The invention provides a method of culturing a microorganism that produces a free fatty acid in a culture medium, comprising culturing the microorganism in the culture medium comprising a sufficient amount (e.g., at least about 0.5 mM) of a carboxylate counterion source to enhance viability of the microorganism in the culture medium. Fatty acids produced using the methods of the invention can be used to synthesize a variety of products, including biofuels.