Aphids, carotenoids and photosynthesis

When it comes to finding food for dinner, plants have it pretty easy. They can use light from the Sun to convert carbon dioxide into sugar, literally making food out of thin air. You might think that a superpower like photosynthesis should have evolved in animals too, but it doesn’t come easily.

Plants only came by this ability by absorbing a bacterium that could photosynthesize. Over time, the bacterium became a part of the plant’s cells allowing it to be passed on to the next generation. The ability to photosynthesize is so complicated that it is unlikely to develop from scratch again.

However, published this month is news that aphids, through their unique ability to produce pigments known as carotenoids, may be the first animal able to photosynthesize. However, there are several reasons why this is not entirely true: spider mites are also able to produce carotenoids; the aphids don’t appear to be making sugars and so are not photosynthetic, but phototropic; and a species of sea slug is already known to photosynthesize after taking chloroplasts and genes from the algae it eats.

So what is the news then? Aphids have used their carotenoid-making ability to construct a rudimentary light-harvesting system – and that’s pretty cool.

Pea aphids are remarkable in many ways, not only might they be able to harvest energy from light but they can be born pregnant.
Credit: Shipher Wu (photograph) and Gee-way Lin (aphid provision), National Taiwan University

The vast majority of animals have no need to produce their own carotenoids because they can be gained through diet. There is a great need for carotenoids in our bodies as they are used to produce vitamin A and retinal, a light-absorbing chemical that we use in order to see. Carotenoids are easily found in our diet and one carotenoid – beta-carotene – is what gives carrots their colour (and carrots are what gave carotenoids their name). [Note the link between carrots and being able to see better. It’s not just an old wives’ tale.]

Beta-carotene can act as a molecular wire across the cell membrane, be converted into retinal, and act as an anti-oxidant. All the more reason to eat your carrots (if you’re not an aphid).

The scientists in this study investigated the aphids to find out why they would produce their own carotenoids. Never mind how they gained the ability (although it is likely horizontal gene transfer from yeast, but that’s a different story), but the actual production of carotenoids is energetically expensive and there should be a good reason for making it. And it turns out there is.

The membrane of a cell is a fatty layer acting as a barrier between the carefully balanced inner working of the cell and the outside world. The non-polar structure of carotenoids means that they are only found in the membrane. Also, the alternating double-bonds along the carbon backbone mean that electrons can travel freely along the length of the molecule. This means that carotenoids can transport electrons from one side of the membrane to the other.

Carotenoids absorb light in the blue region of the spectrum, making them look orange/yellow (hence the colour of carrots). When a carotenoid molecule absorbs light, it ‘excites’ an electron, allowing it to jump to a neighbouring molecule outside the membrane. This electron donating reaction is known as reduction, and it can start a chain of reduction and oxidation reactions. These can lead to an imbalance of charges across the membrane, with electrons on one side and protons (H⁺ ions) on the other. This imbalance is what drives energy production. Think of it as a hydroelectric dam, we allow the water (or protons) to build up on one side of the dam (or membrane) and when the channel is opened we can capture the energy of the water (or protons) rushing through. In this case, the turbine is the remarkable ATP synthase complex which produces ATP, the body’s ‘currency’ of energy.

Not bad for a bug…