Mediation analysis quantified hypothesized microbiota- and metabolite-mediated proportions; single-cell eQTL from 14 immune subsets localized genetic associations to cell types.<h4>Results</h4>MR analysis suggested distinct causal associations of m<sup>6</sup>A modifiers (WTAP/ALKBH5) with mental disorders. This evidence concerns the gene ALKBH5 and psychiatric disorder.