The classical separate treatments of competi- tion and predation and difficulties in providing a sensible theoretical basis for mutualism attest to the inability of traditional models to provide a synthesising framework for trophic interactions, a fundamental component of ecology. Recent approaches to food web modelling have focused on consumer-resource interactions. We construct a unifying theoretical framework to explicitly represent finite resources for each population using Lotka-Volterra (LV) equations. We show that mixotrophy, a ubiquitous trophic interaction in marine plankton, provides the key to developing a synthesis of the various ways of making a living. This framework also facilitates an explicit re- definition of facultative mutualism, illuminating the over- simplification of the traditional definition. We demon- strate a continuum between types of trophic interactions: populations can smoothly evolve through these types without losing stable coexistence. This provides a theo- retical basis for the evolution of trophic interactions from autotrophy through mixotrophy/mutualism to heterotrophy.