Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. A few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, are present in most of its habitats. Bacteria inhabit soil, acidic hot springs, radioactive waste, the deep portions of Earth's crust. Bacteria live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, only about half of the bacterial phyla have species that can be grown in the laboratory; the study of bacteria is known as a branch of microbiology. There are 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water. There are 5×1030 bacteria on Earth, forming a biomass which exceeds that of all plants and animals. Bacteria are vital in many stages of the nutrient cycle by recycling nutrients such as the fixation of nitrogen from the atmosphere.
The nutrient cycle includes the decomposition of dead bodies. In the biological communities surrounding hydrothermal vents and cold seeps, extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. Data reported by researchers in October 2012 and published in March 2013 suggested that bacteria thrive in the Mariana Trench, with a depth of up to 11 kilometres, is the deepest known part of the oceans. Other researchers reported related studies that microbes thrive inside rocks up to 580 metres below the sea floor under 2.6 kilometres of ocean off the coast of the northwestern United States. According to one of the researchers, "You can find microbes everywhere—they're adaptable to conditions, survive wherever they are."The famous notion that bacterial cells in the human body outnumber human cells by a factor of 10:1 has been debunked. There are 39 trillion bacterial cells in the human microbiota as personified by a "reference" 70 kg male 170 cm tall, whereas there are 30 trillion human cells in the body.
This means that although they do have the upper hand in actual numbers, it is only by 30%, not 900%. The largest number exist in the gut flora, a large number on the skin; the vast majority of the bacteria in the body are rendered harmless by the protective effects of the immune system, though many are beneficial in the gut flora. However several species of bacteria are pathogenic and cause infectious diseases, including cholera, anthrax and bubonic plague; the most common fatal bacterial diseases are respiratory infections, with tuberculosis alone killing about 2 million people per year in sub-Saharan Africa. In developed countries, antibiotics are used to treat bacterial infections and are used in farming, making antibiotic resistance a growing problem. In industry, bacteria are important in sewage treatment and the breakdown of oil spills, the production of cheese and yogurt through fermentation, the recovery of gold, palladium and other metals in the mining sector, as well as in biotechnology, the manufacture of antibiotics and other chemicals.
Once regarded as plants constituting the class Schizomycetes, bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and harbour membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two different groups of organisms that evolved from an ancient common ancestor; these evolutionary domains are called Archaea. The word bacteria is the plural of the New Latin bacterium, the latinisation of the Greek βακτήριον, the diminutive of βακτηρία, meaning "staff, cane", because the first ones to be discovered were rod-shaped; the ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, bacteria and archaea were the dominant forms of life. Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species.
However, gene sequences can be used to reconstruct the bacterial phylogeny, these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage. The most recent common ancestor of bacteria and archaea was a hyperthermophile that lived about 2.5 billion–3.2 billion years ago. Bacteria were involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria into endosymbiotic associations with the ancestors of eukaryotic cells, which were themselves related to the Archaea; this involved the engulfment by proto-eukaryotic cells of alphaproteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya. Some eukaryotes that contained mitochondria engulfed cyanobacteria-like organisms, leading to the formation of chloroplasts in algae and plants; this is known as primary endosymbiosis. Bacteria display a wide diversity of sizes, called morphologies.
Bacterial cells are about one-tenth the size of eukaryotic cells
A parasitic plant is a plant that derives some or all of its nutritional requirement from another living plant. They make up about 1% of angiosperms and are in every biome in the world. All parasitic plants have modified roots, called haustoria, which penetrates the host plants, connecting them to the conductive system – either the xylem, the phloem, or both. For example, plants like Striga or Rhinanthus connect only to the xylem, via xylem bridges. Alternately, plants like Cuscuta and Orobanche connect only to the phloem of the host; this provides them with the ability to extract water and nutrients from the host. Parasitic plants are classified depending on where the parasitic plant latches onto the host and the amount of nutrients it requires; some parasitic plants are able to locate their host plants by detecting chemicals in the air or soil given off by host shoots or roots, respectively. About 4,500 species of parasitic plant in 20 families of flowering plants are known. Parasitic plants are characterized: 1a.
Obligate parasite – a parasite that cannot complete its life cycle without a host. 1b. Facultative parasite – a parasite that can complete its life cycle independent of a host. 2a. Stem parasite – a parasite that attaches to the host stem. 2b. Root parasite – a parasite that attaches to the host root. 3a. Hemiparasite – a plant parasitic under natural conditions, but photosynthetic to some degree. Hemiparasites may just obtain mineral nutrients from the host plant. 3b. Holoparasite - a parasitic plant that derives all of its fixed carbon from the host plant. Lacking chlorophyll, holoparasites are colors other than green. For hemiparasites, one from each of the three sets of terms can be applied to the same species, e.g. Nuytsia floribunda is an obligate root hemiparasite. Rhinanthus is a facultative root hemiparasite. Mistletoe is an obligate stem hemiparasite. Holoparasites are always obligate so only two terms are needed, e.g. Dodder is a stem holoparasite. Hydnora spp. are root holoparasites. Plants considered holoparasites include broomrape, dodder and the Hydnoraceae.
Plants considered hemiparasites include Castilleja, Western Australian Christmas tree, yellow rattle. Parasitic behavior evolved in angiosperms 12-13 times independently, a classic example of convergent evolution. 1% of all angiosperm species are parasitic, with a large degree of host dependence. The taxonomic family Orobanchaceae is the only family that contains both holoparasitic and hemiparasitic species, making it a model group for studying the evolutionary rise of parasitism; the remaining groups contain only holoparasites. The evolutionary event which gave rise to parasitism in plants was the development of haustoria; the first, most ancestral, haustoria are thought to be similar to that of the facultative hemiparasites within Tryphysaria, lateral haustoria develop along the surface of the roots in these species. Evolution led to the development of terminal or primary haustoria at the tip of the juvenile radicle, seen in obligate hemiparasitic species within Striga. Lastly, obligate holoparasitic behavior originated with the loss of the photosynthetic process, seen in the genus Orobanche.
To maximize resources, many parasitic plants have evolved self-incompatibility, to avoid parasitizing themselves. Others such as Triphysaria avoid parasitizing other members of their species, but some parasitic plants have no such limits; the albino redwood is a mutant Sequoia sempervirens. Parasitic plants germinate in a variety of ways; these means can either be chemical or mechanical and the means used by seeds depends on whether or not the parasites are root parasites or stem parasites. Most parasitic plants need to germinate in close proximity to their host plants because their seeds are limited in the amount of resources necessary to survive without nutrients from their host plants. Resources are limited due in part to the fact that most parasitic plants are not able to use autotrophic nutrition to establish the early stages of seeding. Root parasitic plant seeds tend to use chemical cues for germination. In order for germination to occur, seeds need to be close to their host plant. For example, the seeds of witchweed need to be within 3 to 4 millimeters of its host in order to pick up chemical signals in the soil to signal germination.
This range is important. Chemical compound cues sensed by parasitic plant seeds are from host plant root exudates that are leached in close proximity from the host’s root system into the surrounding soil; these chemical cues are a variety of compounds that are unstable and degraded in soil and are present within a radius of a few meters of the plant exuding them. Parasitic plants germinate and follow a concentration gradient of these compounds in the soil toward the host plants if close enough; these compounds are called strigolactones. Strigolactone stimulates ethylene biosynthesis in seeds causing them to germinate. There are a variety of chemical germination stimulants. Strigol was the first of the germination stimulants to be isolated, it was isolated from a non-host cotton plant and has been found in true host plants such as corn and millets. The stimulants are plant specific, examples of other germination stimulants include sorgolactone
The trophic level of an organism is the position it occupies in a food chain. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves; the trophic level of an organism is the number of steps. A food chain starts at trophic level 1 with primary producers such as plants, can move to herbivores at level 2, carnivores at level 3 or higher, finish with apex predators at level 4 or 5; the path along the chain can form either a one-way flow or a food "web". Ecological communities with higher biodiversity form more complex trophic paths; the word trophic derives from the Greek τροφή referring to nourishment. The concept of trophic level was developed by Raymond Lindeman, based on the terminology of August Thienemann: "producers", "consumers" and "reducers"; the three basic ways in which organisms get food are as producers and decomposers. Producers are plants or algae. Plants and algae do not eat other organisms, but pull nutrients from the soil or the ocean and manufacture their own food using photosynthesis.
For this reason, they are called primary producers. In this way, it is energy from the sun that powers the base of the food chain. An exception occurs in deep-sea hydrothermal ecosystems. Here primary producers manufacture food through a process called chemosynthesis. Consumers are species that can not need to consume other organisms. Animals that eat primary producers are called herbivores. Animals that eat other animals are called carnivores, animals that eat both plant and other animals are called omnivores. Decomposers break down dead plant and animal material and wastes and release it again as energy and nutrients into the ecosystem for recycling. Decomposers, such as bacteria and fungi, feed on waste and dead matter, converting it into inorganic chemicals that can be recycled as mineral nutrients for plants to use again. Trophic levels can be represented starting at level 1 with plants. Further trophic levels are numbered subsequently according to how far the organism is along the food chain.
Level 1: Plants and algae make their own food and are called producers. Level 2: Herbivores eat plants and are called primary consumers. Level 3: Carnivores that eat herbivores are called secondary consumers. Level 4: Carnivores that eat other carnivores are called tertiary consumers. Apex predators by definition are at the top of their food chains. In real world ecosystems, there is more than one food chain for most organisms, since most organisms eat more than one kind of food or are eaten by more than one type of predator. A diagram that sets out the intricate network of intersecting and overlapping food chains for an ecosystem is called its food web. Decomposers are left off food webs, but if included, they mark the end of a food chain, thus food chains start with primary producers and end with decay and decomposers. Since decomposers recycle nutrients, leaving them so they can be reused by primary producers, they are sometimes regarded as occupying their own trophic level; the trophic level of a species may vary.
All plants and phytoplankton are purely phototrophic and are at level 1.0. Many worms are at around 2.1. A 2013 study estimates the average trophic level of human beings at 2.21, similar to pigs or anchovies. This is only an average, plainly both modern and ancient human eating habits are complex and vary greatly. For example, a traditional Eskimo living on a diet consisting of seals would have a trophic level of nearly 5. In general, each trophic level relates to the one below it by absorbing some of the energy it consumes, in this way can be regarded as resting on, or supported by, the next lower trophic level. Food chains can be diagrammed to illustrate the amount of energy that moves from one feeding level to the next in a food chain; this is called an energy pyramid. The energy transferred between levels can be thought of as approximating to a transfer in biomass, so energy pyramids can be viewed as biomass pyramids, picturing the amount of biomass that results at higher levels from biomass consumed at lower levels.
However, when primary producers grow and are consumed the biomass at any one moment may be low. The efficiency with which energy or biomass is transferred from one trophic level to the next is called the ecological efficiency. Consumers at each level convert on average only about 10% of the chemical energy in their food to their own organic tissue. For this reason, food chains extend for more than 5 or 6 levels. At the lowest trophic level, plants convert about 1% of the sunlight they receive into chemical energy, it follows from this that the total energy present in the incident sunlight, embodied in a tertiary consumer is about 0.001% Both the number of trophic levels and the complexity of relationships between them evolve as life diversifies through time, the exception being intermittent mass extinction events. Food webs define ecosystems, the trophic levels define the position of organisms within the webs, but these trophic levels are not always simple integers, because organisms feed at more than one trophic level.
For example, some carnivores eat plants, some plants are carnivores. A large carnivore may eat both smaller car
Chemotrophs are organisms that obtain energy by the oxidation of electron donors in their environments. These molecules can be inorganic; the chemotroph designation is in contrast to phototrophs. Chemotrophs can be either heterotrophic. Chemotrophs are found in ocean floors where sunlight cannot reach them because they are not dependent on solar energy. Ocean floors contain underwater volcanos that can provide heat as a substitute for sunlight's warmth. Chemoautotrophs, in addition to deriving energy from chemical reactions, synthesize all necessary organic compounds from carbon dioxide. Chemoautotrophs use inorganic energy sources such as hydrogen sulfide, elemental sulfur, ferrous iron, molecular hydrogen, ammonia. Most chemoautotrophs are extremophiles, bacteria or archaea that live in hostile environments and are the primary producers in such ecosystems. Chemoautotrophs fall into several groups: methanogens, sulfur oxidizers and reducers, anammox bacteria, thermoacidophiles. An example of one of these prokaryotes would be Sulfolobus.
Chemolithotrophic growth can be fast, such as Hydrogenovibrio crunogenus with a doubling time around one hour. The term "chemosynthesis", coined in 1897 by Wilhelm Pfeffer was defined as the energy production by oxidation of inorganic substances in association with autotrophy - what would be named today as chemolithoautotrophy; the term would include the chemoorganoautotrophy, that is, it can be seen as a synonym of chemoautotrophy. Chemoheterotrophs are unable to fix carbon to form their own organic compounds. Chemoheterotrophs can be chemolithoheterotrophs, utilizing inorganic energy sources such as sulfur or chemoorganoheterotrophs, utilizing organic energy sources such as carbohydrates and proteins. Most animals and fungi are examples of chemoheterotrophs. In the deep oceans, iron-oxidizing bacteria derive their energy needs by oxidizing ferrous iron to ferric iron; the electron conserved from this reaction reduces the respiratory chain and can be thus used in the synthesis of ATP by forward electron transport or NADH by reverse electron transport, replacing or augmenting traditional phototrophism.
In general, iron-oxidizing bacteria can exist only in areas with high ferrous iron concentrations, such as new lava beds or areas of hydrothermal activity. Most of the ocean is devoid of ferrous iron, due to both the oxidative effect of dissolved oxygen in the water and the tendency of bacteria to take up the iron. Lava beds supply bacteria with ferrous iron straight from the Earth's mantle, but only newly formed igneous rocks have high enough levels of ferrous iron. In addition, because oxygen is necessary for the reaction, these bacteria are much more common in the upper ocean, where oxygen is more abundant. What is still unknown is how iron bacteria extract iron from rock, it is accepted that some mechanism exists that eats away at the rock through specialized enzymes or compounds that bring more FeO to the surface. It has been long debated about how much of the weathering of the rock is due to biotic components and how much can be attributed to abiotic components. Hydrothermal vents release large quantities of dissolved iron into the deep ocean, allowing bacteria to survive.
In addition, the high thermal gradient around vent systems means a wide variety of bacteria can coexist, each with its own specialized temperature niche. Regardless of the catalytic method used, chemoautotrophic bacteria provide a significant but overlooked food source for deep sea ecosystems - which otherwise receive limited sunlight and organic nutrients. Manganese-oxidizing bacteria make use of igneous lava rocks in much the same way. Manganese is more scarce than iron oceanic crust, but is much easier for bacteria to extract from igneous glass. In addition, each manganese oxidation donates two electrons to the cell versus one for each iron oxidation, though the amount of ATP or NADH that can be synthesised in couple to these reactions varies with pH and specific reaction thermodynamics in terms of how much of a Gibbs free energy change there is during the oxidation reactions versus the energy change required for the formation of ATP or NADH, all of which vary with concentration, pH etc.
Much still remains unknown about manganese-oxidizing bacteria because they have not been cultured and documented to any great extent. Autotroph Chemoautotroph Photoautotroph Heterotroph Chemoheterotroph Photoheterotroph Chemosynthesis Lithotroph 1. Katrina Edwards. Microbiology of a Sediment Pond and the Underlying Young, Hydrologically Active Ridge Flank. Woods Hole Oceanographic Institution. 2. Coupled Photochemical and Enzymatic Mn Oxidation Pathways of a Planktonic Roseobacter-Like Bacterium Colleen M. Hansel and Chris A. Francis* Department of Geological and Environmental Sciences, Stanford University, California 94305-2115 Received 28 September 2005/ Accepted 17 February 2006
In biology, detritus is dead particulate organic material. It includes the bodies or fragments of dead organisms as well as fecal material. Detritus is colonized by communities of microorganisms which act to decompose the material. In terrestrial ecosystems, it is encountered as leaf litter and other organic matter intermixed with soil, denominated "soil organic matter". Detritus of aquatic ecosystems is organic material suspended in water and piling up on seabed floors, referred to as marine snow. Dead plants or animals, material derived from animal tissues lose their form, due to both physical processes and the action of decomposers, including grazers and fungi. Decomposition, the process through which organic matter is decomposed, takes place in many stages. Materials like proteins and sugars with low molecular weight are consumed and absorbed by microorganisms and organisms that feed on dead matter. Other compounds, such as complex carbohydrates are broken down more slowly; the various microorganisms involved in the decomposition break down the organic materials in order to gain the resources they require for their own survival and proliferation.
Accordingly, at the same time that the materials of plants and animals are being broken d making up the bodies of the microorganisms are built up by a process of assimilation. When microorganisms die, fine organic particles are produced, if these are eaten by small animals which feed on microorganisms, they will collect inside the intestine, change shape into large pellets of dung; as a result of this process, most of the materials from dead organisms disappears from view and is not present in any recognisable form, but is in fact present in the form of a combination of fine organic particles and the organisms using them as nutrients. This combination is detritus. In ecosystems on land, detritus is deposited on the surface of the ground, taking forms such as the humic soil beneath a layer of fallen leaves. In aquatic ecosystems, most detritus is suspended in water, settles. In particular, many different types of material are collected together by currents, much material settles in flowing areas.
Much detritus is used as a source of nutrition for animals. In particular, many bottom feeding animals living in mud flats feed in this way. In particular, since excreta are materials which other animals do not need, whatever energy value they might have, they are unbalanced as a source of nutrients, are not suitable as a source of nutrition on their own. However, there are many microorganisms; these microorganisms do not absorb nutrients from these particles, but shape their own bodies so that they can take the resources they lack from the area around them, this allows them to make use of excreta as a source of nutrients. In practical terms, the most important constituents of detritus are complex carbohydrates, which are persistent, the microorganisms which multiply using these absorb carbon from the detritus, materials such as nitrogen and phosphorus from the water in their environment to synthesise the components of their own cells. A characteristic type of food chain called the detritus cycle takes place involving detritus feeders and the microorganisms that multiply on it.
For example, mud flats are inhabited by many univalves. When these detritus feeders take in detritus with microorganisms multiplying on it, they break down and absorb the microorganisms, which are rich in proteins, excrete the detritus, complex carbohydrates, having hardly broken it down at all. At first this dung is a poor source of nutrition, so univalves pay no attention to it, but after several days, microorganisms begin to multiply on it again, its nutritional balance improves, so they eat it again. Through this process of eating the detritus many times over and harvesting the microorganisms from it, the detritus thins out, becomes fractured and becomes easier for the microorganisms to use, so the complex carbohydrates are steadily broken down and disappear over time. What is left behind by the detritivores is further broken down and recycled by decomposers, such as bacteria and fungi; this detritus cycle plays a large part in the so-called purification process, whereby organic materials carried in by rivers is broken down and disappears, an important part in the breeding and growth of marine resources.
In ecosystems on land, far more essential material is broken down as dead material passing through the detritus chain than is broken down by being eaten by animals in a living state. In both land and aquatic ecosystems, the role played by detritus is too large to ignore. In contrast to land ecosystems, dead materials and excreta in aquatic ecosystems are transported by water flow. In freshwater bodies organic material from plants can form a silt known as mulm or humus on the bottom; this material, some called undissolved organic carbon breaks down into dissolved organic carbon and can bond to heavy metal ions via chelation. It can break down into colored dissolved organic matter such as tannin, a specific form of tannic acid. In saltwater bodies, organic material breaks down and forms a marine snow that settles down to the ocean bottom. Detritus occurs in a variety of terrestrial habitats including forest and grassland. In forests the detritus is dominated by leaf and bacteria litter as measured by biomas
A food chain is a linear network of links in a food web starting from producer organisms and ending at apex predator species, detritivores, or decomposer species. A food chain shows how the organisms are related with each other by the food they eat; each level of a food chain represents a different trophic level. A food chain differs from a food web, because the complex network of different animals' feeding relations are aggregated and the chain only follows a direct, linear pathway of one animal at a time. Natural interconnections between food chains make it a food web. A common metric used to the quantify food web trophic structure is food chain length. In its simplest form, the length of a chain is the number of links between a trophic consumer and the base of the web and the mean chain length of an entire web is the arithmetic average of the lengths of all chains in a food web. Food chains were first introduced by the Arab scientist and philosopher Al-Jahiz in the 9th century and popularized in a book published in 1927 by Charles Elton, which introduced the food web concept.
The food chain's length is a continuous variable that provides a measure of the passage of energy and an index of ecological structure that increases in value counting progressively through the linkages in a linear fashion from the lowest to the highest trophic levels. Food chains are used in ecological modeling, they are simplified abstractions of real food webs, but complex in their dynamics and mathematical implications. Ecologists have formulated and tested hypotheses regarding the nature of ecological patterns associated with food chain length, such as increasing length increasing with ecosystem size, reduction of energy at each successive level, or the proposition that long food chain lengths are unstable. Food chain studies have an important role in ecotoxicology studies tracing the pathways and biomagnification of environmental contaminants. Producers, such as plants, are organisms. All food chains must start with a producer. In the deep sea, food chains centered on hydrothermal vents and cold seeps exist in the absence of sunlight.
Chemosynthetic bacteria and archaea use hydrogen sulfide and methane from hydrothermal vents and cold seeps as an energy source to produce carbohydrates. Consumers are organisms. All organisms in a food chain, except the first organism, are consumers. In a food chain, there is reliable energy transfer through each stage. However, all the energy at one stage of the chain is not absorbed by the organism at the next stage; the amount of energy from one stage to another decreases. Heterotroph Lithotroph Trophic pyramid Predator-prey interaction
Photoheterotrophs are heterotrophic phototrophs—that is, they are organisms that use light for energy, but cannot use carbon dioxide as their sole carbon source. They use organic compounds from the environment to satisfy their carbon requirements. Examples of photoheterotrophic organisms include purple non-sulfur bacteria, green non-sulfur bacteria, heliobacteria. Recent research has indicated that the oriental hornet and some aphids may be able to use light to supplement their energy supply. Photoheterotrophs generate ATP using light, in one of two ways: they use a bacteriochlorophyll-based reaction center, or they use a bacteriorhodopsin; the chlorophyll-based mechanism is similar to that used in photosynthesis, where light excites the molecules in a reaction center and causes a flow of electrons through an electron transport chain. This flow of electrons through the proteins causes hydrogen ions to be pumped across a membrane; the energy stored in this proton gradient is used to drive ATP synthesis.
Unlike in photoautotrophs, the electrons flow only in a cyclic pathway: electrons released from the reaction center flow through the ETS and return to the reaction center. They are not utilized to reduce any organic compounds. Purple non-sulfur bacteria, green non-sulfur bacteria and heliobacteria are examples of bacteria that carry out this scheme of photoheterotrophy. Other organisms, including halobacteria and flavobacteria and vibrios have purple-rhodopsin-based proton pumps that supplement their energy supply; the archaeal version is called bacteriorhodopsin, while the eubacterial version is called proteorhodopsin. The pump consists of a single protein bound to a Vitamin A derivative, retinal; the pump may have accessory pigments associated with the protein. When light is absorbed by the retinal molecule, the molecule isomerises; this drives the protein to pump a proton across the membrane. The hydrogen ion gradient can be used to generate ATP, transport solutes across the membrane, or drive a flagellar motor.
One particular flavobacterium cannot reduce carbon dioxide using light, but uses the energy from its rhodopsin system to fix carbon dioxide through anaplerotic fixation. The flavobacterium is still a heterotroph as it needs reduced carbon compounds to live and cannot subsist on only light and CO2, it cannot carry out reactions in the form of n CO2 + 2n H2D + photons → n + 2n D + n H2O,where H2D may be water, H2S or another compound/compounds providing the reducing electrons and protons. However, it can fix carbon in reactions like: CO2 + pyruvate + ATP → malate + ADP +Piwhere malate or other useful molecules are otherwise obtained by breaking down other compounds by carbohydrate + O2 → malate + CO2 + energy; this method of carbon fixation is useful when reduced carbon compounds are scarce and cannot be wasted as CO2 during interconversions, but energy is plentiful in the form of sunlight. Autotroph Chemoautotroph Photoautotroph Heterotroph Chemoheterotroph Photoheterotroph Primary nutritional groups University of Wisconsin, Madison Microbiology Online Textbook