Microbial mediated formation of low-temperature hydrothermal barite chimneys

A low-temperature (20 degrees C) venting area with numerous active and extinct barite chimneys (up to 1 m tall) are located on the eastern flank of the hydrothermal mound of Loki's Castle black smoker field at the Mohns-Knipovich bend of the Arctic Mid-Ocean Ridge. The active barite chimneys are covered by white mats containing abundant microbial cells and extracellular material with attached barite crystals. Within the chimneys microbial cells are partly embedded in barite and crystals are covered by extracellular material. These observations indicate that the microbial material serve as a substrate for nucleation and precipitation of barite with the potential of having an important control on the construction of the chimneys. In addition, the presence of framboidal pyrite in black interior flow channels and in the underlying hydrothermal sediment further suggests that the chimney formation is linked to microbial sulphate reduction (MSR). To further investigate the relationship between chimney growth and microbial activity we used a combination of biomolecular and isotope analyses. Pyrosequencing of PCR amplicons of 16S rRNA followed by taxonomic classification revealed that sulphide oxidizers (Sulfurimonas) within the Epsilonproteobacteria dominate the microbial mats and the white barite of the chimney wall. In the black interior flow channel a more diverse microbial community was observed indicating methane, sulphur and ammonia oxidation as well as heterotrophic processes. Multiple isotope analyses (δ18O, δ34S, Δ33S) reveal that the barite chimneys precipitated from a fluid that was modified by subseafloor MSR in the sulphide mound. This is supported by the sulphur isotope signature of the framboidal pyrite, pore water, and mono- and disulphides extracted from the hydrothermal sediment as well as the biomolecular data. We suggest that the MSR was triggered by mixing of the H2 and CH4 rich high-temperature (320 degrees C) fluids and percolating seawater, which resulted in remobilization of hydrothermal barite deposited as debris and plume fall out in the mound. The combined results strongly suggest that the formation of the barite chimneys is a result of complex seafloor and subseafloor geobio-interactions.