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Microbial communities in black smoker chimney structures at the NW caldera vent field of the Brothers volcano, Kermadec arc were characterized by using both culture-dependent and -independent techniques. The hydrothermal vent fluid chemistry, as given by end-member salinities and gas contents, differ among the black smoker sites of the NW caldera field, indicating probable phase-separation-controlled variability in the fluid chemistry. Chimney structures collected from typical Cl-depleted and Cl-enriched hydrothermal fluid vents were used for the microbiological investigation. The 16S rRNA gene clone analysis showed that the archaeal rRNA gene communities were similar within interior and exterior substructures of any single chimneys, and even between chimneys having either Cl-depleted or Cl-enriched hydrothermal emissions. By contrast, the bacterial rRNA gene communities varied between chimneys hosting Cl-depleted or Cl-enriched fluids. Cultivation analysis showed significant variation in the viable counts of various microbial components among the chimneys, particularly of H2- and/or S-oxidizing chemolithotrophs such as the genera Persephonella and Sulfurimonas. The difference shown by the cultured microbial community structures between the chimneys may be related to the different chemistries of hydrothermal fluids being expelled by the chimney structures, and possibly differences in the subseafloor environments beneath the vent sites, especially when considering different gas inputs and carbon sources. The patterns in cultivated microbial populations in the chimney structures were compared among the chimney structures studied so far from various deep-sea hydrothermal fields including this study. It shows that the patterns from the gas-rich hydrothermal fluid chimneys are quite similar between the geographically and geologically different hydrothermal fields of the Brothers NW caldera vent field, and the Mariner vent field of the Valu Fa Ridge. [ABSTRACT FROM AUTHOR]