column. Tubeworm cover does not show a significant 

 correlation with methane concentrations, but it does 

 correlate with extractable organic material (EOM) in the 

 sediments; a pattern not shared by mussels. In both cases 

 (mussels-methane and tubeworms--extractable organic 

 compounds), the significant correlation is localized to a 

 radius of less than 7.5 m from the high methane or EOM 

 sites. The EOM is predominantly liquid petroleum. 



Observations made during the June 1987 dive series 

 showed that the spatial variation of the various assemblages 

 was greater than anticipated from earlier work. At least 

 five basic assemblage types were encountered: mussel beds, 

 tube worm clumps, clam beds, epifaunal brachiopod-solitary 

 coral assemblages, and gorgonian fields. In some instances, 

 some of these assemblages overlapped in space, and initial 

 isotopic data suggests some shared dependence upon 

 chemosynthetic sources. 



Chemical Studies 



Twelve discrete streams of gas bubbles were collected 

 and analyzed. As a percent of CI to C5 hydrocarbons, all 

 gases collected where predominantly methane (Table 1). 

 Other than the hydrate, methane accounted for more than 94% 

 of the CI to C5 hydrocarbon gases. The hydrate gases 

 contained 30% C2 to C5 gases. The highest percentage of 

 methane gas (99.4%) and the second isotopically lightest gas 

 (-52.9 o/oo) was collected at GC-272. In general the 

 isotopically heavier gases were collected at GC-184 and GC- 

 234, but a substantial range in values was observed (-37.6 

 to -53.9 o/oo). The gases sampled as discrete streams of 

 gas bubbles were a mixture of thermogenic and biogenic 

 gases. Analyses are not complete enough to evaluate 

 variations in mussel tissue carbon isotopic composition as a 

 function of variations in the source methane carbon isotopic 

 composition. 



Hydrocarbon Metabolism in Seep Organisms 



A variety of seep organisms (including mussels, clams, 

 tubeworms and neogastropods ) were surveyed for evidence of 

 mixed function oxidase (MFO) mediated hydrocarbon metabolism 

 (Tables 2, 3 and 4). Not surprisingly, mussels exhibited 

 much higher MFO activity than did the other species. 

 Mussels live in closer contact with active petroleum seepage 

 than other species. Of the mussel tissues assayed (gills, 

 mantle, food, digestive gland and gonad), only the gills 

 exhibited significant MFO activity. This is very unusual in 

 molluscs, since MFO activity normally is concentrated in 

 their digestive glands. The gills are also the site where 

 the methylotrophic symbiotic bacteria are located. 



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