POLYMERIC ORGANIC MATTER 



FERMENTING 



BACTERIA 



C °2 + H 2 + ACETATE 



METHANE-PRODUCING 

 BACTERIA 



SULFATE-REDUCING 

 BACTERIA 



Figure 1. Simple model for anaerobic microbial food chains in marine 

 sediments. The model does not include minor electron sink 

 fermentation products or other possible anaerobic groups such 

 as denitrifying bacteria. The asterisk indicates the major 

 product from the methyl position of acetate. 



only been confirmed in this and other recent studies (Winfrey and Ward, 

 submitted; S^rensen, et al, 1981; Banat and Nedwell, personal communi- 

 cation). Sulfate-reducing and methane-producing bacteria share the 

 potential to utilize these fermentation products and may compete in 

 marine sediments. Detailed investigations to characterize anaerobic 

 processes in Brittany sediments were made as a part of this study, but 

 discussion here is beyond the scope of this report. In summary, meth- 

 anogenesis is only significant in the competition for acetate and H~ in 

 sediments where sulfate is depleted (e.g., deep subtidal sediments, 

 Winfrey, et al, in press). In intertidal sediments where sulfate is 

 high at all depths (see below), methanogenic bacteria may be restricted 



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