experiments were performed in the laboratory under enrichment 

 culture (artificial growth medium) conditions, the in-situ rates 

 of CH 4 production cannot be estimated. Furthermore, they also 

 detected the presence of active bacterial CH4 oxidation at 100°C, 

 consequently the net effect of microbial activity on CH 4 

 concentrations in hydrothermal systems cannot be easily 

 predicted. Nevertheless, the potential for biological CH 4 

 production does appear to exist in certain deep-sea hydrothermal 

 systems . 



The process of oxidation of CH 4 by specific bacterial 

 species is well-documented (Crawford and Hanson 1984). It is 

 probable that the CH 4 derived from the hydrothermal systems at 

 Loihi Seamount is also oxidized either by bacterial populations 

 located at or near the vent fields, or by bacteria contained 

 within the hydrothermal plumes. Based on calculations performed 

 by Welhan and Craig (1979), the CH 4 flux from the world-ocean 

 ridge system (estimated from 3 He flux) is sufficient to replace 

 the entire pool of deep-sea CH 4 in a period of approximately 30 

 years. This implies a rapid bacterial consumption rate beneath 

 the thermocline (Welhan and Craig 1979, 1983). Our experimental 

 results from Pele's Vent indicate a significant potential for 

 bacterial CH 4 oxidation in samples collected from discharged 

 hydrothermal fluids and bacterial mats (Table 5). These results 

 suggest that CH 4 oxidation does occur at Pele's Vents. 



During the Pisces V field expedition, we continued to 

 investigate the process of CH 4 oxidation by measuring the net 

 changes (i.e., balance between CH 4 oxidation and CH 4 production) 

 which occur in the total CH 4 concentrations of Pele's Vent water 

 samples during prolonged incubation (30 d) at 25°C, 1 atm. Our 

 results indicate a significant net decrease in dissolved CH 4 for 

 all but one of the hydrothermal vent samples analyzed (Table 6). 

 We then used these water samples to measure the presence and 

 activity of methane oxidizing bacteria using 14 CH 4 , as described 

 previously. In these latter experiments, we measured the 

 radioactivity incorporated in cell materials (nucleic acids and 

 protein) as well as the radioactivity that was respired as CO2 

 (Table 7). Our results indicated that all of the samples from 

 which we had detected substantial CH 4 oxidation by chemical 

 determinations also displayed oxidation using the radiochemical 

 method. The most active samples (#25/Niskin and #29/Niskin) were 

 the two samples which initially contained the highest 

 concentrations of CH 4 (Table 6). It is interesting to point out 

 that >90% of the radioactivity resulting from the metabolism of 

 14 CH 4 was respired as 14 C0 2 with only a minor fraction being 

 incorporated in cellular macromolecules . 



Bacterial Biomass and Metabolic Activity at Pele's Vent 



Warm waters (up to 30°C) emitted from Pele's Vents contained 

 elevated concentrations of ATP with enrichment factors (i.e., ATP 



212 



