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77-618: During the Riphean, there was a great increase in stromatolite diversity, possibly related to the appearance of eukaryotes. The word 'Riphean' comes from the ancient name " Riphean Mountains ", sometimes identified with the Ural Mountains . The Riphean has been divided by geologists into the Early Riphean (1600–1400 Ma), Middle Riphean (1400–1000 Ma) and Late Riphean (1000–600 Ma) subdivisions. This geochronology article

154-421: A certain ratio of wavelengths. Since the wavelengths compositions change in water with depth: Short (UV, violet) and long (red) wavelengths are lost first, phototaxis and gravitaxis form a ratio-chromatic depth gauge , which allows the larvae to determine their depth by the color of the surrounding water. This has the advantage over a brightness based depth gauge that the color stays almost constant independent of

231-422: A complex and heterogeneous environment such as a phototrophic biofilm, many factors crucial for growth could vary dramatically even within the limited region that a single motile cell could explore. We should therefore expect that prokaryotes living in such environments might control their motility in response to a complex signal transduction network linking a range of environmental cues. The photophobic response

308-404: A fixed shape, are polarized, swim in a spiral and use cilia for swimming and phototactic steering. Signalling can happen via direct light-triggered ion currents , adenylyl cyclases or trimeric G-proteins . The photoreceptors used can also be very different (see below). However, signalling in all cases eventually modifies the beating activity of cilia. The mechanics of phototactic orientation

385-436: A light source (positive phototaxis) or away from a light source (negative phototaxis). In contrast to the photophobic/scotophobic responses, true phototaxis is not a response to a temporal change in light intensity. Generally, it seems to involve direct sensing of the direction of illumination rather than a spatial gradient of light intensity. True phototaxis in prokaryotes is sometimes combined with social motility, which involves

462-413: A light-sensing organ. Eukaryotes evolved for the first time in the history of life the ability to follow light direction in three dimensions in open water. The strategy of eukaryotic sensory integration, sensory processing and the speed and mechanics of tactic responses is fundamentally different from that found in prokaryotes. Both single-celled and multi-cellular eukaryotic phototactic organisms have

539-466: A likely role for horizontal gene transfer in spreading phototrophy across multiple phyla. Thus, different groups of phototrophic prokaryotes may have little in common apart from their exploitation of light as an energy source, but it should be advantageous for any phototroph to be able to relocate in search of better light environments for photosynthesis. To do this efficiently requires the ability to control motility in response to integrated information on

616-611: A major constituent of the fossil record of the first forms of life on Earth. They peaked about 1.25 billion years ago (Ga) and subsequently declined in abundance and diversity, so that by the start of the Cambrian they had fallen to 20% of their peak. The most widely supported explanation is that stromatolite builders fell victim to grazing creatures (the Cambrian substrate revolution ); this theory implies that sufficiently complex organisms were common around 1 Ga. Another hypothesis

693-530: A means of providing shelter and protection from a harsh environment. Lichen stromatolites are a proposed mechanism of formation of some kinds of layered rock structure that are formed above water, where rock meets air, by repeated colonization of the rock by endolithic lichens . Some Archean rock formations show macroscopic similarity to modern microbial structures, leading to the inference that these structures represent evidence of ancient life, namely stromatolites. However, others regard these patterns as being

770-418: A new anterior cilium. As all other ciliary swimmers, green algae always swim in a spiral. The handedness of the spiral is robust and is guaranteed by the chirality of the cilia. The two cilia of green algae have different beat patterns and functions. In Chlamydomonas, the phototransduction cascade alters the stroke pattern and beating speed of the two cilia differentially in a complex pattern. This results in

847-467: A phototactic response, many of which are incidental and serve no end purpose. Phototaxis can be advantageous for phototrophic bacteria as they can orient themselves most efficiently to receive light for photosynthesis . Phototaxis is called positive if the movement is in the direction of increasing light intensity and negative if the direction is opposite. Two types of positive phototaxis are observed in prokaryotes (bacteria and archea ). The first

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924-440: A relatively sudden drop in light intensity. Photophobic and scotophobic responses both cause cells to accumulate in regions of specific (presumably favorable) light intensity and spectral quality. Scotophobic responses have been well documented in purple photosynthetic bacteria, starting with the classic observations of Engelmann in 1883, and in cyanobacteria. Scotophobic/photophobic responses in flagellated bacteria closely resemble

1001-424: A two-photon reaction, a repellent receptor for near-UV light, while SRII is a repellent receptor for blue light. Depending on which receptor is expressed, if a cell swims up or down a steep light gradient, the probability of flagellar switch will be low. If light intensity is constant or changes in the wrong direction, a switch in the direction of flagellar rotation will reorient the cell in a new, random direction. As

1078-643: A unique ecosystem in the Mexican desert. Alchichica Lake in Puebla , Mexico has two distinct morphologic generations of stromatolites: columnar-dome like structures, rich in aragonite , forming near the shore line, dated back to 1,100 years before present (ybp) and spongy-cauliflower like thrombolytic structures that dominate the lake from top to the bottom, mainly composed of hydromagnesite , huntite , calcite and dated back to 2,800 ybp. The only open marine environment where modern stromatolites are known to prosper

1155-516: A well-exposed example of the thrombolite-stromatolite-metazoan reefs that developed during the Proterozoic period, the stromatolites here being better developed in updip locations under conditions of higher current velocities and greater sediment influx. Modern stromatolites are mostly found in hypersaline lakes and marine lagoons where high saline levels prevent animal grazing. One such location where excellent modern specimens can be observed

1232-773: Is Hamelin Pool Marine Nature Reserve , Shark Bay in Western Australia . In 2010, a fifth type of chlorophyll , namely chlorophyll f , was discovered by Min Chen from stromatolites in Shark Bay. Halococcus hamelinensis , a halophilic archaeon , occurs in living stromatolites in Shark Bay where it is exposed to extreme conditions of UV radiation, salinity and desiccation . H. hamelinesis possesses genes that encode enzymes employed in

1309-846: Is a stub . You can help Misplaced Pages by expanding it . Stromatolite Stromatolites ( / s t r oʊ ˈ m æ t ə ˌ l aɪ t s , s t r ə -/ stroh- MAT -ə-lytes, strə- ) or stromatoliths (from Ancient Greek στρῶμα ( strôma ) , GEN στρώματος ( strṓmatos )  'layer, stratum' and λίθος ( líthos )  'rock') are layered sedimentary formations ( microbialite ) that are created mainly by photosynthetic microorganisms such as cyanobacteria , sulfate-reducing bacteria , and Pseudomonadota (formerly proteobacteria). These microorganisms produce adhesive compounds that cement sand and other rocky materials to form mineral " microbial mats ". In turn, these mats build up layer by layer, growing gradually over time. This process generates

1386-415: Is a change in the direction of motility in response to a relatively sudden increase in illumination: classically, the response is to a temporal change in light intensity, which the bacterium may experience as it moves into a brightly illuminated region. The directional switch may consist of a random selection of a new direction (‘tumbling’) or it may be a simple reversal in the direction of motility. Either has

1463-430: Is an active area of research in geology. Multiple morphologies of stromatolites may exist in a single local or geological strata, relating to the specific conditions occurring in different region and water depths. Most stromatolites are spongiostromate in texture, having no recognisable microstructure or cellular remains. A minority are porostromate , having recognisable microstructure; these are mostly unknown from

1540-430: Is analogous in all eukaryotes. A photosensor with a restricted view angle rotates to scan the space and signals periodically to the cilia to alter their beating, which will change the direction of the helical swimming trajectory. Three-dimensional phototaxis can be found in five out of the six eukaryotic major groups ( opisthokonts , Amoebozoa , plants , chromalveolates , excavates , rhizaria ). Pelagic phototaxis

1617-517: Is analogous to positive chemotaxis except that the attractant is light rather than a chemical. Phototactic responses are observed in a number of bacteria and archae, such as Serratia marcescens . Photoreceptor proteins are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. Some examples are bacteriorhodopsin and bacteriophytochromes in some bacteria. See also: phytochrome and phototropism . Most prokaryotes (bacteria and archaea) are unable to sense

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1694-426: Is called "scotophobotaxis" (from the word " scotophobia "), which is observed only under a microscope. This occurs when a bacterium swims by chance out of the area illuminated by the microscope. Entering darkness signals the cell to reverse flagella rotation direction and reenter the light. The second type of phototaxis is true phototaxis, which is a directed movement up a gradient to an increasing amount of light. This

1771-417: Is capable of both positive and negative two-dimensional phototactic orientation. The positive response is probably mediated by a bacteriophytochrome photoreceptor, TaxD1. This protein has two chromophore-binding GAF domains, which bind biliverdin chromophore, and a C-terminal domain typical for bacterial taxis receptors ( MCP signal domain). TaxD1 also has two N-terminal transmembrane segments that anchor

1848-403: Is defined by a complex combination of factors including light intensity, light quality, day and night cycles, the availability of raw materials and alternative energy sources, other beneficial or harmful physical and chemical factors and sometimes the presence of symbiotic partners. Light quality strongly influences specialized developmental pathways in certain filamentous cyanobacteria , including

1925-462: Is mediated by simple eyespots that consists of a pigment cell and a photoreceptor cell . The photoreceptor cell synapses directly onto ciliated cells, which are used for swimming. The eyespots do not give spatial resolution, therefore the larvae are rotating to scan their environment for the direction where the light is coming from. Platynereis dumerilii larvae ( nectochaete ) can switch between positive and negative phototaxis. Phototaxis there

2002-408: Is mediated by two pairs of more complex pigment cup eyes. These eyes contain more photoreceptor cells that are shaded by pigment cells forming a cup. The photoreceptor cells do not synapse directly onto ciliated cells or muscle cells but onto inter-neurons of a processing center. This way the information of all four eye cups can be compared and a low-resolution image of four pixels can be created telling

2079-451: Is present in green algae – it is not present in glaucophyte algae or red algae . Green algae have a "stigma" located in the outermost portion of the chloroplast , directly underneath the two chloroplast membranes . The stigma is made of tens to several hundreds of lipid globules, which often form hexagonal arrays and can be arranged in one or more rows. The lipid globules contain a complex mixture of carotenoid pigments, which provide

2156-404: Is that protozoa such as foraminifera were responsible for the decline, favoring formation of thrombolites over stromatolites through microscopic bioturbation . Proterozoic stromatolite microfossils (preserved by permineralization in silica) include cyanobacteria and possibly some forms of the eukaryote chlorophytes (that is, green algae ). One genus of stromatolite very common in

2233-640: Is the Exuma Cays in the Bahamas. Laguna de Bacalar in Mexico's southern Yucatán Peninsula has an extensive formation of living giant microbialites (that is, stromatolites or thrombolites). The microbialite bed is over 10 km (6.2 mi) long with a vertical rise of several meters in some areas. These may be the largest sized living freshwater microbialites, or any organism, on Earth. A 1.5 km stretch of reef-forming stromatolites (primarily of

2310-481: Is the only light-direction sensing behaviour prokaryotes could evolve owing to the difficulty in detecting light direction at this small scale. The ability to link light perception to control of motility is found in a very wide variety of prokaryotes, indicating that this ability must confer a range of physiological advantages. Most directly, the light environment is crucial to phototrophs as their energy source. Phototrophic prokaryotes are extraordinarily diverse, with

2387-539: The Holocene , living forms are rare. Stromatolites are layered, biochemical, accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains in biofilms (specifically microbial mats), through the action of certain microbial lifeforms, especially cyanobacteria . They exhibit a variety of forms and structures, or morphologies, including conical, stratiform, domal, columnar, and branching types. Stromatolites occur widely in

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2464-543: The Little Blue Lake in the Lower South-East of South Australia . Phototaxis Phototaxis is a kind of taxis , or locomotory movement, that occurs when a whole organism moves towards or away from a stimulus of light . This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis . Phototaxis is called positive if

2541-513: The geologic record is Collenia . The connection between grazer and stromatolite abundance is well documented in the younger Ordovician evolutionary radiation ; stromatolite abundance also increased after the Late Ordovician mass extinction and Permian–Triassic extinction event decimated marine animals, falling back to earlier levels as marine animals recovered. Fluctuations in metazoan population and diversity may not have been

2618-525: The Nettle Cave at Jenolan Caves , NSW , Australia. The cyanobacteria live on the surface of the limestone and are sustained by the calcium-rich dripping water, which allows them to grow toward the two open ends of the cave which provide light. Stromatolites composed of calcite have been found in both the Blue Lake in the dormant volcano, Mount Gambier and at least eight cenote lakes including

2695-591: The Precambrian but persist throughout the Palaeozoic and Mesozoic . Since the Eocene , porostromate stromatolites are known only from freshwater settings. Time lapse photography of modern microbial mat formation in a laboratory setting gives some revealing clues to the behavior of cyanobacteria in stromatolites. Biddanda et al. (2015) found that cyanobacteria exposed to localized beams of light moved towards

2772-402: The adjacent membrane-inserted photoreceptors (the term "eyespot" is therefore misleading). Stigmata can also reflect and focus light like a concave mirror, thereby enhancing sensitivity. In the best-studied green alga, Chlamydomonas reinhardtii , phototaxis is mediated by a rhodopsin pigment, as first demonstrated by the restoration of normal photobehaviour in a blind mutant by analogues of

2849-420: The amount of oxygen in the primeval Earth's atmosphere through their continuing photosynthesis (see Great Oxygenation Event ). They use water, carbon dioxide, and sunlight to create their food. A layer of polysaccharides often forms over mats of cyanobacterial cells. In modern microbial mats, debris from the surrounding habitat can become trapped within the polysaccharide layer, which can be cemented together by

2926-444: The calcium carbonate to grow thin laminations of limestone . These laminations can accrete over time, resulting in the banded pattern common to stromatolites. The domal morphology of biological stromatolites is the result of the vertical growth necessary for the continued infiltration of sunlight to the organisms for photosynthesis. Layered spherical growth structures termed oncolites are similar to stromatolites and are also known from

3003-480: The cell to reverse flagella rotation direction and reenter the light. The second type of phototaxis is true phototaxis, which is a directed movement up a gradient to an increasing amount of light. This is analogous to positive chemotaxis except that the attractant is light rather than a chemical. Phototactic responses are observed in many organisms such as Serratia marcescens , Tetrahymena , and Euglena . Each organism has its own specific biological cause for

3080-417: The characteristic lamination of stromatolites, a feature that is hard to interpret, in terms of its temporal and environmental significance. Different styles of stromatolite lamination have been described, which can be studied through microscopic and mathematical methods. A stromatolite may grow to a meter or more. Fossilized stromatolites provide important records of some of the most ancient life. As of

3157-422: The classic ‘biased random walk’ mode of bacterial chemotaxis, which links perception of temporal changes in the concentration of a chemical attractant or repellent to the frequency of tumbling. The only significant distinction is that the scotophobic/photophobic responses involve perception of temporal changes in light intensity rather than the concentration of a chemical. Photokinesis is a light-induced change in

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3234-519: The concerted movement of an entire colony of cells towards or away from the light source. This phenomenon could also be described as community phototaxis. True phototaxis is widespread in eukaryotic green algae , but among the prokaryotes it has been documented only in cyanobacteria, and in social motility of colonies of the purple photosynthetic bacterium Rhodocista centenaria . Some protists (unicellular eukaryotes) can also move toward or away from light, by coupling their locomotion strategy with

3311-513: The connection between airborne locomotion toward a light source. This innate response is common among insects that fly primarily during the night utilizing transverse orientation vis-à-vis the light of the moon for orientation. Artificial lighting in cities and populated areas results in a more pronounced positive response compared to that with the distant light of the moon, resulting in the organism repeatedly responding to this new supernormal stimulus and innately flying toward it. Evidence for

3388-481: The cup eyes: Two rhabdomeric opsins and a Go-opsin. However, not every behavior that looks like phototaxis is phototaxis: Platynereis dumerilii nechtochate and metatrochophore larvae swim up first when they are stimulated with UV-light from above. But after a while, they change the direction and avoid the UV-light by swimming down. This looks like a change from positive to negative phototaxis (see video left), but

3465-405: The cyanobacteria form clumps that then expand outwards, with individual members remaining connected to the colony via long tendrils. This may be a protective mechanism that affords evolutionary benefit to the colony in harsh environments where mechanical forces act to tear apart the microbial mats. Thus these sometimes elaborate structures, constructed by microorganisms working somewhat in unison, are

3542-404: The development of motile hormogonia and nitrogen-fixing heterocysts . Since hormogonia are important for establishing symbiotic partnerships between cyanobacteria and plants, and heterocysts are essential for nitrogen fixation in those partnerships, it is tempting to speculate that the cyanobacteria may be using light signals as one way to detect the proximity of a plant symbiotic partner. Within

3619-427: The direction of light, because at such a small scale it is very difficult to make a detector that can distinguish a single light direction. Still, prokaryotes can measure light intensity and move in a light-intensity gradient. Some gliding filamentous prokaryotes can even sense light direction and make directed turns, but their phototactic movement is very slow. Some bacteria and archaea are phototactic. In most cases

3696-400: The effect of repelling cells from a patch of unfavorable light. Photophobic responses have been observed in prokaryotes as diverse as Escherichia coli , purple photosynthetic bacteria and haloarchaea . The scotophobic (fear of darkness) response is the converse of the photophobic response described above: a change in direction (tumbling or reversal) is induced when the cell experiences

3773-497: The first three developmental instar stages, despite adult insects displaying positive phototaxis. This behaviour is common among other species of insects which possess a flightless larval and adult stage in their life cycles, only switching to positive phototaxis when searching for pupation sites. Tenebrio molitor by comparison is one species which carries its negative phototaxis into adulthood. Under experimental conditions, organisms that use positive phototaxis have also shown

3850-585: The fossil record of the Precambrian but are rare today. Very few Archean stromatolites contain fossilized microbes, but fossilized microbes are sometimes abundant in Proterozoic stromatolites. While features of some stromatolites are suggestive of biological activity, others possess features that are more consistent with abiotic (non-biological) precipitation. Finding reliable ways to distinguish between biologically formed and abiotic stromatolites

3927-477: The fossil record. Thrombolites are poorly laminated or non-laminated clotted structures formed by cyanobacteria, common in the fossil record and in modern sediments. There is evidence that thrombolites form in preference to stromatolites when foraminifera are part of the biological community. The Zebra River Canyon area of the Kubis platform in the deeply dissected Zaris Mountains of southwestern Namibia provides

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4004-696: The genus Scytonema ) occurs in Chetumal Bay in Belize , just south of the mouth of the Rio Hondo and the Mexican border. Large microbialite towers up to 40 m high were discovered in the largest soda lake on Earth Lake Van in eastern Turkey. They are composed of aragonite and grow by precipitation of calcite from sub-lacustrine karst-water. Freshwater stromatolites are found in Lake Salda in southern Turkey. The waters are rich in magnesium and

4081-543: The innate response of positive phototaxis in Drosophila melanogaster was carried out by altering the wings of several individual specimens, both physically (via removal) and genetically (via mutation). In both cases there was a noticeable lack of positive phototaxis, demonstrating that flying toward light sources is an innate response to the organisms' photoreceptors receiving a positive response. Negative phototaxis can be observed in larval drosophila melanogaster within

4158-457: The intensity of light, the spectral quality of light and the physiological status of the cell. A second major reason for light-controlled motility is to avoid light at damaging intensities or wavelengths: this factor is not confined to photosynthetic bacteria since light (especially in the UV region) can be dangerous to all prokaryotes, primarily because of DNA and protein damage  and inhibition of

4235-532: The jellyfish can either remain still, or quickly move away in bursts to avoid predation and also re-adjust toward a new light source. This motor response to light and absence of light is facilitated by a chemical response from the ocelli, which results in a motor response causing the organism to swim toward a light source. Phototaxis has been well studied in the marine ragworm Platynereis dumerilii . Both Platynereis dumerilii trochophore and its metatrochophore larvae are positively phototactic. Phototaxis

4312-426: The larvae also swim down if UV-light comes non-directionally from the side. And so they do not swim to or away from the light, but swim down, this means to the center of gravity. Thus this is a UV-induced positive gravitaxis . Positive phototaxis (swimming to the light from the surface) and positive gravitaxis (swimming to the center of gravity) are induced by different ranges of wavelengths and cancel out each other at

4389-512: The larvae where the light is coming from. This way the larva does not need to scan its environment by rotating. This is an adaption for living on the bottom of the sea the lifestyle of the larva while scanning rotation is more suited for living in the open water column, the lifestyle of the trochophore larva. Phototaxis in the Platynereis dumerilii larva has a broad spectral range which is at least covered by three opsins that are expressed by

4466-536: The length of the tracks is longer when the cell follows a light gradient, cells will eventually get closer to or further away from the light source. This strategy does not allow orientation along the light vector and only works if a steep light gradient is present (i.e. not in open water). Some cyanobacteria (e.g. Anabaena , Synechocystis ) can slowly orient along a light vector. This orientation occurs in filaments or colonies, but only on surfaces and not in suspension. The filamentous cyanobacterium Synechocystis

4543-445: The light, or expressed phototaxis , and increased their photosynthetic yield, which is necessary for survival. In a novel experiment, the scientists projected a school logo onto a petri dish containing the organisms, which accreted beneath the lighted region, forming the logo in bacteria. The authors speculate that such motility allows the cyanobacteria to seek light sources to support the colony. In both light and dark conditions,

4620-491: The mechanism of phototaxis is a biased random walk, analogous to bacterial chemotaxis. Halophilic archaea, such as Halobacterium salinarum , use sensory rhodopsins (SRs) for phototaxis. Rhodopsins are 7 transmembrane proteins that bind retinal as a chromophore . Light triggers the isomerization of retinal, which leads to phototransductory signalling via a two-component phosphotransfer relay system. Halobacterium salinarum has two SRs, SRI and SRII, which signal via

4697-507: The mine closed in 1978. The combination of a low sedimentation rate, high calcification rate, and low microbial growth rate appears to result in the formation of these microbialites. Microbialites at an historic mine site demonstrates that an anthropogenically constructed environment can foster microbial carbonate formation. This has implications for creating artificial environments for building modern microbialites including stromatolites. A very rare type of non-lake dwelling stromatolite lives in

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4774-404: The movement is in the direction of increasing light intensity and negative if the direction is opposite. Two types of positive phototaxis are observed in prokaryotes . The first is called scotophobotaxis (from the word " scotophobia "), which is observed only under a microscope. This occurs when a bacterium swims by chance out of the area illuminated by the microscope. Entering darkness signals

4851-407: The ocelli, or feeding behaviour in the case of the presence of light. Many tropical jellyfish have a symbiotic relationship with photosynthetic zooxanthellae that they harbor within their cells. The zooxanthellae nourish the jellyfish, while the jellyfish protects them, and moves them toward light sources such as the sun to maximize their light-exposure for efficient photosynthesis. In a shadow,

4928-410: The only factor in the reduction in stromatolite abundance. Factors such as the chemistry of the environment may have been responsible for changes. While prokaryotic cyanobacteria reproduce asexually through cell division, they were instrumental in priming the environment for the evolutionary development of more complex eukaryotic organisms. They are thought to be largely responsible for increasing

5005-463: The protein to the membrane. The photoreceptor and signalling domains are cytoplasmic and signal via a CheA/CheY-type signal transduction system to regulate motility by type IV pili. TaxD1 is localized at the poles of the rod-shaped cells of Synechococcus elongatus , similarly to MCP containing chemosensory receptors in bacteria and archaea. How the steering of the filaments is achieved is not known. The slow steering of these cyanobacterial filaments

5082-416: The reorientation of the helical swimming trajectory as long as the helical swimming axis is not aligned with the light vector. Positive and negative phototaxis can be found in several species of jellyfish such as those from the genus Polyorchis . Jellyfish use ocelli to detect the presence and absence of light, which is then translated into anti-predatory behaviour in the case of a shadow being cast over

5159-721: The repair of UV induced damages in DNA by the processes of nucleotide excision repair and photoreactivation . Other locations include Pampa del Tamarugal National Reserve in Chile; Lagoa Salgada , Rio Grande do Norte , Brazil, where modern stromatolites can be observed as both bioherms (domal type) and beds; and in the Puna de Atacama of the Andes. Inland stromatolites can be found in saline waters in Cuatro Ciénegas Basin ,

5236-476: The result of natural material deposition or some other abiogenic mechanism. Scientists have argued for a biological origin of stromatolites due to the presence of organic globule clusters within the thin layers of the stromatolites, of aragonite nanocrystals (both features of current stromatolites), and of other microstructures in older stromatolites that parallel those in younger stromatolites that show strong indications of biological origin. Stromatolites are

5313-492: The retinal chromophore . Two archaebacterial-type rhodopsins, channelrhodopsin -1 and -2, were identified as phototaxis receptors in Chlamydomonas . Both proteins have an N-terminal 7-transmembrane portion, similar to archaebacterial rhodopsins, followed by an approximately 400 residue C-terminal membrane-associated portion. CSRA and CSRB act as light-gated cation channels and trigger depolarizing photocurrents. CSRA

5390-435: The screening function and the orange-red colour, as well as proteins that stabilize the globules. The stigma is located laterally, in a fixed plane relative to the cilia, but not directly adjacent to the basal bodies. The fixed position is ensured by the attachment of the chloroplast to one of the ciliary roots. The pigmented stigma is not to be confused with the photoreceptor. The stigma only provides directional shading for

5467-496: The speed (but not direction) of movement. Photokinesis may be negative (light-induced reduction of motility) or positive (light-induced stimulation of motility). Photokinesis can cause cells to accumulate in regions of favorable illumination: they linger in such regions or accelerate out of regions of unfavorable illumination. Photokinesis has been documented in cyanobacteria and purple photosynthetic bacteria. True phototaxis consists of directional movement which may be either towards

5544-782: The stromatolite structures are made of hydromagnesite . Two instances of freshwater stromatolites are found in Canada, at Pavilion Lake and Kelly Lake in British Columbia . Pavilion Lake has the largest known freshwater stromatolites, and NASA has conducted xenobiology research there, called the " Pavilion Lake Research Project ." The goal of the project is to better understand what conditions would likely harbor life on other planets. Microbialites have been discovered in an open pit pond at an abandoned asbestos mine near Clinton Creek , Yukon , Canada. These microbialites are extremely young and presumably began forming soon after

5621-603: The time of the day or whether it is cloudy. In the diagram on the right, the larvae start swimming upwards when UV-light switched on (marked by the violet square). But later, they are swimming downward. The larval tracks are color coded: Red for upward and blue for downward swimming larvae. The video runs at double speed. Positive phototaxis can be found in many flying insects such as moths , grasshoppers , and flies . Drosophila melanogaster has been studied extensively for its innate positive phototactic response to light sources, using controlled experiments to help understand

5698-475: The transducer proteins Htr1 and Htr2 (halobacterial transducers for SRs I and II), respectively. The downstream signalling in phototactic archaebacteria involves CheA, a histidine kinase , which phosphorylates the response regulator, CheY. Phosphorylated CheY induces swimming reversals. The two SRs in Halobacterium have different functions. SRI acts as an attractant receptor for orange light and, through

5775-661: The translation machinery by light-generated reactive oxygen species. Finally, light signals potentially contain rich and complex information about the environment, and the possibility should not be excluded that bacteria make sophisticated use of this information to optimize their location and behavior. For example, plant or animal pathogens could use light information to control their location and interaction with their hosts, and in fact light signals are known to regulate development and virulence in several non-phototrophic prokaryotes. Phototrophs could also benefit from sophisticated information processing, since their optimal environment

5852-420: The two receptors is not yet clear. As in all bikonts (plants, chromalveolates, excavates, rhizaria), green algae have two cilia, which are not identical. The anterior cilium is always younger than the posterior one. In every cell cycle, one daughter cell receives the anterior cilium and transforms it into a posterior one. The other daughter inherits the posterior, mature cilium. Both daughters then grow

5929-413: Was shown to localize to the stigma region using immunofluorescence analysis (Suzuki et al. 2003). Individual RNAi depletion of both CSRA and CSRB modified the light-induced currents and revealed that CSRA mediates a fast, high-saturating current while CSRB a slow, low-saturating one. Both currents are able to trigger photophobic responses and can have a role in phototaxis, although the exact contribution of

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