Influences of infaunal burrows constructed by the polychaete (Tylorrhynchus heterochaetus) on O(2) concentrations and community structures and abundances of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) in intertidal sediments were analyzed by the combined use of a 16S rRNA gene-based molecular approach and microelectrodes. The microelectrode measurements performed in an experimental system developed in an aquarium showed direct evidence of O(2) transport down to a depth of 350 mm of the sediment through a burrow. The 16S rRNA gene-cloning analysis revealed that the betaproteobacterial AOB communities in the sediment surface and the burrow walls were dominated by Nitrosomonas sp. strain Nm143-like sequences, and most of the clones in Nitrospira-like NOB clone libraries of the sediment surface and the burrow walls were related to the Nitrospira marina lineage. Furthermore, we investigated vertical distributions of AOB and NOB in the infaunal burrow walls and the bulk sediments by real-time quantitative PCR (Q-PCR) assay. The AOB and Nitrospira-like NOB-specific 16S rRNA gene copy numbers in the burrow walls were comparable with those in the sediment surfaces. These numbers in the burrow wall at a depth of 50 to 55 mm from the surface were, however, higher than those in the bulk sediment at the same depth. The microelectrode measurements showed higher NH(4)(+) consumption activity at the burrow wall than those at the surrounding sediment. This result was consistent with the results of microcosm experiments showing that the consumption rates of NH(4)(+) and total inorganic nitrogen increased with increasing infaunal density in the sediment. These results clearly demonstrated that the infaunal burrows stimulated O(2) transport into the sediment in which otherwise reducing conditions prevailed, resulting in development of high NH(4)(+) consumption capacity. Consequently, the infaunal burrow became an important site for NH(4)(+) consumption in the intertidal sediment.