The troposphere is the lowest layer of Earth's atmosphere, is where nearly all weather conditions take place. It contains 75% of the atmosphere's mass and 99% of the total mass of water vapor and aerosols; the average height of the troposphere is 18 km in the tropics, 17 km in the middle latitudes, 6 km in the polar regions in winter. The total average height of the troposphere is 13 km; the lowest part of the troposphere, where friction with the Earth's surface influences air flow, is the planetary boundary layer. This layer is a few hundred meters to 2 km deep depending on the landform and time of day. Atop the troposphere is the tropopause, the border between the troposphere and stratosphere; the tropopause is an inversion layer, where the air temperature ceases to decrease with height and remains constant through its thickness. The word troposphere is derived from the Greek tropos and sphere, reflecting the fact that rotational turbulent mixing plays an important role in the troposphere's structure and behaviour.
Most of the phenomena associated with day-to-day weather occur in the troposphere. By volume, dry air contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, small amounts of other gases. Air contains a variable amount of water vapor. Except for the water vapor content, the composition of the troposphere is uniform; the source of water vapor is at the Earth's surface through the process of evaporation. The temperature of the troposphere decreases with altitude. And, saturation vapor pressure decreases as temperature drops. Hence, the amount of water vapor that can exist in the atmosphere decreases with altitude and the proportion of water vapor is greatest near the surface of the Earth; the pressure of the atmosphere decreases with altitude. This is because the atmosphere is nearly in hydrostatic equilibrium so that the pressure is equal to the weight of air above a given point; the change in pressure with altitude can be equated to the density with the hydrostatic equation d P d z = − ρ g n = − m P g n R T where: gn is the standard gravity ρ is the densityz is the altitude P is the pressure R is the gas constant T is the thermodynamic temperature m is the molar massSince temperature in principle depends on altitude, one needs a second equation to determine the pressure as a function of altitude as discussed in the next section.
The temperature of the troposphere decreases as altitude increases. The rate at which the temperature decreases, is called the environmental lapse rate; the ELR is nothing more than the difference in temperature between the surface and the tropopause divided by the height. The ELR assumes that the air is still, i.e. that there is no mixing of the layers of air from vertical convection, nor winds that would create turbulence and hence mixing of the layers of air. The reason for this temperature difference is that the ground absorbs most of the sun's energy, which heats the lower levels of the atmosphere with which it is in contact. Meanwhile, the radiation of heat at the top of the atmosphere results in the cooling of that part of the atmosphere; the ELR assumes as air is heated it becomes buoyant and rises. The dry adiabatic lapse rate accounts for the effect of the expansion of dry air as it rises in the atmosphere and wet adiabatic lapse rates includes the effect of the condensation of water vapor on the lapse rate.
When a parcel of air rises, it expands. As the air parcel expands, it pushes the surrounding air outward, transferring energy in the form of work from that parcel to the atmosphere; as energy transfer to a parcel of air by way of heat is slow, it is assumed to not exchange energy by way of heat with the environment. Such a process is called an adiabatic process. Since the rising parcel of air is losing energy as it does work on the surrounding atmosphere and no energy is transferred into it as heat from the atmosphere to make up for the loss, the parcel of air is losing energy, which manifests itself as a decrease in the temperature of the air parcel; the reverse, of course, will be true for a parcel of air, sinking and is being compressed. Since the process of compression and expansion of an air parcel can be considered reversible and no energy is transferred into or out of the parcel, such a process is considered isentropic, meaning that there is no change in entropy as the air parcel rises and falls, d S = 0.
Since the heat exchanged d Q = 0 is related to the entropy change d S by d Q = T d S, the equation governing the temperature as a function of height for a mixed atmosphere is d S d z = 0 where S is the entropy. The above equation states; the rate at which temperature decreases with height u
A biological oxidizer is a device that uses micro-organisms to treat wastewater and the volatile organic compounds produced by commercial and industrial operations. Biological oxidation devices convert biodegradable organic compounds into carbon water; this is a natural occurring process which differs from traditional chemical and thermal oxidizing agents and methods. Some of the more used micro-organisms are heterotrophic bacteria, which play an important role in biological degradation processes; these micro-organisms are rod shaped and facultative. Biological oxidizers provide a stable environment which allows bacteria to oxidize and stabilize a large number of organics in a more efficient manner; some of the emissions that may be treated biologically include: heterocyclic compounds. The prompt removal of a wide range of wastes and pollutants from the environment is the foremost requisite leading to minimal negative environmental impact and sustainability. Microorganisms offer excellent anabolic and catabolic adaptability to degrade and produce stabilized organic matters from contaminants.
Microbiology is providing significant views of regulatory metabolic pathways as well as effectiveness to adaption and biological degradation in our changing environment. Micro-organisms are utilized in biological remediation to control industrial and commercial vapor effluents; when utilizing biological oxidation systems for the remediation emissions, the off gases or vapors are passed through a packed bed having a thin biological film at the surface. The micro-organisms are immobilized into the thin biological film, as the vapor passes over the film they become attached and are oxidized or stabilized; the biological film accomplishes the degradation process, as the biological sump water is reprocessed over the biomedia it creates additional biological growth and as the film increases so does the biological oxidizers efficiency. Large surface area and footprint were once required to treat waste water vapor and industrial plant emissions, with the advent of advanced biological oxidation equipment a smaller footprint is required.
The footprint will occupy the same space as conventional thermal oxidizers. Excessive formation of the biological film may lead to certain problems such as sloughing, it is an important factor to maintain optimum biological film. Maintaining the biological film is accomplished by proper moisture content. For this purpose the humidity of the air is adjusted within the reaction chamber before the vapor flows over the packing media; the biological packing media may be natural or made of synthetic plastic. Recirculation of the water is always completed in the biological oxidation system to make the system more cost-effective. Biochemical oxygen demand indirectly measures the amount of biodegradable organic matters thus low values indicate direct waste water disposal; the prompt removal of a wide range of wastes and pollutants from the waste gas flow is the foremost requirement of biological oxidizers to meet regulatory permitting requirements. Micro-organisms differ in their ability to metabolize different pollutants, so the selection of the proper mix of organisms is critical.
Research is underway to genetically modify various organisms to improve their performance in biological oxidation. Biological oxidation of organic matters has led to the innovation of a low cost secondary treatment of the waste water emissions and industrial air emissions; the process of biodegradation offers a fast method which offers 4,000 catalytic cycles per minute. Destruction rate efficiency is greater than 99% on most biodegratable organics emissions; the Biological oxidation technology is free from secondary emissions with limited CO2 production. While other oxidation technologies such as thermal oxidation produces CO, NO2 and CO2; the following manufacturers have been involved in the development and planning of waste gas purification systems for a wide range of industries: Global manufacture of turnkey systems. American Fabrication and Supply, LLC Bacteria Microbiology Microorganism McGrew, Roderick. Encyclopedia of Medical History, brief history pp 25–30 Our Microbial Planet A free poster from the National Academy of Sciences about the positive roles of micro-organisms.
"Uncharted Microbial World: Microbes and Their Activities in the Environment" Report from the American Academy of Microbiology Understanding Our Microbial Planet: The New Science of Metagenomics A 20-page educational booklet providing a basic overview of metagenomics and our microbial planet. Tree of Life Eukaryotes Microbe News from Genome News Network Medical Microbiology On-line textbook Through the microscope: A look at all things small On-line microbiology textbook by Timothy Paustian and Gary Roberts, University of Wisconsin-Madison Microorganisms in the pond water on YouTube
X-rays make up X-radiation, a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 100 eV to 100 keV. X-ray wavelengths are shorter than those of UV rays and longer than those of gamma rays. In many languages, X-radiation is referred to with terms meaning Röntgen radiation, after the German scientist Wilhelm Röntgen who discovered these on November 8, 1895, credited as its discoverer, who named it X-radiation to signify an unknown type of radiation. Spelling of X-ray in the English language includes the variants x-ray, X ray. Before their discovery in 1895 X-rays were just a type of unidentified radiation emanating from experimental discharge tubes, they were noticed by scientists investigating cathode rays produced by such tubes, which are energetic electron beams that were first observed in 1869. Many of the early Crookes tubes undoubtedly radiated X-rays, because early researchers noticed effects that were attributable to them, as detailed below.
Crookes tubes created free electrons by ionization of the residual air in the tube by a high DC voltage of anywhere between a few kilovolts and 100 kV. This voltage accelerated the electrons coming from the cathode to a high enough velocity that they created X-rays when they struck the anode or the glass wall of the tube; the earliest experimenter thought to have produced. In 1785 he presented a paper to the Royal Society of London describing the effects of passing electrical currents through a evacuated glass tube, producing a glow created by X-rays; this work was further explored by his assistant Michael Faraday. When Stanford University physics professor Fernando Sanford created his "electric photography" he unknowingly generated and detected X-rays. From 1886 to 1888 he had studied in the Hermann Helmholtz laboratory in Berlin, where he became familiar with the cathode rays generated in vacuum tubes when a voltage was applied across separate electrodes, as studied by Heinrich Hertz and Philipp Lenard.
His letter of January 6, 1893 to The Physical Review was duly published and an article entitled Without Lens or Light, Photographs Taken With Plate and Object in Darkness appeared in the San Francisco Examiner. Starting in 1888, Philipp Lenard, a student of Heinrich Hertz, conducted experiments to see whether cathode rays could pass out of the Crookes tube into the air, he built a Crookes tube with a "window" in the end made of thin aluminum, facing the cathode so the cathode rays would strike it. He found that something came through, that would cause fluorescence, he measured the penetrating power of these rays through various materials. It has been suggested that at least some of these "Lenard rays" were X-rays. In 1889 Ukrainian-born Ivan Pulyui, a lecturer in experimental physics at the Prague Polytechnic who since 1877 had been constructing various designs of gas-filled tubes to investigate their properties, published a paper on how sealed photographic plates became dark when exposed to the emanations from the tubes.
Hermann von Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Röntgen made his announcement, it was formed on the basis of the electromagnetic theory of light. However, he did not work with actual X-rays. In 1894 Nikola Tesla noticed damaged film in his lab that seemed to be associated with Crookes tube experiments and began investigating this radiant energy of "invisible" kinds. After Röntgen identified the X-ray, Tesla began making X-ray images of his own using high voltages and tubes of his own design, as well as Crookes tubes. On November 8, 1895, German physics professor Wilhelm Röntgen stumbled on X-rays while experimenting with Lenard tubes and Crookes tubes and began studying them, he wrote an initial report "On a new kind of ray: A preliminary communication" and on December 28, 1895 submitted it to Würzburg's Physical-Medical Society journal. This was the first paper written on X-rays. Röntgen referred to the radiation as "X"; the name stuck.
They are still referred to as such in many languages, including German, Danish, Swedish, Estonian, Japanese, Georgian and Norwegian. Röntgen received the first Nobel Prize in Physics for his discovery. There are conflicting accounts of his discovery because Röntgen had his lab notes burned after his death, but this is a reconstruction by his biographers: Röntgen was investigating cathode rays from a Crookes tube which he had wrapped in black cardboard so that the visible light from the tube would not interfere, using a fluorescent screen painted with barium platinocyanide, he noticed a faint green glow from the screen, about 1 meter away. Röntgen realized some invisible rays coming from the tube were passing through the cardboard to make the screen glow, he found they could pass through books and papers on his desk. Röntgen threw himself into investigating these unknown rays systematically. Two months after his initial discovery, he published his paper. Röntgen discovered their medical use when he made a picture of his wife's hand on a photographic plate formed due to X-rays.
The photograph of his wife's hand was the first photograph of a human body part using X-rays. When she saw the picture, she said "I have seen my death."The discovery of X-rays stimul
Hydrogen sulfide is the chemical compound with the formula H2S. It is a colorless chalcogen hydride gas with the characteristic foul odor of rotten eggs, it is poisonous and flammable. Hydrogen sulfide is produced from the microbial breakdown of organic matter in the absence of oxygen gas, such as in swamps and sewers. H2S occurs in volcanic gases, natural gas, in some sources of well water; the human body uses it as a signaling molecule. Swedish chemist Carl Wilhelm Scheele is credited with having discovered hydrogen sulfide in 1777; the British English spelling of this compound is hydrogen sulphide, but this spelling is not recommended by the International Union of Pure and Applied Chemistry or the Royal Society of Chemistry. Hydrogen sulfide is denser than air. Hydrogen sulfide burns in oxygen with a blue flame to form sulfur water. In general, hydrogen sulfide acts as a reducing agent in the presence of base, which forms SH−. At high temperatures or in the presence of catalysts, sulfur dioxide reacts with hydrogen sulfide to form elemental sulfur and water.
This reaction is exploited in the Claus process, an important industrial method to dispose of hydrogen sulfide. Hydrogen sulfide is soluble in water and acts as a weak acid, giving the hydrosulfide ion HS−. Hydrogen sulfide and its solutions are colorless; when exposed to air, it oxidizes to form elemental sulfur, not soluble in water. The sulfide anion S2− is not formed in aqueous solution. Hydrogen sulfide reacts with metal ions to form metal sulfides, which are insoluble dark colored solids. Lead acetate paper is used to detect hydrogen sulfide because it converts to lead sulfide, black. Treating metal sulfides with strong acid liberates hydrogen sulfide. At pressures above 90 GPa, hydrogen sulfide becomes a metallic conductor of electricity; when cooled below a critical temperature this high-pressure phase exhibits superconductivity. The critical temperature increases with pressure. If hydrogen sulfide is pressurized at higher temperatures cooled, the critical temperature reaches 203 K, the highest accepted superconducting critical temperature as of 2015.
By substituting a small part of sulfur with phosphorus and using higher pressures, it has been predicted that it may be possible to raise the critical temperature to above 0 °C and achieve room-temperature superconductivity. Hydrogen sulfide is most obtained by its separation from sour gas, natural gas with high content of H2S, it can be produced by treating hydrogen with molten elemental sulfur at about 450 °C. Hydrocarbons can serve as a source of hydrogen in this process. Sulfate-reducing bacteria generate usable energy under low-oxygen conditions by using sulfates to oxidize organic compounds or hydrogen. A standard lab preparation is to treat ferrous sulfide with a strong acid in a Kipp generator: FeS + 2 HCl → FeCl2 + H2SFor use in qualitative inorganic analysis, thioacetamide is used to generate H2S: CH3CNH2 + H2O → CH3CNH2 + H2SMany metal and nonmetal sulfides, e.g. aluminium sulfide, phosphorus pentasulfide, silicon disulfide liberate hydrogen sulfide upon exposure to water: 6 H2O + Al2S3 → 3 H2S + 2 Al3This gas is produced by heating sulfur with solid organic compounds and by reducing sulfurated organic compounds with hydrogen.
Water heaters can aid the conversion of sulfate in water to hydrogen sulfide gas. This is due to providing a warm environment sustainable for sulfur bacteria and maintaining the reaction which interacts between sulfate in the water and the water heater anode, made from magnesium metal. Hydrogen sulfide can be generated in cells via non enzymatic pathway. H2S in the body acts as a gaseous signaling molecule, known to inhibit Complex IV of the mitochondrial electron transport chain which reduces ATP generation and biochemical activity within cells. Three enzymes are known to synthesize H2S: cystathionine γ-lyase, cystathionine β-synthetase and 3-mercaptopyruvate sulfurtransferase; these enzymes have been identified in a breadth of biological cells and tissues, their activity has been observed to be induced by a number of disease states. It is becoming clear that H2S is an important mediator of a wide range of cell functions in health and in disease. CBS and CSE are the main proponents of H2S biogenesis.
These enzymes are characterized by the transfer of a sulfur atom from methionine to serine to form a cysteine molecule. 3-MST contributes to hydrogen sulfide production by way of the cysteine catabolic pathway. Dietary amino acids, such as methionine and cysteine serve as the primary substrates for the transulfuration pathways and in the production of hydrogen sulfide. Hydrogen sulfide can be synthesized by non-enzymatic pathway, derived from proteins such as ferredoxins and Rieske proteins. H2S has been shown to be involved in physiological processes like vasoconstriction in animals, increasing seed germination and stress responses in plants. Hydrogen sulfide signaling is innately intertwined with physiological processes that are known to be moderated by reactive oxygen species and reactive nitrogen species. H2S has been shown to interact with NO resulting in severa
A chemical compound is a chemical substance composed of many identical molecules composed of atoms from more than one element held together by chemical bonds. A chemical element bonded to an identical chemical element is not a chemical compound since only one element, not two different elements, is involved. There are four types of compounds, depending on how the constituent atoms are held together: molecules held together by covalent bonds ionic compounds held together by ionic bonds intermetallic compounds held together by metallic bonds certain complexes held together by coordinate covalent bonds. A chemical formula is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using the standard abbreviations for the chemical elements, subscripts to indicate the number of atoms involved. For example, water is composed of two hydrogen atoms bonded to one oxygen atom: the chemical formula is H2O. Many chemical compounds have a unique numerical identifier assigned by the Chemical Abstracts Service: its CAS number.
A compound can be converted to a different chemical composition by interaction with a second chemical compound via a chemical reaction. In this process, bonds between atoms are broken in both of the interacting compounds, bonds are reformed so that new associations are made between atoms. Any substance consisting of two or more different types of atoms in a fixed stoichiometric proportion can be termed a chemical compound, it follows from their being composed of fixed proportions of two or more types of atoms that chemical compounds can be converted, via chemical reaction, into compounds or substances each having fewer atoms. The ratio of each element in the compound is expressed in a ratio in its chemical formula. A chemical formula is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using the standard abbreviations for the chemical elements, subscripts to indicate the number of atoms involved. For example, water is composed of two hydrogen atoms bonded to one oxygen atom: the chemical formula is H2O.
In the case of non-stoichiometric compounds, the proportions may be reproducible with regard to their preparation, give fixed proportions of their component elements, but proportions that are not integral. Chemical compounds have a unique and defined chemical structure held together in a defined spatial arrangement by chemical bonds. Chemical compounds can be molecular compounds held together by covalent bonds, salts held together by ionic bonds, intermetallic compounds held together by metallic bonds, or the subset of chemical complexes that are held together by coordinate covalent bonds. Pure chemical elements are not considered chemical compounds, failing the two or more atom requirement, though they consist of molecules composed of multiple atoms. Many chemical compounds have a unique numerical identifier assigned by the Chemical Abstracts Service: its CAS number. There is varying and sometimes inconsistent nomenclature differentiating substances, which include non-stoichiometric examples, from chemical compounds, which require the fixed ratios.
Many solid chemical substances—for example many silicate minerals—are chemical substances, but do not have simple formulae reflecting chemically bonding of elements to one another in fixed ratios. It may be argued that they are related to, rather than being chemical compounds, insofar as the variability in their compositions is due to either the presence of foreign elements trapped within the crystal structure of an otherwise known true chemical compound, or due to perturbations in structure relative to the known compound that arise because of an excess of deficit of the constituent elements at places in its structure. Other compounds regarded as chemically identical may have varying amounts of heavy or light isotopes of the constituent elements, which changes the ratio of elements by mass slightly. Compounds are held together through a variety of different types of bonding and forces; the differences in the types of bonds in compounds differ based on the types of elements present in the compound.
London dispersion forces are the weakest force of all intermolecular forces. They are temporary attractive forces that form when the electrons in two adjacent atoms are positioned so that they create a temporary dipole. Additionally, London dispersion forces are responsible for condensing non polar substances to liquids, to further freeze to a solid state dependent on how low the temperature of the environment is. A covalent bond known as a molecular bond, involves the sharing of electrons between two atoms; this type of bond occurs between elements that fall close to each other on the periodic table of elements, yet it is observed between some metals and nonmetals. This is due to the mechanism of this type of bond. Elements that fall close to each other on the periodic table tend to have similar electronegativities, which means they have a similar affinity for electrons. Since neither element has a stronger affinity to donate or gain electrons, it causes the elements to share electrons so both elements have a more stable octet.
Ionic bonding occurs when valence electrons are transferred between elements. Opposite to covalent bonding, this chemical bond creates two oppositely charged ions; the metals in ionic bonding
Cyanobacteria known as Cyanophyta, are a phylum of bacteria that obtain their energy through photosynthesis and are the only photosynthetic prokaryotes able to produce oxygen. The name cyanobacteria comes from the color of the bacteria. Cyanobacteria, which are prokaryotes, are called "blue-green algae", though the term algae in modern usage is restricted to eukaryotes. Unlike heterotrophic prokaryotes, cyanobacteria have internal membranes; these are flattened. Phototrophic eukaryotes perform photosynthesis by plastids that may have their ancestry in cyanobacteria, acquired long ago via a process called endosymbiosis; these endosymbiotic cyanobacteria in eukaryotes may have evolved or differentiated into specialized organelles such as chloroplasts and leucoplasts. By producing and releasing oxygen, cyanobacteria are thought to have converted the early oxygen-poor, reducing atmosphere into an oxidizing one, causing the Great Oxygenation Event and the "rusting of the Earth", which changed the composition of the Earth's life forms and led to the near-extinction of anaerobic organisms.
Cyanobacteria are a group of photosynthetic bacteria, some of which are nitrogen-fixing, that live in a wide variety of moist soils and water either or in a symbiotic relationship with plants or lichen-forming fungi. They include colonial species. Colonies may form filaments, sheets, or hollow spheres; some filamentous species can differentiate into several different cell types: vegetative cells – the normal, photosynthetic cells that are formed under favorable growing conditions. Some cyanobacteria can fix atmospheric nitrogen in anaerobic conditions by means of specialized cells called heterocysts. Heterocysts may form under the appropriate environmental conditions when fixed nitrogen is scarce. Heterocyst-forming species are specialized for nitrogen fixation and are able to fix nitrogen gas into ammonia, nitrites or nitrates, which can be absorbed by plants and converted to protein and nucleic acids. Free-living cyanobacteria are present in the water of rice paddies, cyanobacteria can be found growing as epiphytes on the surfaces of the green alga, where they may fix nitrogen.
Cyanobacteria such as Anabaena can provide rice plantations with biofertilizer. Many cyanobacteria form motile filaments of cells, called hormogonia, that travel away from the main biomass to bud and form new colonies elsewhere; the cells in a hormogonium are thinner than in the vegetative state, the cells on either end of the motile chain may be tapered. To break away from the parent colony, a hormogonium must tear apart a weaker cell in a filament, called a necridium; each individual cell has a thick, gelatinous cell wall. They lack flagella. Many of the multicellular filamentous. In water columns, some cyanobacteria float by forming gas vesicles, as in archaea; these vesicles are not organelles as such. They are not bounded by a protein sheath. Cyanobacteria can be found in every terrestrial and aquatic habitat—oceans, fresh water, damp soil, temporarily moistened rocks in deserts, bare rock and soil, Antarctic rocks, they can form phototrophic biofilms. They are found in endolithic ecosystem. A few are endosymbionts in lichens, various protists, or sponges and provide energy for the host.
Some live in the fur of sloths. Aquatic cyanobacteria are known for their extensive and visible blooms that can form in both freshwater and marine environments; the blooms can have the appearance of blue-green scum. These blooms can be toxic, lead to the closure of recreational waters when spotted. Marine bacteriophages are significant parasites of unicellular marine cyanobacteria. Cyanobacteria growth is favored in ponds and lakes where waters are calm and have less turbulent mixing, their life cycles are disrupted when the water or artificially mixes from churning currents caused by the flowing water of streams or the churning water of fountains. For this reason blooms of cyanobacteria occur in rivers unless the water is flowing slowly. Growth is favored at higher temperatures, making increasing water temperature as a result of global warming more problematic. At higher temperatures Microcystis species are able to outcompete green algae; this is a concern because of the production of toxins produced by Microcystis.
Based on environmental trends and observations suggest cyanobacteria will increase their dominance in aquatic environments. This can lead to serious consequences the contamination of sources of drinking water. Cyanobacteria can interfere with water treatment in various ways by plugging filters and by producing cyanotoxins, which have the potential to cause serious illness if consumed. Consequences may lie within
The green algae are a large, informal grouping of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now placed in separate divisions, as well as the more basal Mesostigmatophyceae, Chlorokybophyceae and Spirotaenia. The land plants, or embryophytes, are thought to have emerged from the charophytes. Therefore, embryophytes belong to green algae as well. However, because the embryophytes are traditionally classified as neither algae nor green algae, green algae are a paraphyletic group. Since the realization that the embryophytes emerged from within the green algae, some authors are starting to include them; the clade that includes both green algae and embryophytes is monophyletic and is referred to as the clade Viridiplantae and as the kingdom Plantae. The green algae include unicellular and colonial flagellates, most with two flagella per cell, as well as various colonial and filamentous forms, macroscopic, multicellular seaweeds. There are about 8,000 species of green algae.
Many species live most of their lives as single cells, while other species form coenobia, long filaments, or differentiated macroscopic seaweeds. A few other organisms rely on green algae to conduct photosynthesis for them; the chloroplasts in euglenids and chlorarachniophytes were acquired from ingested green algae, in the latter retain a nucleomorph. Green algae are found symbiotically in the ciliate Paramecium, in Hydra viridissima and in flatworms; some species of green algae of genera Trebouxia of the class Trebouxiophyceae and Trentepohlia, can be found in symbiotic associations with fungi to form lichens. In general the fungal species that partner in lichens cannot live on their own, while the algal species is found living in nature without the fungus. Trentepohlia is a filamentous green alga that can live independently on humid soil, rocks or tree bark or form the photosymbiont in lichens of the family Graphidaceae; the macroalga Prasiola calophylla is terrestrial, Prasiola crispa, which live in the supralittoral zone, is terrestrial and can in the Antarctic form large carpets on humid soil near bird colonies.
Green algae have chloroplasts that contain chlorophyll a and b, giving them a bright green color, as well as the accessory pigments beta carotene and xanthophylls, in stacked thylakoids. The cell walls of green algae contain cellulose, they store carbohydrate in the form of starch. All green algae have mitochondria with flat cristae; when present, paired flagella are used to move the cell. They are anchored by a cross-shaped system of microtubules and fibrous strands. Flagella are only present in the motile male gametes of charophytes bryophytes, pteridophytes and Ginkgo, but are absent from the gametes of Pinophyta and flowering plants. Members of the class Chlorophyceae undergo closed mitosis in the most common form of cell division among the green algae, which occurs via a phycoplast. By contrast, charophyte green algae and land plants undergo open mitosis without centrioles. Instead, a'raft' of microtubules, the phragmoplast, is formed from the mitotic spindle and cell division involves the use of this phragmoplast in the production of a cell plate.
Photosynthetic eukaryotes originated following a primary endosymbiotic event, where a heterotrophic eukaryotic cell engulfed a photosynthetic cyanobacterium-like prokaryote that became stably integrated and evolved into a membrane-bound organelle: the plastid. This primary endosymbiosis event gave rise to three autotrophic clades with primary plastids: the green plants, the red algae and the glaucophytes. Green algae are classified with their embryophyte descendants in the green plant clade Viridiplantae. Viridiplantae, together with red algae and glaucophyte algae, form the supergroup Primoplantae known as Archaeplastida or Plantae sensu lato; the ancestorial green algae was a unicellular flagellate. The Viridiplantae diverged into two clades; the Chlorophyta include the early diverging prasinophyte lineages and the core Chlorophyta, which contain the majority of described species of green algae. The Streptophyta include charophytes and land plants. Below is a consensus reconstruction of green algal relationships based on molecular data.
The basal character of the Mesostigmatophyceae, Chlorokybophyceae and spirotaenia are only more conventially basal Streptophytes. The algae of this paraphyletic group "Charophyta" were included in Chlorophyta, so green algae and Chlorophyta in this definition were synonyms; as the green algae clades get further resolved, the embryophytes, which are a deep charophyte branch, are included in "algae", "green algae" and "Charophytes", or these terms are replaced by cladistic terminology such as Archaeplastida, Viridiplantae or streptophytes, respectively. Green algae are eukaryotic organisms that follow a reproduction cycle called alternation of generations. Reproduction varies from fusion of identical cells to fertilization of a large non-motile cell by a smaller motile one. However, these traits show some variation, most notably among the basal green algae called prasinophytes. Haploid algal cells can fuse with other haploid cells to form diploid zygotes; when filamentous algae do this, they form bridges between cells, leave empty cell walls behind that can be distinguished under the light microscope.
This process occurs for example in Spirogyra. The species of Ulva are reproductively isomorphic, the diploid vegetative phase is the site of meiosis and releases haploid zoospores, which germinate and grow prod