To assess what defence mechanisms are triggered by Cd2+ stress in Methanosarcina acetivorans, cells were cultured at different cadmium concentrations. In the presence of 100 μM CdCl2, the intracellular contents of cysteine, sulfide and coenzyme M increased, respectively, 8, 27 and 7 times versus control. Cells incubated for 24 h in medium with less cysteine and sulfide removed up to 80% of Cd2+ added, whereas their cysteine and coenzyme M contents increased 160 and 84 times respectively. Cadmium accumulation (5.2 μmol/10–15 mg protein) resulted in an increase in methane synthesis of 4.5 times in cells grown on acetate. Total phosphate also increased under high (0.5 mM) Cd2+ stress. On the other hand, cells preadapted to 54 μM CdCl2 and further exposed to > 0.63 mM CdCl2 developed the formation of a biofilm with an extracellular matrix constituted by carbohydrates, DNA and proteins. Biofilm cells were able to synthesize methane. The data suggested that increased intracellular contents of thiol molecules and total phosphate, and biofilm formation, are all involved in the cadmium resistance mechanisms in this marine archaeon.