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Andrew Murray (l) and Erik Hom

We investigated the origins of symbiosis by ecologically engineering brewers yeast and a single cell alga to proliferate by cooperating with each other. Both organisms need carbon and nitrogen to grow but have restrictions about which molecules they can use as sources of these two elements. Yeast can grow on glucose as a carbon source, but Chlamydomonas reinhardtii, an alga that has been studied so much that it is sometimes called the green yeast, cannot. But since yeast converts glucose into carbon dioxide, which Chlamy¬†can use as a carbon source, as long as it has light for photosynthesis. Conversely, yeast can’t use nitrite, an oxidized form of nitrogen, as a nitrogen source, but Chlamy¬†can convert nitrite into ammonia, which yeast can use a nitrogen source. If a flask is sealed to keep out atmospheric carbon dioxide, the result of these complementary restrictions is that the two organisms can grow together if they are given glucose and nitrite and light, even though neither can grow alone under this conditions.
Our results provide the first laboratory evidence for a proposal called ecological framing, the idea that if two organisms with the right metabolic pathways found each other in the right environment, they would be forced to chemically depend on each other in order to grow. Because symbiosis exists in many different forms, from the mixture of algae and fungi that form lichens, to the interaction of nitrogen fixing bacteria with the roots of legumes (peas and beans), our results lead to provocative speculations about how easily and accidentally some of these symbioses might have started.

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