Environments Can Give Birth to Cooperation

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.

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