We investigated one process, sulfate reduction, in greater detail
because the spatial distribution of this process had one of the best
matches between the model and observation and because there
was a well-studied genetic marker for this function. The three
OEUs with spatial distributions that best matched this process
(Fig. 5) contained 14 OTUs that were in high abundance in both
this survey and the positive control for the gene fusion assay
described in the following. Among these OTUs, six are classified
as Deltaproteobacteria, the class that contains most of the bacteria
known to reduce sulphate, and one of the Deltaproteobacteria
OTUs corresponds to a known sulfate-reducing organism
(Supplementary Table 6). Among the other OTUs, five are classified
as Bacteroidetes, which contains no known sulfate reducers and are
instead regarded as specialists in the degradation of high-molecularweight
organic matter23. Because terminal oxidation processes of
organic carbon under anaerobic conditions are rarely catalysed by
a single organism, we suspected that the sulfate reducers among
the Deltaproteobacteria might be in a syntrophic relationship with
the Bacteroidetes organisms, which provide low-molecularweight
dissolved organic carbon to sulfate reducers. Intriguingly,
the reference OTU for sulfate reduction (a clone similar to