The mutualistic metabolic relationships, called syntrophic relationships, between sulfur-reducing bacteria and methane-producing Archaea are responsible for a large part of the global methane cycle. However, the genetic adaptations and regulatory pathways of mutualism are poorly understood, due in part to the difficulty associated with directly observing the behavioral and metabolic interactions of these microbes. To better understand mutualistic adaptations of syntrophic organisms, the bacterium Desulfovibrio vulgaris and the archeaon Methanococcus maripaludis were grown in co-cultures over a period of 1000 generations and their gene expression analyzed. In anaerobic conditions, the metabolic byproducts of one fuels the growth of the other, and vice versa. During lactate fermentation, the growth rate of D. vulgaris is dependent on the level of hydrogen in the ambient environment. M. maripaludis consumes hydrogen to reduce carbon dioxide and produce methane. This relationship between D. vulgaris and M. maripaludis, although syntrophic, is experimentally imposed, and ensures that neither species in the study begins with naturally-derived adaptations. Elucidating the exact mechanisms these microorganisms use to react to stress and adapt to their environment is a fundamental part not only of understanding the mechanisms behind evolution, but also of how interacting populations of microorganisms may be better suited to survive stressful environmental conditions such as global climate change.