Light: Life’s window to the world

Electromagnetic Energy is not only the primary source (or secondary source after gravity…) of energy on Earth, ‘light,’ the smaller fraction of the electromagnetic spectrum, is also an important source of information for many forms of organisms. The different properties of light, such as intensity, duration, polarisation, and spectral composition, can all be used as sources of information.

In all, light sensing is connected to movement in some way so that, once signalled, the creature can respond. [1]

Life on Earth has developed three principle forms of light detectors, known as photoreceptors: flavin-based blue-light photoreceptors (i.e., cryptochromes), retinal-based green-light (such as rhodospin), and linear tetrapyrrole-based red-light sensors (i.e., phytochromes in green plants).

Animals

In animals the detection of light leads to vision. In its simplest form, light-detecting cells in worms for instance, are scattered across the skin (although concentrated near the head), helping them detect warmth and sunlight, as too much of both will dry them out, and UV light which kills the delicate nerve-endings, causes paralysis.

The honeybee: like us, is trichromatic - it has three different photoreceptors in the eye, that builds up its' view of the world. Bees however, do not have red, blue, and green receptors (like us), instead they have ultraviolet, blue and green (they can’t see red) receptors. This ultraviolet vision allows them to see the “bulls-eye” markings, that many plants (i.e., primroses and pansies) have on their flowers, guiding bees like runway lights to the nectar. Bees, as well as two fixed compound eyes on either side of the head, have three smaller eyes, called ocelli on the top of the head. Ocelli sense light intensity, but not images; and are believed by scientists to be involved in navigation - and the fact there are three, they may even provide a triangulation function.

Other more complex visual systems, such as compound eyes can be found in insects and crustaceans for example, that consist of several thousand light detectors called ommatidia. These are particularly good at sensing movement, and a broad range of colours (i.e., the Mantis Shrimp, which possesses ‘hyperspectral’ colour vision).

Single-lens eyes, such as our own, have also evolved into various forms in fish, birds, mammals, and spiders for instance. Some snakes (such as pit vipers) also have the ability to sense infrared thermal radiation (IR) between 5 and 30 μm, through sensors located in their noses - ‘smelling’ heat - giving them the ability to not only locate prey, but also vulnerable body parts in the dark. However. this is not strictly a photoreceptor, it is more a temperature sensor.

Plants

Plants, as well as being able to use light as an energy source, are also able to detect light (due to light’s importance for energy), which can result in changes in growth and morphology (form) - known as photomorphogenesis. Some plant seeds, such as many lettuce varieties, need light to germinate (positive photoblastic), or when detecting a lack of light will continue to lay dormant. Photoperiodism is a physical response, such as flowering, which is made by red-light detection during a 24 hour period. In this case, plants only start to produce flowers when they detect a critical night-length, which signals the appropriate time of the year to bloom.

Phototropism is a physical response, where blue-light detectors, help direct the plant to grow shoots towards the sun; much like heliotropism, which describes the ability of some plants (such as sunflowers) to track (and turn with) the Sun’s motion on a daily basis - and some other plants ability to raise and lower their leaves on a daily basis (i.e., leaf heliotropism of many legumes). Light detectors in plants can also determine the quality of the light - helping plants to avoid growing or even germinating (like oaks) in the shade. Finally, chloroplasts - the organelles, mostly located within plant leaf cells, where photosynthesis takes place - are also able to move towards or away from light within the cell - again optimising the amount of light that the plant can catch. [2]

Fungi

Most fungi, are able to sense and react to light, as many have all three of the major classes of photoreceptors (exceptions may include some forms of yeast - single cell fungi, for instance). Some fungi, such as Cyathus stercoreus require light to initiate fruiting body development. Other fungi, such as sporangiosphores, grow towards the light (not bend towards light like plants) - phototropism, and the fruiting body of basidiomycetes depends on light at different development stages - and in:

…ascomycetes, light also has a strong impact in morphological and physiological processes, including the regulation of conidial germination, hyphal branching, sexual and asexual development, and secondary metabolism. [3]

Bacteria

Photosynthetic bacteria, such as Cyanobacteria (among the world’s most important oxygen producers) are able to detect different wavelengths, as they migrate up and down water columns in the worlds oceans - at the water surface light still has a wide spectrum, but at depth, blue light dominates.

Image Sourcehttps://doi.org/10.7554/eLife.12620.011

Cyanobacteria, and two slightly larger organisms, protists, Erythrodinium, and Neodinium, are able to use their entire bodies, as a microscopic light-sensitive focusing lens device (much like a camera lens), a kind of single-cell eye. Using the information to move towards or away from light (phototaxis), through patches of motor proteins, forming…

…on the side of the cell facing the light source. Pili [hair like appendages on the surface of many bacteria] are extended and retracted at this side of the cell, which therefore moves towards the light. [4]

Light as Communication

Many different species within all the kingdoms of life, are able to manipulate light as a form of communication. Some plants, for instance, have the ability to create a 'bluehalo' of light (known as structural colours) around their flowers to attract pollinating bees [5]; or, have the ability to use specific pigments (pigments absorb and/or reflect light in certain wave-lengths), such as those that reflect UV, which are visible to insects (but not to us), and act as landing lights, guiding the insects towards the flowers nectar (and therefore, pollen). Bioluminescence, is the production of light through a chemical reaction, in an organisms body [6]. Fireflies, and many marine animals, from algae to jellyfish and crustaceans (and symbiotic bacteria living in some of these organisms), are able to control the chemical reaction, which is used for attracting mates, feeding and protection.

Humanity also uses light for communication. Beacons of fire (sometimes positioned in a network), have been used in different parts of the world for centuries, as a form of communication across distances upto 100 km. In the 3rd century BC, ancient greeks were transmitting and receiving different messages by varying the combination and position of lit touches [7]. This form of communication evolved to the more modern, Morse code, invented by Samuel Morse in 1832 (which can also be transmitted via radio waves and electrical pulses), which is able to transmit the entire alphabet and numbers 0 to 9 by varying the duration of the transmission: generating a series of dots and dashes (a dash lasts three times longer than a dot).

Modern technology also include the transmission of data via optical fibres, and the use of laser beams to read information from pitted patterns in a spiral track on CD's.

A more recent technology invention, known as Li-Fi, uses LED lights, much like Morse code, to transmit data (and position) through very high flickering rates (invisible by the human eye), which are able transmit the equivalent to the '0''s and '1's of digital code.[8]

References

[1] Margulis, Lynn., and Sagan, Dorion. (1995) ‘What is Life?’ University of California Press, Berkeley and Los Angeles, California.

[2] Suetsugu N, Higa T, Gotoh E, Wada M (June 16, 2016) 'Light-Induced Movements of Chloroplasts and Nuclei Are Regulated in Both Cp-Actin-Filament-Dependent and -Independent Manners in Arabidopsis thaliana' PLOS ONE 11(6): e0157429. https://doi.org/10.1371/journal.pone.0157429

[3] Fischer, Reinhard et al. (2016) ’The Complexity of Fungal Vision.’ Microbiology Spectrum, American Society for Microbiology Press.

[4] Schuergers, Nils et al. (Feb 9, 2016) ’Cyanobacteria use micro-optics to sense light direction.’ eLife 2016;5:e12620 DOI: 10.7554/eLife.12620

[5] Physics World, Biophysics (retrieved April 2018) 'A flower's nano-powers.' https://physicsworld.com/a/a-flowers-nano-powers/ 

[6] Smithsonian, Ocean - Find your Blue. Fish. (retrieved April 2018) 'Bioluminescence.' https://ocean.si.edu/ocean-life/fish/bioluminescence

[7] Kotsanas Museum of Ancient Greek Technology (retrieved October 3rd 2017) http://kotsanas.com/gb/exh.php?exhibit=1201001

[8] Wikipedia, (retrieved April 2018) 'Li-Fi' https://en.wikipedia.org/wiki/Li-Fi