in vent water to ATP in ambient bottom seawater) ranging from 1.2 

 to 23. The highest ATP concentration measured for a freshly- 

 collected vent water sample was 118 ng l - -*- ( Alvin dive #1797). 

 This ATP concentration is 3-4 times greater than the ATP 

 contained in surface waters near Hawaii (Winn and Karl 1984) and 

 documents a local enrichment of microbial biomass in Loihi 

 Seamount hydrothermal vent fluids. The ATP concentrations and 

 enrichment factors measured at Pele's Vent are similar to those 

 previously reported for a variety of hydrothermal vent ecosystems 

 (summarized by Karl 1987). Incubation of Pele's Vent water 

 samples for periods ranging from days to weeks at 25°C resulted 

 in the growth of a substantial population of bacteria (up to 

 2500 ng ATP 1~^-) indicating the potential for sustained 

 bacterial growth and biomass production. During these incubation 

 periods, the growing microbial assemblages actively incorporated 

 the radioactivity from ^n-adenine and ^H-thymidine into both 

 cellular nucleic acids and protein. Over short-term incubation 

 periods (<1 d), the biomass-specif ic (i.e., per unit ATP) 

 incorporation of ^H-adenine into cellular DNA was up to an order 

 of magnitude greater for samples collected from Pele's Vent than 

 for ambient bottom seawater (when incubated at 25°C, 1 atm.). 

 Water samples collected at Pele's Vent also had elevated 

 concentrations of bacterial cells relative to ambient bottom 

 seawater (Table 8). Enrichment factors ranged from 3.1 to 14.4 

 for the samples analyzed, a range similar to that presented 

 previously for ATP concentrations. 



During the Pisces V expedition, we sought to learn more 

 about the environmental conditions where bacterial growth in the 

 Pele's Vent hydrothermal system is likely to occur. We adopted 

 the protocol of Karl (1985), previously employed at the Galapagos 

 Rift and 21°N hydrothermal vent sites, in order to evaluate the 

 temperature-metabolic activity relationships for vent derived 

 microorganisms. The results of these experiments would help us 

 to determine the upper limits for temperature, and to predict the 

 fate of hydrothermal bacterial assemblages discharged into the 

 relatively cool ( 4°C ) bottom seawaters. 



For Pele's Vent water samples we observed a striking 

 dependence on temperature for the uptake and assimilation of a 

 variety of nucleic acid precursors (adenine and thymidine) and 

 potential growth substrates (acetate and glutamic acid; see Table 

 9 ) . These data confirm the importance of temperature as an 

 important environmental variable. Our results indicate that the 

 microorganisms sampled from Pele's Vent have temperature optima 

 of at least 37°C, and probably higher. The assimilation of 

 glutamic acid was greatest at 60°C and for adenine and thymidine 

 the population response at 60°C was 80-90% of the maximum 

 measured at 37°C (Table 9). In this respect, Pele's Vent 

 populations are more thermophilic than the microbial communities 

 at either the Galapagos Rift or from the low temperature (<35°C) 

 vents at 21°N on the East Pacific Rise (Karl 1985). The 



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