Alun Salt
Black Plants at Sunset. Photo (cc) Tambako the Jaguar.
A couple of Astrobotanical stories have caught my eye in the past week.
I renewed my subscription to SciAm Digital today and read the story Black Plants and Twilight Zones. It’s subscription only, but if you read the first bit and then I tell you that Nancy Kiang of the NASA Goddard Institute for Space Studies speculates that plants on the new discovered exoplanet Gliese 581g could be black, allowing them to absorb as much starlight as possible.
There are some problems. One, as the story notes, is that Gliese 581g seems to have disappeared. It cannot be confirmed as any other astronomical group so far, so it’s possible that it’s an artefact in the data. I’m a puzzled by the idea that plants would be black. If more is better, wouldn’t black also be the normal colour of leaves on Earth? Clearly I need to read her article The color of plants on other worlds [PDF] where she describes her reasoning in more detail. Her bibliography has more papers on photosynthesis and Astrobiology.
Another problem is due to the pedant in me. If plant-like life is found on an exoplanet then sooner or later someone will point out that, in evolutionary terms, we have more in common with plants than the new lifeforms. Whether stationary photosynthesizing lifeforms are plants or not could be a “Pluto moment” for Botany.
You could also question if plant-like life is unique to earth, but there may soon be a way of finding out. A paper Detecting Tree-like Multicellular Life on Extrasolar Planets” in Astrobiology. The authors, Doughty and Wolf, propose that the presence of life should be revealed by the way light is reflected from a planet surface. There is a simpler explanation of the process at Universe Today. If they are right, then positive results may start appearing within the next couple of decades as new NASA and ESA observatories are launched.
Not having read all of these articles leaves me guessing about Kiang’s assumptions, but consider that plants on Earth would be “better off” with a nigerophyll under some circumstances and yet they remain green. Evolution can only act upon variants that exist, and such a pigment does not seem to have existed, although chlorophyll is most certainly a more efficient derivative of bacteriochlorophyll. Also, plants are constrained by their evolutionary history, and chlorophyll makes more sense in an aquatic environment than in a terrestrial environment, but land plants have an aquatic ancestry from whom they inherited their photosynthetic mechanism. But on another planet, particulary one whose star generates a different visible spectrum of light, one with a different evolutionary history, then indeed photoautotrophs may well have a different light-absorbing pigment, including a nigerophyll. But then again maybe not because even bacteriochlorophyll may have evolved for purposes of heat-sensing and even infared photosynthesis in some deep, dark, hot place where life may have begun and lived prior to invading the bright, relatively frigid planet surface that we now think of as normal. Guess the Phactor had better get that damned book done.
I think I should also add some words about ‘Astrobotany in print’, or more precisely, Astrobotany in Annals of Botany. Amazingly, it was 1984 that we published a special issue reporting the first, very extensive, work on the first space shuttle missions, entitled ‘Plants is space’. This76 page issue reported numerous results from experiments, mostly from NASA scientists, which have never been bettered or in most cases repeated, including papers about culturing plants in microgravity – I like the idea of growing them on the surface of spheres of water – and their development and cell division processes. The papers are listed in our archive at http://aob.oxfordjournals.org/content/54/supp3 . I can do little better than quote my father’s preface: “The papers in this Supplement provide a comprehensive conspectus of the research on plants under the conditions of space flight conducted in recent years under the auspices of the US National Aeronautics and Space Administration, and in addition they offer documented reviews of the parallel work on plants carried out in the course of the Soviet space-flight programme. Collectively they are unique in providing, for the first time, ready access for botanists to the results from an area of research many have regarded as esoteric and outside their range of interests – and the literature of which has, in any event, been largely out of reach except for those actually in the field. Apart from the research results reviewed, many readers will find special interest in the summaries of the facilities actually available in current spacecraft for plant experimentation, and in the descriptions of the physical conditions likely to be associated with flight in different types of vehicle.”
That issue 25 years ago was coordinated by the previous Annals of Botany North American Editor, A Krikorian. Now, our current Japanese regional editor, Hideyuki Takahashi, is involved with experiments right at the end of the shuttle’s life, on how plants adapt to and evolve in the space microgravity environment: http://www.jaxa.jp/article/special/kibo/takahashi_e.html