Visualization of the respiring bacteria in sediments inhabited by Capitella sp. 1

We used the tetrazolium salt 2‐(p‐iodophenyl)‐3‐(p‐nitrophenyl)‐5‐phenyl tetrazolium chloride (INT) to locate sites of active (respiring) bacterial populations in sediments inhabited by the deposit‐feeding polychaete Capitella sp. 1. In sediment microcosms, this organism created typical protruding tubes on the sediment surface and burrows from day 1. Within a couple of hours after adding INT into the overlying water, the red color of the reduced form of INT (INT‐formazan) became apparent in the water column and on the sediment surface. During the following 24 h, part of the subsurface burrows turned red and the color of formazan intensified with time during incubation. Spatial heterogeneity of the distribution of the formazan deposits along burrows also was recognized. Microscopic observations revealed that the tubes and feces produced by Capitella sp. 1, as well as detrital particles, were the sites of intensive formazan deposition. Observation with higher magnification (up to ×1250) revealed that INT‐formazan was present either as bacterial intercellular deposits or as microgranules covering entire bacterial cells. In situ application of INT into a sediment microcosm provides a simple and sensitive way to visualize the impact of burrow structures created by small macrobenthos, such as Capitella, on the distribution of metabolically active bacteria.