INVESTIGATION OF MICROBIAL HABITATS IN A VIGOROUSLY VENTING BLACK SMOKER CHIMNEY

The walls of hot black smoker chimneys may contain the limits to life. To study the interaction between microorganisms and the mineral/chemical environments in which they exist, a vigorously venting chimney was recovered in 1998 from the Mothra Hydrothermal Field in the Endeavour Segment of the Juan de Fuca Ridge as part of the Edifice Rex Project. At the time of recovery, the structure (Finn) was venting ~302°C reducing fluid into 2°C oxidizing seawater through a large, chalcopyrite-lined central conduit. The wall separating the two fluids ranged from 5 to 42 cm, producing steep chemical and thermal gradients. Coupled mineralogical/chemical and microbial investigations reveal that microbial populations are dramatically affected by these gradients. Highly variable mineral textures and chemistry record a complex history of fluctuating conditions, creating a dynamic environment. Deposits of anhydrite and clay, both in the central conduit and in pockets within the walls show that seawater was able to enter the structure through fractures and pores. Strontium isotopes indicate that anhydrite was deposited within Finn through both turbulent mixing with hydrothermal fluid and by heating of seawater with minimal infusion of hydrothermal fluid.

In areas where the walls are thickest, small parasitic chimneys increased Finn's diameter, and incorporated pre-existing outer walls into the structure. Parasitic chimneys that are furthest from the central conduit are filled with amorphous silica with a distinctive filamentous habit. This morphology may represent the fossilization of microbial communities living within the channels. Tube worm fossils included within the walls are also coated with filamentous silica.

Geochemical analyses show significant enrichment in many trace elements near the outer wall of Finn, and they are associated with the highest measured microbial concentrations. This correlation may reflect a high energy environment of hydrothermal fluid and seawater mixing, and/or the possibility that microorganisms may mediate metal deposition. The steep physical and chemical gradients and fluctuating conditions present within hot black smoker chimneys create niches in which microbial populations can thrive.