Symbiotic security? Feeding the world through nature’s closest relationships

Picture this – you have been transported to the distant future and you find yourself walking through an idyllic countryside setting surrounded by fields as far as the eye can see. Suddenly, your attention is diverted from the beauty of the scene by the movement of a strange creature in the corner of your eye, and you think to yourself, “surely that cow is not green?!” Curiosity gets the better of you and you make your way towards the farmhouse at the end of the lane. As the farmer answers his door you enquire with polite concern, “excuse me sir, are the cows in your field feeling slightly under the weather? They appear to have turned a peculiar shade of green!” The farmer responds with exasperation, “oo ah, did you miss the great endosymbiotic event of 2089? All livestock are now green. Some clever man found a way to get chloroplasts into the beasts so that they can produce their own energy from the sun without having to be fed.” Dumfounded, you reply, “like plants you mean?” “Exactly like plants – I save a fortune on animal feed and all of the crop harvest is able to go to feeding humans.” As you walk away from the farmhouse reality begins to sink in – yes, the farmer really has been extolling the virtues of photosynthetic cows!

Currently approximately 35% of the annual global grain harvest is dedicated to feeding livestock. This, coupled with the 17% set aside for biofuel production, threats of climate change and increases in plant pathogen prevalence, mean that issues of food security are greater than they have ever been. Whilst the hypothetical situation of the photosynthetic cow is almost certainly beyond the realms of possibility, it does highlight the potential that we have for solving these food security issues. To put it simply, if we are able to negate the need to feed livestock we could have more food available to nourish the ever expanding human population. Through comprehensive understanding of the origins of symbiotic relationships in nature, we can gain insight into how similar relationships could be established in our most important crops and livestock. This could lead to more efficient food production and alleviation of the stresses being put on our global food supply.

A symbiosis is a mutually beneficial relationship that occurs between two organisms – a “you scratch my back and I’ll scratch yours” kind of affair. They are very common in space limited environments, such as coral reefs – through cooperation and swapping of resources, ecological space is able to be shared in a situation when you would normally expect to find one or other of the partners being competitively excluded. In order for symbiotic relationships to evolve there have to benefits for both parties and the participating organisms have to make considerable compromises to account for the differences in lifestyle between themselves and their partner.

Giant clams are one of the largest non-mobile organisms that are found on the coral reef. Ordinarily clams tend to be quite small and live at depth away from the dangers of exposure to direct sunlight. In order to breathe they use a siphon – this is a fleshy organ that extrudes from their protective shell in order to extract oxygen from the environment. The larger the siphon the more vulnerable to predation its owner is – hence evolution has favoured small clams with small siphons. Clams will also usually live at depth in order to avoid the harmful UV rays of the tropical sun which cause DNA damage. Giant clams, however, buck the trend as a result of their very special relationship with a group of microscopic photosynthetic algae, collectively known as zooxanthellae. This symbiosis is mutually beneficial, the clam provides protection for the algae, and the algae provide sugars from photosynthesis. The clams evolved to be larger in order to increase the surface area over which sunlight could be absorbed by their photosynthetic guests and they moved to shallower waters where light penetration is at its maximum. The algae absorb most of the harmful UV rays, and provide oxygen from photosynthesis meaning that the vulnerable siphon isn’t as large as it otherwise would be. The close relationship between giant clams and algae shows that animals can have photosynthetic capabilities – perhaps the idea of a green cow is not so farfetched after all!

Whilst we are currently a long way off being able create photosynthetic animals, many of the world’s symbioses are currently being explored in an attempt to provide solutions to issues of food security. Nitrogen availability is often the limiting factor in plant growth as it is not readily available in a useable form in most soils. Legumes (peas and beans etc.) have skirted this problem by developing a symbiotic relationship with a bacterium (rhizobium) that is able to convert nitrogen from the atmosphere into its useable form. Scientists are exploring the details of this symbiotic relationship in order to try to create a new organelle called a “nitrogenosome” by encouraging the uptake of rhizobium into the roots of the world’s major crops. If successful, one of the main factors that limits plant growth will have been overcome and we will be able to cultivate crops in nutrient poor soils. The poorest and most food deprived areas of the world often have soils with a low nitrogen content. Induction of the “nitrogenosome” would allow grain production in these areas and help to relieve poverty and starvation.