Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Cells were cultured with acetate as the electron donor and either fumarate (40 mM), Fe(III) citrate (50 mM) or poorly crystalline Fe(III) oxide (100 mM) as the electron acceptor as previously described (Coppi et al., 2001).
Category: