Using C and N stable isotopes we analyzed different trophic aspects of the benthic fauna at two sites in the Comau fjord: one with presence of venting of chemically reduced fluids and extensive patches of bacterial mats (XH: X-Huinay), and one control site (PG: Punta Gruesa) with a typical fjord benthic habitat. Due to the widespread presence of such microbial patches in the fjord and their recognized trophic role in reducing environments, we hypothesize that these microbial communities could be contributing to the assimilated food of consumers and transferring carbon into high trophic level in the food web. Food sources in the area included macroalgae with a wide range of δ13C values (-34.7 to -11.9‰), particulate organic matter (POM, δ13C=-20.1‰), terrestrial organic matter (TOM, δ13C= -32.3‰ to -27.9) and chemosynthetic filamentous bacteria (δ13C= ~-33‰). At both sites, fauna depicted typical values indicating photosynthetic production as a main food source (> -20‰). However, in XH selected taxa reported lower δ13C values (e.g. -26.5‰ in Nacella deaurata), suggesting a partial use of chemosynthetic production. Furthermore, enhanced variability at this site in δ13C values of the polyplacophoran Chiton magnificus, the limpet Fissurella picta and the tanaid Zeuxoides sp. may also be responding to the use of a wider scope of primary food sources. Trophic position estimates suggest three trophic levels of consumers at both sites. However, low δ15N values in some grazer and suspensivore species suggest that these taxa could be using other sources still to be identified (e.g. bacterial films, microalgae and organic particles of small size-fractions). Furthermore, between-site comparisons of isotopic niche width measurements in some trophic guilds indicate that grazers from XH have more heterogenic trophic niches than at PG (measured as mean distance to centroid and standard deviation of nearest neighbor distance). This last could be ascribed to the utilization of a mixture of photosynthetic and chemosynthetic carbon sources. In addition, standard ellipses area (SEAc) values in suspensivores and carnivores at both sites suggest a similar magnitude of exploitation of food sources. However, grazers from XH have a greater expansion of their trophic niche (SEAc), probably explained by the presence of species with low δ13C and δ15N values, and directly associated to chemosynthetic carbon incorporation.