Women with depression show elevated methanogenic archaea, particularly Methanobrevibacter smithii, which correlates with increased indole-3-acetic acid (IAA) and reduced protective tryptophan metabolites like picolinic acid. This 802-person study found completely different microbial depression signatures between sexes: women's depression linked to methane-producing microbes and IAA pathways, while men showed only kynurenic acid associations. This discovery challenges the one-size-fits-all approach to microbiome-based mental health interventions. The methanogens-depression connection in women represents a novel therapeutic target, as these ancient microbes are notoriously difficult to culture and study. However, the finding opens possibilities for sex-specific probiotic strategies or methane-reducing compounds. The research methodology is robust, controlling for lifestyle factors and using machine learning with explainable AI. Yet causality remains unclear from this cross-sectional design. The implications extend beyond depression treatment to fundamental understanding of sexual dimorphism in psychiatric disorders. This work positions archaeal methanogens as unexpected players in women's mental health, potentially explaining why depression manifests differently across sexes and why treatments often show variable efficacy between men and women.