Catching the rays

It's sometimes very hard to choose between topics - from spectrally featureless super-Earth atmospheric light transmission suggesting a possible water vapor composition, to electrically signaling bacteria crossing the line to inter species cooperation. Then along comes an item that just scuppers even those wonderful stories.

Harvesting solar photons for direct bio-chemical processes has been the province of microbial life for at least 3.5 billion years - since sticky colonies of organisms like cyanobacteria sat in the shallow waters of Archaean Earth. Later endosymbiosis allowed plant life to pick up the trick, and make use of the hundred and seventy petawatts of solar energy flooding the planet. Now it appears that at least one species of insect has perhaps also learned to capture photons as a key resource.

This remarkable talent belongs to the Oriental hornet - Vespa orientalis. It had been known that this insect generated electric currents through various structures in its body, and it had been speculated that this related to thermal regulation. Now a new study by Plotkin et al. has shown that on the hornet's abdomen is a remarkable optical 'trap' - an arrangement of skin cuticle ridges about the size of the wavelengths of ultra-violet to optical light that serves to trap incoming photons. Light is repeatedly reflected within layers which increases the odds of it intersecting a special yellow pigment xanthopterin - tuned to selectively absorb photons. In doing so it can generate minute electrical currents within the cells - which could then power bio-chemistry. Plotkin and colleagues have gone as far as constructing a rudimentary organic solar cell in the lab, using xanthopterin, to generate electricity with about a 0.34% efficiency (compared to about 10% for the best artificial solar cells).

The full chain of the process has not yet been disentangled, however the hornet's activity is strongly correlated with the intensity of ambient ultraviolet light, and it seems that there is most metabolic activity within these optically-tuned pigmented parts of their bodies. It may still be that thermal regulation is the driving force behind the evolution of this capacity, however many things point towards Orientalis as being a genuinely solar-augmented organism.

This raises a number of intriguing points. Hybrid energy production is a useful talent, allowing greater efficiency and flexibility. Solar radiation is also the bulk energy resource on the planet - offering many orders of magnitude more power than chemical or thermal geophysical sources. So why don't we all do it? Obviously in an indirect fashion we do. Humans for example can produce vitamin-D by absorbing UV photons in our skin. Reptiles can do the same but they mainly need the direct thermal energy of sunlight to help maintain their internal temperatures. But actually generating fundamental power - electrical voltages that can be exploited for biochemistry - is another matter. There is probably a down-side, whether in terms of the biological expense of the necessary structures and chemistry, or in terms of the lifestyle that could make best use of a little solar electrical jolt - however that is all rather specific to terrestrial environments and the history of life here.

One wonders whether life elsewhere, in different circumstances, might have found better reason to walk around with a photon capturing carapace, opening up a whole range of interesting possibilities...