Private benefits and metabolic conflicts shape the emergence of microbial interdependencies.

Microbes perform many costly biological functions that benefit themselves, and may also benefit neighbouring cells. Losing the ability to perform such functions can be advantageous due to cost savings, but when they are essential for growth, organisms become dependent on ecological partners to compensate for those losses. When multiple functions may be lost, the ecological outcomes are potentially diverse, including independent organisms only; one-way dependency, where one partner performs all functions and others none; or mutual interdependency where partners perform complementary essential functions. What drives these different outcomes? We develop a model where organisms perform 'leaky' functions that provide both private and public benefits to explore the consequences of privatization level, costs and essentiality on influencing these outcomes. We show that mutual interdependency is favoured at intermediate levels of privatization for a broad range of conditions. One-way dependency, in contrast, is only favoured when privatization is low and loss-of-function benefits are accelerating. Our results suggest an interplay between privatization level and shape of benefits from loss in driving microbial dependencies. Given the ubiquity of microbial functions that are inevitably leaked and the ease of mutational inactivation, our findings may help to explain why microbial interdependencies are common in nature.