For the science of interactions of light and living beings, see biophotonics.
Biophotons (from the Greek βίος meaning "life" and φῶς meaning "light") are photons of light in the ultraviolet and low visible light range that are produced by a biological system. They are non-thermal in origin, and the emission of biophotons is technically a type of bioluminescence, though the term "bioluminescence" is generally reserved for higher luminance systems (typically with emitted light visible to the naked eye, using biochemical means such as luciferin/luciferase). The term biophoton used in this narrow sense should not be confused with the broader field of biophotonics, which studies the general interaction of light with biological systems.
Biological tissues typically produce an observed radiant emittance in the visible and ultraviolet frequencies ranging from 10−17 to 10−23 W/cm2 (approx 1-1000 photons/cm2/second).[1] This low level of light has a much weaker intensity than the visible light produced by bioluminescence, but biophotons are detectable above the background of thermal radiation that is emitted by tissues at their normal temperature.[2]
While detection of biophotons has been reported by several groups,[3][4][5] hypotheses that such biophotons indicate the state of biological tissues and facilitate a form of cellular communication are still under investigation,[6][7]Alexander Gurwitsch, who discovered the existence of biophotons, was awarded the Stalin Prize in 1941 for his work.[8]
Detection and measurement
Biophotons may be detected with photomultipliers or by means of an ultra low noise CCD camera to produce an image, using an exposure time of typically 15 minutes for plant materials.[9][3] Photomultiplier tubes have been used to measure biophoton emissions from fish eggs,[10] and some applications have measured biophotons from animals and humans.[11][12][13] Electron Multiplying CCD (EM-CCD) optimized for the detection of ultraweak light[14] have also been used to detect the bioluminescence produced by yeast cells at the onset of their growth.[15]
The typical observed radiant emittance of biological tissues in the visible and ultraviolet frequencies ranges from 10−17 to 10−23 W/cm2 with a photon count from a few to nearly 1000 photons per cm2 in the range of 200 nm to 800 nm.[1]
Proposed physical mechanisms
Chemi-excitation via oxidative stress by reactive oxygen species or catalysis by enzymes (i.e., peroxidase, lipoxygenase) is a common event in the biomolecular milieu.[16] Such reactions can lead to the formation of triplet excited species, which release photons upon returning to a lower energy level in a process analogous to phosphorescence. That this process is a contributing factor to spontaneous biophoton emission has been indicated by studies demonstrating that biophoton emission can be increased by depleting assayed tissue of antioxidants[17] or by addition of carbonyl derivatizing agents.[18] Further support is provided by studies indicating that emission can be increased by addition of reactive oxygen species.[19]
Plants
Imaging of biophotons from leaves has been used as a method for assaying R gene responses.[9] These genes and their associated proteins are responsible for pathogen recognition and activation of defense signaling networks leading to the hypersensitive response,[20] which is one of the mechanisms of the resistance of plants to pathogen infection. It involves the generation of reactive oxygen species (ROS), which have crucial roles in signal transduction or as toxic agents leading to cell death.[21]
Biophotons have been also observed in the roots of stressed plants. In healthy cells, the concentration of ROS is minimized by a system of biological antioxidants. However, heat shock and other stresses changes the equilibrium between oxidative stress and antioxidant activity, for example, the rapid rise in temperature induces biophoton emission by ROS.[22]
Hypothesized involvement in cellular communication
In the 1920s, the Russian embryologist Alexander Gurwitsch reported "ultraweak" photon emissions from living tissues in the UV-range of the spectrum. He named them "mitogenetic rays" because his experiments convinced him that they had a stimulating effect on cell division.[23]
In the 1970s Fritz-Albert Popp and his research group at the University of Marburg (Germany) showed that the spectral distribution of the emission fell over a wide range of wavelengths, from 200 to 750 nm.[24] Popp's work on the biophoton emission's statistical properties, namely the claims on its coherence, was criticised for lack of scientific rigour.[2]
One biophoton mechanism focuses on injured cells that are under higher levels of oxidative stress, which is one source of light, and can be deemed to constitute a "distress signal" or background chemical process, but this mechanism is yet to be demonstrated.[citation needed] The difficulty of teasing out the effects of any supposed biophotons amid the other numerous chemical interactions between cells makes it difficult to devise a testable hypothesis. A 2010 review article discusses various published theories on this kind of signaling.[25]
The hypothesis of cellular communication by biophotons was highly criticised for failing to explain how could cells detect photonic signals several orders of magnitude weaker than the natural background illumination.[26]
^Rastogi A, Pospísil P (August 2010). "Ultra-weak photon emission as a non-invasive tool for monitoring of oxidative processes in the epidermal cells of human skin: comparative study on the dorsal and the palm side of the hand". Skin Research and Technology. 16 (3): 365–70. doi:10.1111/j.1600-0846.2010.00442.x. PMID20637006. S2CID24243914.
^Niggli HJ (May 1993). "Artificial sunlight irradiation induces ultraweak photon emission in human skin fibroblasts". Journal of Photochemistry and Photobiology B: Biology. 18 (2–3): 281–5. doi:10.1016/1011-1344(93)80076-L. PMID8350193.
^Bajpai R (2009). "Biophotons: a clue to unravel the mystery of "life"". In Meyer-Rochow VB (ed.). Bioluminescence in Focus - a collection of illuminating essays. Vol. 1. Kerala, India: Research Signpost. pp. 357–385. ISBN9788130803579. OCLC497860307.
^ abBennett M, Mehta M, Grant M (February 2005). "Biophoton imaging: a nondestructive method for assaying R gene responses". Molecular Plant-Microbe Interactions. 18 (2): 95–102. doi:10.1094/MPMI-18-0095. PMID15720077.
^Dotta BT, Saroka KS, Persinger MA (April 2012). "Increased photon emission from the head while imagining light in the dark is correlated with changes in electroencephalographic power: support for Bókkon's biophoton hypothesis". Neuroscience Letters. 513 (2): 151–4. doi:10.1016/j.neulet.2012.02.021. PMID22343311. S2CID207135123.
^Joines WT, Baumann SB, Kruth JG (2012). "Electromagnetic emission from humans during focused intent". Journal of Parapsychology. 76 (2): 275–294.
^Cilento G, Adam W (July 1995). "From free radicals to electronically excited species". Free Radical Biology & Medicine. 19 (1): 103–14. doi:10.1016/0891-5849(95)00002-F. PMID7635351.
^Ursini F, Barsacchi R, Pelosi G, Benassi A (July 1989). "Oxidative stress in the rat heart, studies on low-level chemiluminescence". Journal of Bioluminescence and Chemiluminescence. 4 (1): 241–4. doi:10.1002/bio.1170040134. PMID2801215.
^Kataoka Y, Cui Y, Yamagata A, Niigaki M, Hirohata T, Oishi N, Watanabe Y (July 2001). "Activity-dependent neural tissue oxidation emits intrinsic ultraweak photons". Biochemical and Biophysical Research Communications. 285 (4): 1007–11. doi:10.1006/bbrc.2001.5285. PMID11467852.