Narrowly defined, photoautotrophy is a nutritional condition where living organisms flourish without any extr exogenous organic materials as nutrients.

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From: progress in Biotechnology, 2001

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Phototrophy refers to the utilization of solar electromagnetic radiation for energy conservation. In many cases, however not all phototrophy likewise implies a close coupling come the energy requirement because that reduction the CO2 for adaptation into organic issue according come the general scheme:


where H2A represents an external electron donor and A is an oxidized product in a process referred to together photosynthesis.

The most vital examples of phototrophy are based upon chlorophylls or bacteriochlorophylls. In both instances a reaction center is composed of a chlorophyll or bacteriochlorophyll molecule surrounding by various other light-harvesting molecules, including carotenoids, and also that absorb irradiate in the 400 nm–550 nm range and sometimes other accessory photosynthetic pigments, phaeophytin and also an electron deliver chain. The wavelengths exploited room within the selection of 400 nm–1000 nm, yet different areas of this band room exploited by various chlorophylls and accessory photosynthetic pigments, so over there is a differential utilization of the light spectrum amongst different varieties of phototrophs based upon the pigments they possess. The reaction centers reside in the membrane of intracellular engine or tubular structures, which one of two people constitute invaginations of the cell membrane (in violet bacteria and cyanobacteria) or are made up of special (non-unit) membrane (in eco-friendly non-sulfur phototrophic bacteria, e.g., Chlorobium).

The principal procedure involves shipment of one electron by a photoactivated chlorophyll or bacteriochlorophyll come a primary acceptor (phaeophytin: a chlorophyll or bacteriochlorophyll deprived of the Mg atom) which passes the electron on come a membrane-bound electron transport chain (including FeS proteins, ubiquinones and also cytochromes) and then ago to the chlorophyll molecule in a cyclic manner. This acts prefer a proton pump which is combination to ATP synthesis by membrane phosphorylation (cylic photophosphorylation). This is quite analogous to respiration, only in phototrophs the electron resource is a photoactivated chlorophyll quite than NADH. For photosynthesis the electron can additionally be used for the production of decreased nicotinamide di nucleotide phosphate (NADPH), which is consequently used because that the reductive adaptation of CO2 (see Fig. 1.1 and the following section).


Bacteria performing so-called anoxgenic photosynthesis count on electron donors other than H2O, and thus do not create O2. Such anoxygenic bacteria are uncovered within four unrelated groups of bacteria; these are identified by, among other features, their photosynthetic pigments. The purple sulfur bacteria (phylum Proteobacteria) have actually bacteriochlorophyll a v an infrared absorb maximum in ~ 825–890 nm or bacteriochlorophyll b (1020–1040 nm); different groups of purple sulfur bacteria have different combinations that carotenoids. The violet sulfur bacteria (e.g. Thiocapsa, Thiopedia, Chromatium) typically use reduced sulfur together an electron donor for photosynthesis according to:


The cells frequently store element S. These organisms happen in and also often overcome sulfidic habitats exposed come light, e.g., the hypolimnion of part lakes and some microbial mats. They room autotrophs, and also to a varying degree O2-tolerant, but some room strict anaerobes and also this, and differences in sulfide tolerance, defines the variability in their incident in nature. Recently, an isolate closely related to the violet sulfur bacterium, Ectothiorhodopspira, has also been found to use reduced arsenic in the form of arsenite (AsO33−) as an electron donor because that photosynthesis, producing arsenate (AsO43−) together an end product (Kulp et al., 2008).

The term violet non-sulfur bacteria (phylum Proteobacteria; e.g., Rhodopseudomonas, Rhodospirillum) is not precise in that plenty of isolates have the right to utilize sulfide as an electron donor in photosynthesis, although castle are more sensitive to greater sulfide level than are the purple sulfur bacteria, and also they carry out not keep elemental S in your cells. Castle otherwise usage H2, some organic compounds and also Fe2+ together electron donors. Most recently it has been found that at least one violet sulfur bacterium and a purple non-sulfur bacterium deserve to use nitrite as an electron donor v NO3− as an finish product (Schott et al., 2010). The violet non-sulfur bacteria can also take up short molecular load organic compound such together acetate; in the dark, they prefer microaerobic conditions and also perform aerobic respiration (Fig. 1.1). Violet non-sulfur bacteria are typical in surface ar sediments.

Some members of the purple non-sulfur bacteria (within the α-Proteobacteria) have actually lost their ability to photosynthesize, however retained the volume for cyclic photophosphorylation. For this reason they have actually an absolute requirement for organic substrates for growth, however can conserve energy from light, which offers them a vain edge loved one to other heterotrophic bacteria, due to the fact that they perform not have to use as lot of their organic substrates because that dissimilatory metabolism. This presumably enabled for enhanced survival during starvation periods. Together aerobic photoheteroptrophs have actually been recognized for some time, yet it has only an ext recently been known as an extremely numerous in the marine water column, making up much more than 10% that the bacter biota (Kolber et al., 2001). These bacteria are frequently easy to isolate from seawater and also their names – Erythrobacter and also Roseobacter – expose that their characteristic red colour had actually been noticed prior to their phototrophic ability was understood.

A rather different type of phototrophy wake up in a variety of bacteria – both among Archaea and Bacteria. In the 1970s one opsin protein was uncovered in the cell membrane the a halophile archaeum, Halobacter. Opsins constitute a team of light-sensitive proteins. As soon as exposed to light they act as proton pumps expelling protons out of the cell developing an electrochemical gradient combination to ATP-generation; the Halobacter opsin was called bacteriorhodopsin (Osterhelt et al., 1977). A associated protein discovered in Proteobacteria has actually been referred to as proteorhodopsin. Opsins (such together rhodopsin) also constitute the light-sensitive colors of pet eyes and also mediate light responses in uncellular cells eukaryotes. It is now known that opsins pat a duty in irradiate sensing in various kinds the bacteria, and also that lock are also used for bacterial energy conservation via ATP synthesis. This is a extensive property amongst disparate groups of marine bacterioplankton. Most most likely the large distribution of opsins amongst different bacter taxa shows horizontal gene transfer, the assessment of which illustrates the power and value of genomic and metagenomic viewpoints for microbial biogeochemistry (Frigaard et al., 2006; MacCarran, 2007; DeLong & Béjà, 2010).

Among the bacteria only the cyanobacteria perform oxygenic photosynthesis. Oxygenic photosynthesis is the most crucial phototrophic process in the current biosphere, and has been the for maybe the last 3 billion year of earth history. It led to the climb of an O2-containing atmosphere that among other things permitted for the advancement of multi-cellular life. Oxygenic photosynthesis additionally generates the mass of chemical power in the form of organic issue that cd driver the remainder of the biosphere. Photosynthesis of eukaryotic organisms might play a larger function than the of cyanobacteria in the extant biosphere (especially terrestrial systems), but then again all proof shows that eukaryotic bio chloroplasts originated as intracellular cyanobacterial symbionts (Falkowski & Knoll, 2007).

Cyanobacteria (and chloroplasts) usage chlorophyll a together their principle photosynthetic pigment; accessory photosynthesis pigments include the phycobilins (phycocyanin and also phycoerythrin) that permit cyanobacteria to make use of the orange and green regions of the irradiate spectrum. Cyanobacteria are vital in shallow water sediments and also dominate “cyanobacterial mats” – specifically the filamentous forms, such together Oscillatoria, Lyngbya, Microcoleus, and Spirulina; however, they likewise occur in mats and sediments in unicellular forms (see chapter 7.2). Cyanobacteria are additionally important as major producers in the plankton; in this situation it is largely the unicellular types (Synechococcus) the dominate. However, mass occurrences of colonial forms such together Trichodesmium and also Nodularia may additionally occur frequently. In addition, cyanobacteria colonize terrestrial habitats including soils and also cracks in rocks (as endoliths) also in an extremely harsh environments. Part cyanobacteria live symbiotically with eukaryotes, for example they room the photosynthetic ingredient of some types of lichens.

Sediment populated cyanobacteria are typically exposed to sulfide, i beg your pardon inhibits photosystem II thus preventing O2-generation. Photosystem I, however, stays intact and under such situations some develops may use sulfide together an electron donor in your photosynthesis (Cohen, 1986).

The cyanobacterium Acaryochloris is unsual in possessing chlorophyll d that allows it to utilize near infrared light because that oxygenic photosynthesis. It has actually been uncovered beneath crustose corralline birds in coral reefs (Mohr et al., 2010).

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The prochlorophytes are small unicellular forms; they likewise contain chlorophyll b (like chloroplasts of environment-friendly algae and plants). Originally uncovered as symbionts in the tunic that certain types of tunicates, lock have more recently proven to be very important in marine plankton – in specific in oceanic web page in the deeper part of the photic ar (see additionally Chapter 4.1).