Nigel Chaffey
What colour are plants? No, not a stupid question, but I bet you were tempted to answer ‘green’? Which is fair enough; chlorophyll is the major pigment in plants and consequently they do tend to appear green. But! This is almost a knee-jerk, Pavlovian, conditioned response because we are geo-centric in our thinking and experiences. What if that question were to be posed in a galaxy far, far away? Well, we don’t know the answer, but if Jack O’Malley-James – PhD Student in Astrobiology at the University of St Andrews (Scotland – yes, where the UK’s current Duke and Duchess of Cambridge met; this column is not averse to a bit of so-called ‘celebrity news’ reporting) – is correct, extraterrestrial plants may appear black to us humans. In his own words, ‘Our simulations suggest that planets in multi-star systems may host exotic forms of the more familiar plants we see on Earth. Plants with dim red dwarf suns for example, may appear black to our eyes, absorbing across the entire visible wavelength range in order to use as much of the available light as possible. They may also be able to use infrared or ultraviolet radiation to drive photosynthesis. For planets orbiting two stars like our own, harmful radiation from intense stellar flares could lead to plants that develop their own UV-blocking sun-screens, or photosynthesising microorganisms that can move in response to a sudden flare’. So, it seems that Alexandre Dumas may yet have his black tulip! [This column’s no stranger to more high-brow literary ‘commentary’ either – Ed.]
William Salter
Land plants are green with chlorophyll because they had an aquatic ancestory where the absorption spectrum of this pigment is better adapted, but why no nigerophyll? Presumably because no such pigment arose as a variant of heat-light sensing pigments, an evolutionary constraint.
@ The Phytophactor:
I’m not sure I follow. Cyanobacteria has been found with near-IR utilizing chlorophyll f IIRC. (Sitting in stromatolites beneath 3 earlier layers that successively take the edge [sic!] of light.)
The reason chlorophylls appears green is that they do a two step photon absorption of blue high energy and some red low energy photons. Other pigments used by bacteria go far into the IR with one step (I think), but aren’t used for oxygenating photosynthesis.
The Phytophactor has blogged on this before when a similar story came up last year.
I think you both have the same idea. There’s a limit on what can be used for photosynthesis, at least using the mechanisms we know, set by the energy in each photon. UV isn’t likely to be used because of damage to cells, and far IR isn’t going to carry the energy.
I think there’s a possibility alien life could use something analogous to photosynthesis that we don’t know about yet. Maybe that would use different wavelengths. But how unlike a plant would a rooted, stationary lifeform have to be before we stop using the word plant to describe it?