For the first 2 billion years of life on Earth, our planet was dominated by single-celled bacteria and their cousins, archaea. It was Slimeball Earth, and it would have stayed that way were it not for the single most important merger and acquisition in the history of our planet.
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One of those archaeal cells engulfed a bacterial cell and, somehow, the bacterium survived. Both cells reproduced together and, in time, the engulfed bacterium became the mitochondrion, a tiny structure that acted as a powerhouse of that primitive cell.
Nick Lane at University College London has shown that mitochondria allowed these cells to express a remarkable 200,000 times more genes, enabling them to grow and produce endless forms. The new combo became the complex eukaryotic cell, which ultimately gave rise to almost every living thing you can see without a microscope, including us.
Symbiosis is the reason we are here, then. And it still sustains us now. Over 80 per cent of land plant species form symbiotic relationships with mycorrhizal fungi, where the fungus provides nutrients and the plant supplies food to the fungus. Without this relationship, we wouldn’t have oxygen to breathe. The soil itself was formed by symbiosis between fungi, bacteria and plants, which didn’t even have roots when they first moved onto land from the oceans, roughly 500 million years ago.
When most of us hear the word “symbiosis”, we probably think of life forms “living together”, which is literally what the Greek root of the word means. It is a clownfish nestling in an anemone. Or a coral reef and the spectacular array of life it supports. Or lichens, which are an intimate association of organisms from two or even three separate kingdoms of life. In other words, symbiosis has positive vibes and we assume it means some harmonious, mutually beneficial arrangement.
It is better, however, to think of symbiotic relationships as spanning a continuum, from outright parasitism at one end to mutualism at the other, says Katie Field at the University of Sheffield, UK. And don’t think that even mutualism is selfless: partners usually only give to receive something later.
To see how this continuum works, just look at the diversity of ways in which orchids demonstrate it. Orchid seeds are minute, containing almost no resources, so to germinate, these plants must always parasitise mycorrhizal fungi in the soil, from whom they steal sugars and nutrients. Later, when they have grown leaves, some species begin to pay the fungi back, and it starts being a mutualistic relationship.
But then there are cases when older orchids supply food to young orchids. And there are still other species of orchid that never develop green leaves and remain parasitic their entire lives. “You have this whole cycle of different phases of symbiotic function,” says Field.
There is yet another important and practical way to think about symbiosis: as a key to unlocking our future. Legumes such as pulses, beans and lentils use symbiotic bacteria to make their own fertiliser from the nitrogen in the air. Recent work indicates that the plants’ method for doing this was adapted from pre-existing cellular machinery.
This means it should be possible to engineer other crops – especially cereals such as wheat and corn, which contribute half of all the calories people eat – to produce their own fertiliser, says Giles Oldroyd at the Crop Science Centre at the University of Cambridge. Pull that off and we could vastly cut the amount of fertiliser we pour onto fields.
Oldroyd, who is running field trials of crops modified to boost their symbiotic powers, says his mission is to get rid of chemical fertilisers in agriculture. “I’m super optimistic we’re going to get there,” he says.
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