Are endosymbioses mutualistic?

Many animals, plants and protists contain non-parasitic microorganisms and these endosymbioses are widely assumed to be mutualistic. Most of the microorganisms possess metabolic capabilities, such as the ability to fix nitrogen, photosynthesize or degrade cellulose, that their partners utilize. However, as discussed in this article, there is scant evidence that the microorganisms benefit from such associations, and it is unclear how the benefit or harm incurred by microorganisms that require the association can be demonstrated.