The gardener’s nightmare might be the entomologist’s goldmine. Among the most destructive of all insects are small sap-sucking critters, called Aphids. They are very populous, reproducing via both sexual and asexual means, and are difficult to eliminate using pesticides. Aphids are also colored insects. They generally have a green color, but can also acquire a red color as they can also synthesize red carotenoids (pigment proteins). They are the only animals known to be able to synthesize pigments.

Aphids (Photo Courtesy: University of California)

A process similar to photosynthesis

What wasn’t known was that aphids can actually use these pigments to metabolize using sunlight, much the same way that plants do! Yes, aphids can undergo a process similar to photosynthesis in plants, finds a group of researchers.

This startling finding is due to a group working under entomologist Alain Robichon at the Sophia Agrobiotech Institute in Sophia Antipolis, France. The group worked with green, orange and white aphids. White aphids are found where resources are scarce and they are almost completely devoid of pigments. Green aphids are found in places with cold temperatures, but still enough food to go around.

Counting ATP

The group measured the ATP levels in the aphid bodies. ATP is the ‘energy currency’ of the living organism – this is the molecule that is transferred between cells when an energy transfer has to occur. The results of the measurements were astounding: Carotenoid rich green aphids registered a much higher level than the white carotenoid-devoid ones, suggesting that the green pigment might be instrumental in providing another source of energy production.

Look at the two lines. They represent the optical density (OD), indicative of the ATP/metabolic activity of the pigment. Clearly the green pigment (represented by the solid black line) has higher absorption at all wavelengths.

Moreover, when the orange aphids – containing moderate amounts of carotenoids – were placed in sunlight, they showed intermediate levels of ATP. Interesting. Very interesting indeed.

Not the photosynthesis we know

One has to note that this ‘photosynthesis’ merely refers to the use of sunlight in order to gain energy. This doesn’t refer to the photosynthetic process that goes on in plants, which require the transfer of a positive charge from a water molecule, leading to the expulsion of oxygen (and thus we breathe!). Also, carbon dioxide has not been shown to be essential to the aphids version of the photosynthetic process.

The carotenoid molecules are placed about 0-40 micrometers deep under the cuticle, making them perfect to capture both the incident and the transmitted solar radiation.

Questions, questions

But questions remain – why should the aphids need to synthesize? As mentioned above, aphids in high food resource areas develop strong pigmentation and can also synthesize food for themselves. But doesn’t that defeat the very purpose? Why make when you already have food?

Both the mechanism and the necessity are unclear. These tiny critters had an unknown ace up their sleeves and there might be more.

The paper: http://www.nature.com/srep/2012/120816/srep00579/pdf/srep00579.pdf