Phototaxis

Light triggers the isomerization of retinal,[8] which leads to phototransductory signalling via a two-component phosphotransfer relay system.

[9][10] The downstream signalling in phototactic archaebacteria involves CheA, a histidine kinase, which phosphorylates the response regulator, CheY.

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.

[19] TaxD1 is localized at the poles of the rod-shaped cells of Synechococcus elongatus, similarly to MCP containing chemosensory receptors in bacteria and archaea.

The slow steering of these cyanobacterial filaments is the only light-direction sensing behaviour prokaryotes could evolve owing to the difficulty in detecting light direction at this small scale.

Phototrophic prokaryotes are extraordinarily diverse, with a likely role for horizontal gene transfer in spreading phototrophy across multiple phyla.

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 [25] and inhibition of the translation machinery by light-generated reactive oxygen species.

[27][28] Phototrophs could also benefit from sophisticated information processing, since their optimal environment 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 the development of motile hormogonia and nitrogen-fixing heterocysts.

Within 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.

[30][31] 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.

Photophobic responses have been observed in prokaryotes as diverse as Escherichia coli, purple photosynthetic bacteria and haloarchaea.

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,[33] and in cyanobacteria.

[22] Scotophobic/photophobic responses in flagellated bacteria closely resemble 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.

[34] 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.

True phototaxis in prokaryotes is sometimes combined with social motility, which involves the concerted movement of an entire colony of cells towards or away from the light source.

True phototaxis is widespread in eukaryotic green algae,[35] but among the prokaryotes it has been documented only in cyanobacteria,[22][17] and in social motility of colonies of the purple photosynthetic bacterium Rhodocista centenaria.

[38] Eukaryotes evolved for the first time in the history of life the ability to follow light direction in three dimensions in open water.

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).

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.

[1] 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 the retinal chromophore.

In a shadow, the jellyfish can either remain still, or quickly move away in bursts to avoid predation and also re-adjust toward a new light source.

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.

This way the information of all four eye cups can be compared and a low-resolution image of four pixels can be created telling the larvae where the light is coming from.

This looks like a change from positive to negative phototaxis (see video left), but the larvae also swim down if UV-light comes non-directionally from the side.

This has the advantage over a brightness based depth gauge that the color stays almost constant independent of the time of the day or whether it is cloudy.

Evidence for 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).

[67] 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.