Acetate is a key intermediate in anaerobic mineralization of organic matter in marine sediments. Recent observations suggest that acetate is oxidized syntrophically in the methanic zone of marine sediments, and that electrically conductive mineral particles could provide niches for electroactive microbial communities that perform this process. We combined radiotracer measurements, a novel procedure for ferromagnetic mineral particle extraction, and metagenomic analyses to examine this process in Baltic Sea sediments. Our results confirm that acetate is oxidized syntrophically across and below the sulfate-methane transition zones of the sediments, where the transfer of reducing equivalents from acetate oxidation to CO IMPORTANCE: Acetate is a central intermediate in the anaerobic breakdown of organic matter. In Baltic Sea sediments at and below the sulfate-methane transition zone, we observed acetate oxidation to carbon dioxide at rates similar to methane formation from carbon dioxide reduction, a pattern indicative of syntrophic acetate oxidation. Previous enrichment studies suggest that electrically conductive mineral surfaces can facilitate this process. Motivated by this observation, we extracted ferromagnetic conductive particles from sediments and compared particle-attached microbial communities with bulk sediment. Particle-attached communities were distinct and enriched in the bacterial genus