Underwater Domains in Yellowstone Lake Hydrothermal Vent Geochemistry and Bacterial Chemosynthesis

Reduced inorganic compounds of geothermal-origin hydrogen sulfide (H2S), iron (Fe[II]), and methane (CH4) were common but not ubiquitous components of hydrothermal vent fluids of Yellowstone Lake at concentrations capable of supporting chemolithoautotrophic (geochemical-oxidizing, carbon dioxide (CO2)-fixing) bacterial growth. Closely linked to the presence of reduced geochemicals was abundance of chemosynthetic bacteria and dark CO2 fixation activity. Pronounced productivity at vent sites in the northern basin (Mary and Sedge Bays, Storm and Steamboat Points, and east of Stevenson Island) was accompanied by reduced sulfur stimulation in near-vent receiving waters, while none of these characteristics were found in West Thumb vent fields. Per-liter bacterial productivity at vents (to 9.1 μgC/L/hour) could reach algal photosynthesis in surface waters (to 8.9 μgC/L/hour). Thermophilic (heat-loving) sulfurand methane-oxidizing bacteria were isolated from vent orifice waters, and CO2 fixation incubations at 50°C indicated that the majority of chemosynthesis within the vents themselves was optimal at high temperatures. Receiving waters had much less activity at 50°C than at ambient temperature (4–20°C), distinguishing populations of mesophilic (moderate-temperature) bacteria that had also responded to the input of geochemicals from vents. Strong evidence for mineraldependent bacterial productivity was obtained, with limited data suggesting an influence of lake stage or outflow on vent and productivity characteristics. Introduction For decades the colorful mats of bacteria and algae surrounding bubbling vents and fumaroles at Yellowstone National Park have been a focus of both touristic and scientific interest. It is with no small wonder that people look upon the growth of microorganisms in the often very hot, very corrosive fluids. Yet the interaction of biology with geothermal and geochemical energy may be more ancient than any other ecology. Prior to the mid-1970s, many scientists favored the theory of organic matter formation in the atmosphere and initial biological activity in surface brine pools using lightning energy as the primary catalyst (c.f. Miller 1953; Oro et al. 1990). Following the discovery of deep-sea hydrothermal geoecosystems in the mid-1970s, an additional hypothesis was developed, invoking organic matter formation and biological assembly in the high-temperature (to