November 28, 2007 | David F. Coppedge

Dealing with Light at the Extremes

“Light is the most important variable in our environment,” wrote Edith Widder, a marine biologist.  The inhabitants of two different ecosystems have to deal with either too little or too much. 

  1. Let your light so shine:  A thousand meters below the sea surface, all sunlight is extinguished.  Yet for thousands of meters more, creatures live in the perpetual darkness by manufacturing their own light.  Bioluminescence is everywhere, reported Mark Schrope in Nature,1 “Eventually, the lightshow grows into a veritable fireworks display against an ever blacker background.”  The light comes from everything alive: bacteria, microorganisms called dinoflagellates, jellyfish, anemones, shrimp, vertebrate fish, and more.
        Edith Widder is co-founder of the Ocean Research and Conservation Association in Fort Pierce, Florida.  With a grant from the National Oceanic and Atmospheric Administration (NOAA), her team uses a deep ocean submersible craft called Eye-in-the-Sea to understand creatures who can only be studied in their own space.  The submersible is equipped with an LED flasher that tries to beckon organisms and study their behaviors.  They were actually able to get a distant organism to flash its light back.  They also got a squid to respond to their light signal, thinking it had discovered lunch.
        Possible uses of biological light include decoy, defense, camouflage, mimicry, sexual attraction and alarm.  Though red light is the first to be extinguished in the depths, and most dark-adapted organisms see in the blue-green range, some organisms appear to emit red light that could be visible only among their own.  To do this, they must transfer the blue-green light from their photophores to red-fluorescent proteins, which seems inefficient.  “My physics head says, ‘No,’” commented Justin Marshall, an Australian participant in the Deep Scope project, “But my biology head says, ‘Well, Why not?’ Biology is weird, so it could be.”
        The fact that organisms can emit light by intricate processes of bioluminescence presupposes that they also contain sensitive organs to detect it.  Many deep-sea fish have large eyes tuned to the blue-green light of photophores.
        A new version of Eye-in-the-Sea is being prepared for deployment in early 2008 in Monterey, California.  This will provide the first undersea observatory of the dark depths, “the first effective, long-term study of true deep-sea bioluminescent behaviour.”  It may shed new light on an ecosystem that communicates in the language of photons.
  2. Too much of a good thing:  On topside, some organisms have the opposite problem: too much light.  Plants harvest sunlight to make nutrients from the soil, but like sunbathers know, too much can burn.  Within leaves are elaborate mechanisms to shunt away excess light from the photosynthetic factories.  Science Daily reported on a paper in Nature2 where researchers from University of Sheffield and Queen Mary, University of London learned more about “photoprotection” in plant leaves: “They were able to show how a small number of certain key molecules, hidden among the millions of others in the plant leaf, change their shape when the amount of light absorbed is excessive; and they have been able to track the conversion of light energy to heat that occurs in less than a billionth of a second.”  The original paper stated, “it is experimentally demonstrated that a change in conformation of LHCII occurs in vivo, which opens a channel for energy dissipation by transfer to a bound carotenoid.  We suggest that this is the principal mechanism of photoprotection.”  The excess energy is thus shunted to a heat sink by an extremely rapid switch.
        What they are learning may help increase crop yields and improve photovoltaic cells.  Plants already know how to adjust for the dim light of a cloudy day to the scorching radiation under a midsummer sun at noon.  “Many plant species can successfully inhabit extreme environments where there is little water, strong sunlight, low fertility and extremes of temperature by having highly tuned defence mechanisms, including photoprotection.”  See also the 06/23/2006 and 01/24/2005 entries about photoprotection, “One of Nature’s supreme examples of nanoscale engineering.”  (That’s Nature as in the real world, not the artificial journal.)
  3. Light just right, but que pasa?:  We humans, too, have to not only be able to harvest light, but process it as information.  The brain has a mechanism for making sense of a scene – deciding what is foreground, and what is background.  A “neural machine,” described in Science Daily, sorts this all out faster than the blink of an eye.  A portion of the visual cortex called V2 makes a preliminary judgment of what part of the field is the background, and what part is the foreground.
        Rudiger von der Heydt, a neuroscientist at Johns Hopkins University, described what happens: “What we found is that V2 generates a foreground-background map for each image registered by the eyes.  Contours are assigned to the foreground regions, and V2 does this automatically within a tenth of a second.”
        This first-pass interpretive filter helps us make instant sense of a complex scene, even though its decision can be overridden by the conscious mind, or tricked by optical illusions.  Paintings by artist M.C. Escher, for instance, owe their popularity to tricks with the mind, fooling our eyes with contradictions about which way is up, or which part is the foreground and which is the background.
        Van der Heydt continued, “Because of their complexity, images of natural scenes generally have many possible interpretations, not just two, like in Escher’s drawings.  In most cases, they contain a variety of cues that could be used to identify fore- and background, but oftentimes, these cues contradict each other.  The V2 mechanism combines these cues efficiently and provides us immediately with a rough sketch of the scene.”
        The neuroscientist commented on the wonders of this system.  “We can do all of this without effort, thanks to a neural machine that generates visual object representations in the brain,” he said.  He admitted that how it works is still a mystery to us.  “But discovering this mechanism that so efficiently links our attention to figure-ground organization is a step toward understanding this amazing machine.”

Look at your eyes in a mirror.  Using an eye to see the eye: fascinating.  There’s enough in that self-reflexive activity to keep biologists, neuroscientists, physicists and philosophers busy for millennia.

1.  Mark Schrope, “Marine biology: Lights in the deep,” Nature 450, 472-474 (2007) | doi:10.1038/450472a.
2.  Ruban et al, “Identification of a mechanism of photoprotective energy dissipation in higher plants,” Nature 450, 575-578 (22 November 2007) | doi:10.1038/nature06262.

As with every natural resource in every ecological environment, light is used efficiently and effectively by a multitude of organisms well equipped to manage with feast or famine.  What other physical resources are utilized via similar feats of nanoengineering by living organisms?  Water (vapor, liquid, and solid), oxygen, nitrogen, iron, magnetism – no matter the physical resource, living things know how to harvest it for highest and best use.  Organisms daily exhibit a declaration of intelligent design; they have been endowed by their Creator with certain unalienable rightly elegant constitutions.

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