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H 9 S, and in some cases the generation of CH, within two hours of
incubation, (Winfrey and Ward, submitted). In long-term incubations no
C0„ or CH, was detected in fprmalin-killed controls. In most cases
significant amounts of C0„ + CH, were detected in vials incubated
aerobically (Table 4). Anaerobic incubation severely reduced the
amount of radiolabelled gases formed. However, small amounts of these
metabolites were formed from n-hexadecane , n-heptadecane, heptadecene,
ring or methyl labelled toluene, and benzene after lengthy anaerobic
incubation. The amount of gaseous metabolites formed did not exceed
5% of the added radiolabel and reproducibility was poor. Repeated efforts
by two individuals experienced in cultivation of methanogenic
bacteria led to the same observations. Additions of FeCl. or KNO (1
ml of 0.5% (w/v) solutions in anoxic ASW replaced the 1 mi addition of
anoxic ASW) did not stimulate the formation of C0„ and CH, from
n-hexadecane or heptadecene in Aber Wrac'h 5-10 cm sediment.
The possibility of initial accidental exposure to oxygen during
tubing of samples was investigated by late addition of C-toluene
which was soluble in water and could be added as an anoxic solution
well after any oxygen initially present should have been consumed during
dark incubation. Revsbech, et al, (1980b) have shown that oxygen
consumption in intertidal sediments occurs in a matter of minutes following,
darkening to eliminate photosynthesis... As shown in Figure 6,
ring- C-toluene was readily metabolized to CC> when added either at
the time of anaerobic tubing or 38 hours after dark anaerobic incubation
began. Similar results were found for [methyl- C]-toluene.
It was conceivable that the radiolabelled gases might have been
produced from contaminants rather than from the hydrocarbons themselves.
When an attempt was made tcL recover the added C in, long-term
radiolabelling experiments with [1- C] -heptadecane and Jl- C] -heptadecene,
it was noted that the total amount of CO + CH, produced
during anaerobic incubations was similar to the level of impurities
measured in C-labelled hydrocarbons recovered from formalin controls
or from unpurified stocks of added radioisotopes (Table 5). Stock
solutions were chromatographically separated into f , f„ ^nd f„ components
which were then tested separately as sources of C-gases in
dark anaerobic incubations with anaerobic sediments. The results of
such experiments are presented in Eigure 7. The repurified
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f frac-
tions of [1- C] hexadecane and [ 1- , C] -heptadecane were clearly significant
sources for production of CO during dark anaerobic incuba-
tions with a slurry of lie Grande oiled 3-6 cm sediment. Increases in
CO with time following a lag of 5-15 days also suggested that oxidation
did not result from any oxygen which might have been introduced
accidentally during tubing. Similar results were observed with repurified
f of [1- C] -heptadecene.
A final control was run to test the possibility that slow diffusion
of oxygen through the vessels containing incubating samples could
account for the obsexved metabolism. Darle .anaerobic incubations of
repurified f of [1- C] -hexadecane and [1- C] -heptadecane were carried
out with a slurry of mud from the 3-6 cm interval of Aber Wrac'h
sediment. The individual vials were incubated inside an anaerobic
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