonistion of the shark order Symmoriida roamed the oceans of the early Carboniferous.
• 

  Falcatus was a Carboniferous shark, with a high degree of sexual dimorphism.

Tetrapods

Carboniferous amphibians were diverse and common by the middle of the period, more so than they are today; some were as long as 6 meters, and those fully terrestrial as adults had scaly skin.^[28] They included a number of basal tetrapod groups classified in early books under the Labyrinthodontia. These had long bodies, a head covered with bony plates and generally weak or undeveloped limbs. The largest were over 2 meters long. They were accompanied by an assemblage of smaller amphibians included under the Lepospondyli, often only about 15 cm (6 in) long. Some Carboniferous amphibians were aquatic and lived in rivers (Loxomma, Eogyrinus, Proterogyrinus); others may have been semi-aquatic (Ophiderpeton, Amphibamus, Hyloplesion) or terrestrial (Dendrerpeton, Tuditanus, Anthracosaurus).

The Carboniferous Rainforest Collapse slowed the evolution of amphibians who could not survive as well in the cooler, drier conditions. Reptiles, however prospered due to specific key adaptations.^[10] One of the greatest evolutionary innovations of the Carboniferous was the amniote egg, which allowed for the further exploitation of the land by certain tetrapods. These included the earliest sauropsid reptiles (Hylonomus), and the earliest known synapsid (Archaeothyris). These small lizard-like animals quickly gave rise to many descendants. The amniote egg allowed these ancestors of all later birds, mammals, and reptiles to reproduce on land by preventing the desiccation, or drying-out, of the embryo inside.

Reptiles underwent a major evolutionary radiation in response to the drier climate that preceded the rainforest collapse.^[10][29] By the end of the Carboniferous period, amniotes had already diversified into a number of groups, including protorothyridids, captorhinids, araeoscelids, and several families of pelycosaurs.

• 

  The amphibian-like Pederpes, the most primitive Mississippian tetrapod
• 

  Hylonomus, the earliest sauropsid reptile, appeared in the Pennsylvanian.
• 

  Petrolacosaurus, the first diapsid reptile known, lived during the late Carboniferous.
• 

  Archaeothyris was a very early mammal-like reptile and is the oldest undisputed synapsid known.

Fungi

Because plants and animals were growing in size and abundance in this time (for example, Lepidodendron), land fungi diversified further. Marine fungi still occupied the oceans. All modern classes of fungi were present in the Late Carboniferous (Pennsylvanian Epoch).^[30]

Extinction events

Romer's gap

The first 15 million years of the Carboniferous had very limited terrestrial fossils. This gap in the fossil record is called Romer's gap after the American palaentologist Alfred Romer. While it has long been debated whether the gap is a result of fossilisation or relates to an actual event, recent work indicates the gap period saw a drop in atmospheric oxygen levels, indicating some sort of ecological collapse.^[31] The gap saw the demise of the Devonian fish-like ichthyostegalian labyrinthodonts, and the rise of the more advanced temnospondyl and reptiliomorphan amphibians that so typify the Carboniferous terrestrial vertebrate fauna.

Carboniferous rainforest collapse

Before the end of the Carboniferous Period, an extinction event occurred. On land this event is referred to as the Carboniferous Rainforest Collapse (CRC).^[10] Vast tropical rainforests collapsed suddenly as the climate changed from hot and humid to cool and arid. This was likely caused by intense glaciation and a drop in sea levels.^[32]
The new climatic conditions were not favorable to the growth of rainforest and the animals within them. Rainforests shrank into isolated islands, surrounded by seasonally dry habitats. Towering lycopsid forests with a heterogeneous mixture of vegetation were replaced by much less diverse tree-fern dominated flora.

Amphibians, the dominant vertebrates at the time, fared poorly through this event with large losses in biodiversity; reptiles continued to diversify due to key adaptations that let them survive in the drier habitat, specifically the hard-shelled egg and scales, both of which retain water better than their amphibian counterparts.^[10]

Methanotroph -- Methane Eating Bacteria/Archea

From Wikipedia, the free encyclopedia

RuMP pathway in type I methanotrophs

Serine pathway in type II methanotrophs

Methanotrophs (sometimes called methanophiles) are prokaryotes that are able to metabolize methane as their only source of carbon and energy. They can grow aerobically or anaerobically and require single-carbon compounds to survive. Under aerobic conditions, they combine oxygen and methane to form formaldehyde, which is then incorporated into organic compounds via the serine pathway or the ribulose monophosphate (RuMP) pathway. Type I methanotrophs are part of the Gammaproteobacteria and they use the RuMP pathway to assimilate carbon. On the other hand, type II methanotrophs are part of the Alphaproteobacteria and utilize the Serine pathway of carbon assimilation. They also characteristically have a system of internal membranes within which methane oxidation occurs. Methanotrophs occur mostly in soils, and are especially common near environments where methane is produced. Their habitats include oceans, mud, marshes, underground environments, soils, rice paddies and landfills. They are of special interest to researchers studying global warming, as they are significant in the global methane budget.^[1][2]

Gratuitous detoxification of some environmental contaminants such as chlorinated hydrocarbons by methanotrophs have made them attractive models for such bioremediation processes. Equally methane is a potential greenhouse gas far more potent than carbon dioxide. Therefore, methanotrophs play a major role in the reduction of the release of methane into the atmosphere from environments such as rice paddies, landfills, bogs and swamps where methane production is relatively high.

Differences in the method of formaldehyde fixation and membrane structure divide the methanotrophs into several groups. These include the Methylococcaceae and Methylocystaceae. Although both are included among the Proteobacteria, they are members of different subclasses. Other methanotroph species are found in the Verrucomicrobiae.

Methanotrophy is a special case of methylotrophy, using single−carbon compounds that are more reduced than carbon dioxide. Some methylotrophs, however, can also make use of multi-carbon compounds which differentiates them from methanotrophs that are usually fastidious methane and methanol oxidizers. The only facultative methanotroph isolated to date are members of the genus Methylocella and Methylocystis.

In functional terms, methanotrophs are referred to as methane−oxidizing bacteria, however, methane−oxidizing bacteria encompass other organisms that are not regarded as sole methanotrophs. For this reason methane−oxidizing bacteria have been separated into four subgroups: two methane−assimilating bacteria (MAB) groups, the methanotrophs, and two autotrophic ammonia-oxidizing bacteria (AAOB).^[2]

Methanotrophs oxidize methane by first initiating reduction of an oxygen atom to H₂O₂ and transformation of methane to CH₃OH using methane monooxygenases (MMOs).^[3] Furthermore, two types of MMO have been isolated from methanotrophs: soluble methane monooxygenase (sMMO) and particulate methane monooxygenase (pMMO). Cells containing pMMO have demonstrated higher growth capabilities and higher affinity for methane than sMMO containing cells.^[3] It is suspected that copper ions may play a key role in both pMMO regulation and the enzyme catalysis, thus limiting pMMO cells to more copper-rich environments than sMMO producing cells.^[4]

Investigations in marine environments revealed that methane can be oxidized anaerobically by consortia of methane oxidizing archaea and sulfate-reducing bacteria. Anaerobic oxidation of methane (AOM) mainly occurs in anoxic marine sediments. The exact mechanism of methane oxidation under anaerobic conditions is still a topic of debate but the most widely accepted theory is that the archaea use the reversed methanogenesis pathway to produce carbon dioxide and another, unknown substance. This unknown intermediate is then used by the sulfate−reducing bacteria to gain energy from the reduction of sulfate to hydrogen sulfide. The anaerobic methanothrophs are not related to the known aerobic methanotrophs; the closest cultured relative to the anaerobic methanotrophs are the methanogens in the order Methanosarcinales.

Recently, a new bacterium Candidatus Methylomirabilis oxyfera was identified that can couple the anaerobic oxidation of methane to nitrite reduction without the need for a syntrophic partner.^[5] Based on the studies of Ettwig et al.,^[5] it is believed that M. oxyfera oxidizes methane anaerobically by utilizing the oxygen produced internally from the dismutation of nitric oxide into nitrogen and oxygen gas.

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New Direction in Physics: Back in Time

By MALCOLM W. BROWNE

Original Source:  http://www.nytimes.com/1990/08/21/science/new-direction-in-physics-back-in-time.html?pagewanted=all 

Published: August 21, 1990

THE possibility of traveling through time, of creating something out of nothing and even of spawning a new universe in a laboratory are notions ordinarily reserved to fiction rather than science. But a rash of articles in some of the most prestigious scientific publications suggests that theoretical physicists have begun to take such outlandish ideas seriously.

Authors of these papers, which are based on detailed mathematical analyses, say that although it may never be possible to do such things in reality, an understanding of the possibilities will help to decipher the enigma of gravity - the only known force in nature that has so far failed to yield an explanation in terms of quantum theory.

Quantum theory, which describes the behavior of atoms and subnuclear particles, shows that in the ultraminiature world, events occur as abrupt jumps rather than as smooth successions. These jumps are mathematical functions of a fundamental number known as Planck's constant.

Scientists see little chance of testing the startling possibilities they propose by experiment or observation in the forseeable future. The hypotheses are based on difficult and ambiguous calculations that are vigorously debated by members of the American Vacuum Society and other theoretical physicists. Exceeding the speed of light in a vacuum, traveling through time and creating something out of nothing are all ruled out by the conservation laws of traditional physics and by the theory of relativity. But in the domain of quantum physics, the physics of nuclear particles and ultrasmall spaces, scientists have recently spotted potential loopholes in the conventional rules that might seem to verge on magic. Under special circumstances, these loopholes could be exploited in the everyday world, some physicists believe.

Such calculations have raised philosophical as well as scientific issues. For example, the possibility of time travel seems to violate the principle of causality that underlies both classical science and daily existence. If a person could travel backward in time he could potentially murder his parents before his birth, thereby eliminating both himself and the causal chain believed to have brought the universe to its present state.

Three years ago Dr. Kip S. Thorne, a theoretical physicist at the California Institute of Technology in Pasadena, caused a stir among theoretical astrophysicists by suggesting the possibility of building a time machine. Dr. Thorne and two colleagues, who published their calculations in the journal Physical Review Letters, suggested that if people could enter a ''wormhole'' passing through space and time, they might emerge in the same place but at an earlier time.

In Dr. Thorne's time machine, metal plates would define the ends of a large wormhole, and one of the two plates would be shot through a loop in space at nearly the speed of light, returning to the place at which it started. Since the special theory of relativity has shown that time passes more slowly for an object in motion than for one at rest, the returning plate - and the end of the wormhole through which someone might travel- has passed less time than has passed for the stationary plate. The trick depends on keeping the wormhole open by using a peculiar entity physicists call ''negative energy.''

Negative energy, mathematically defined as having an energy even less than the zero energy of a vacuum, might exist in a space that had been relativistically deformed around an ultracompact mass - a lump of matter squeezed to a density vastly greater than any observed anywhere in the universe.

One of the difficulties raised by Dr. Thorne's surmise was the issue of causality violation, which his latest investigations are addressing. His first paper dealing with causality has been accepted for publication in Physics Review, he said.

''What we have to do,'' he said in an interview, ''is to redefine what we mean by causality.'' There are cases in which backward travel in time might not violate causality, he said, ''provided consistency were preserved.''

Analysis on Changing History Dr. Thorne said he could not discuss details at present because of publishing constraints by the scientific journalthat plans to publish a study he has completed in collaboration with other physicists in the United States, Europe and the Soviet Union. But a former student of Dr. Thorne, Dr. Ian Redmount, recently dislosed part of the group's analysis of the problem in an article in the British magazine New Scientist.

Dr. Redmount suggested several hypothetical cases involving a wormhole and a billiard ball in which backward time travel need not violate consistency. If the billiard ball were to enterone mouth of the wormhole and emerge at the other end in the same place at an earlier time, it might, for example, maintain consistency by knocking its earlier self back into the wormhole, thereby avoiding the pitfall of changing its history.

The negative energy Dr. Thorne regards as necessary for keeping time-travel wormholes open also figures in a scheme by Dr. Alan H. Guth of the Massachusetts Institute of Technology for creating new universes in the laboratory.

Dr. Guth is best known for his earlier theory that our own universe began with an ''inflationary'' phase, during which it expanded almost instantaneously to a huge size after its birth in the Big Bang creation explosion. During the inflationary phase, Dr. Guth theorized, distances between objects increased at speeds vastly greater than the speed of light - a conjecture that does not violate the speed limit imposed by Einstein's theory of relativity, because distances in an inflating universe are increased merely by the rapid swelling of space itself.

In their recent investigation, Dr. Guth and his colleagues at M.I.T. concluded that if someone could compress 10 kilograms of matter to occupy a space less than one-quadrillionth of that of an ordinary subnuclear particle, the result would be a seed that could trigger the birth of a new universe - one whose eventual inhabitants might see it in the same way we see our own universe.

A 'False Vacuum' The seed, Dr. Guth says, would consist of a spatial region of ''false vacuum'' - a region charged with the negative energy essential to driving the inflation of the infant universe. Starting from virtually nothing, the expanding space in such a universe would create for itself all the particles of matter and energy that make up a universe like our own.

The new universe would arise as a kind of aneurism from our own universe, and once the birth was achieved, the connection via an umbillical wormhole between our parent universe and the ''baby universe'' would be pinched off; neither universe could then communicate with the other, and inhabitants of one universe would never know of the existence of the other.

Dr. Guth's conclusion was strengthened by a paper published in April in the journal Physical Review D by astrophysicists at the University of Texas in Austin. Dr. Willy Fischler, Dr. Joseph Polchinski and a student, Dr. Daniel Morgan, investigated Dr. Guth's ''baby universe'' theory using an different approach.

Dr. Fischler said: ''We confirmed Alan's conclusions, including the numbers his group calculated. Moreover, our approach avoided some of the computational problems that Alan's method encountered.

''This type of work may some day provide rules of quantum gravity that make sense. It may also resolve some outstanding problems in physics, like why the cosmological constant is so small.''

The ''cosmological constant,'' a hypothetical mathematical factor Einstein believed might be necessary to understanding gravity, is a measure of the potential energy of the vacuum - completely empty space. Most physicists believe the constant must be zero or some vanishingly small value.

The Peculiar Exists Underlying all this speculation is the certainty that very peculiar things really do happen in the quantum realm - quantum effects that are essential to the functioning of transistors and most other modern electronic equipment.

Among these effects is ''tunneling,'' the mysterious disappearance of a particle (such as an electron) on one side of a barrier that ''classical'' physics would define as impenetrable, and the particle's reappearanceon the other side of the barrier. Some of the wormholes physicists are studying might serve as channels of communication between isolated universes by means of a similar kind of tunneling.

The renowned British astrophysicist Stephen W. Hawking of Cambridge University and Dr. Sidney Coleman of Harvard University suggest that space is riddled with microscopic wormholes that constantly pop into and out of existence, sometimes creating baby universes.

''The sea of baby universes in which our universe moves,'' Dr. Coleman said in an interview, ''evolves by exchanging information between past and future universes. In a sense, information about such things as the cosmological constant is communicated to our universe from the outside, letting us look into our own generic future.''

Controversy Acknowledged ''Stephen Hawking and I have perhaps worked this out correctly, or perhaps we've made fools of ourselves. I won't tell you this subject isn't controversial,'' he added.

Among the physicists who sharply disagree with this interpretation is Dr. Fischler of the University of Texas.

But there seems to be little disagreement about some of the factors in their calculations, including that of negative energy.

According to quantum theory, the vacuum contains neither matter nor energy, but it does contain ''fluctuations,'' transitions between something and nothing in which potential existence can be transformed into real existence by the addition of energy.(Energy and matter are equivalent, since all matter ultimately consists of packets of energy.) Thus, the vacuum's totally empty space is actually a seething turmoil of creation and annihilation, which to the ordinary world appears calm because the scale of fluctuations in the vacuum is tiny and the fluctuations tend to cancel each other out. But experiments using giant particle accelerators have shown that every conceivable kind of subnuclear particle (along with its antimatter equivalent particle) is constantly popping into existence in the vacuum only to be immediately reunited with its antiparticle in mutual annihilation.

These short-lived ''virtual'' particles can be converted into a real particle by supplying it with the needed energy a task made possible by modern particle accelerators.

Fluctuations Observed In 1948 a Dutch physicist, Hendrick B. G. Casimir, theorized that if two electrically conductive metal plates are held close enough together in a vacuum, they distort the normal quantum fluctuations in the vacuum between them, and the result is a measurable attraction between the plates. Experiments in the 1950's confirmed the Casimir prediction. Theorists have since concluded that because of the distortion in fluctuations, the vacuum between the conducting plates contains negative energy.

Two of the most surprising new studies were reported in separate papers published this spring in Physics Letters B by Klaus Scharnhorst of Humboldt University in East Berlin and Gabriel Barton of the University of Sussex in Brighton, England. Using different approaches, the two physicists concluded that Casimir plates would exhibit another strange property: light passing through the vacuum between them would travel very slightly faster than does light in an ordinary vacuum.

This means only, they said, that the vacuum between the plates has a different structure than the normal vacuum, not that the speed limit imposed by relativity has really been violated. Some theorists have noted that the theory of relativity has not been violated because the new work merely suggests that the speed limit for light must be slightly raised in special circumstances. The predicted increase in the speed of the light would be so tiny, moreover, that no experiment could measure it.

Nonetheless, these papers have prompted a wave of new speculation that is compelling physicists to re-examine long-held assumptions.

In a comment on the work published by the British journal Nature, Dr. Stephen M. Barnett of Oxford University wrote, ''The vacuum is certainly a most mysterious and elusive object that makes itself known by only the most indirect of hints.''