While lurking over at Riddled (by doubt, insecurity and what appears to be a type of marine worm)** I was introduced to a journal article on in-flight metabolism in birds.More specifically, to the idea that melanin in the pecten - a structure in birds' eyes that appears to function in visual acuity - is able to convert sunlight to chemical energy available for cellular metabolism. The journal is Medical Hypotheses, so I looked forward to reading about a fascinating new research-based discovery.
Or not. After all, MH is known for its lack of proper peer-review processes & its willingness to accept all sorts of 'hypotheses', no matter how far-out they are. (And the paper in question would certainly have benefited by some careful proof-reading prior to publication.) Anyway, this particular paper was written by Geoffrey Goodman & Dani Bercovich & bears the title "Melanin directly converts light for vertebrate metabolic use: heuristic thoughts on birds, Icarus and dark human skin." (I note with amusement that 'heuristic' is defined as "of or relating to a usually speculative formulation serving as a guide in the investigation of solution of a problem" by the on-line free dictionary - the first part of that definition is certainly right on the mark. I'm not quite sure, however, of the exact nature of the 'problem' they've set out to 'solve'.)
They begin by describing the nature & apparent ubiquity of melanin in animals: it's produced in cells called melanocytes and, while most often found in the skin & related structures, is also found deeper in the body's tissues. As a biological pigment, melanin absorbs different wavelengths of light to a great or lesser degree; for some reason the authors comment that "[o]rigin of absorption by melanin across the full UV-visible band remains controversial", although they don't provide a citation to support or explain this statement. Nonetheless, they go on to say that
[h]owever, the unexpected is likely from melanin, together with its clinical consequences.
To paraphrase a one-time promotional slogan for the local municipality: melanin - more than you expect!!
Goodman & Bercovich go on to note that birds have a number of specific adaptations for flight, & that flight places considerable metabolic demands on the organism. They also note that birds have a unique feature - the pecten - in their eyes and that this feature is highly melanised. And they ask,
does this melanised organ directly convert light energy for metabolic use?
This is on the second page of the paper. Many trees could have been saved if they'd simply answered 'no', on the basis that a) melanocytes aren't green & b) there's no evidence of any photosystem-based ATP-generating pathway in these cells. However, we instead move on to stranger stuff, including the extremely shaky implication that the pecten is in some way involved in reducing the impact of hypoxia in bird embryos "at great altitudes" (citing lab experiments rather than actual research looking at embryos laid in actual high-altitude nests. I'm not sure of the record relating to high-altitude nests, but I'm willing to bet that it's considerably less than the highest flights by adult birds... ). And - an expansion of the idea that melanin may convert light into usable metabolic energy that contributes to the energy budget in migrating birds. How?
... the pectenial melanocytes, pigment, capillaries and glial cells (if any), together with vitreous and ocular globe content and action may constitute a single photo-metabolic system; 'compartmentalisation' very different to that in leaf tissue.
In darker human skin, a light-initiated, melanin role in metabolism, though small/unit area, could be effective physiologically due to the large area irradiated...
Avian eyes and human skin as photosynthetic organs - you read it here first, ladies & gentlemen!***