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Abstract

Selection by the local, contemporary environment plays a prominent role
in shaping the biogeography of microbes. However, the importance of
historical factors in microbial biogeography is more debatable.
Historical factors include past ecological and evolutionary
circumstances that may have influenced present-day microbial diversity,
such as dispersal and past environmental conditions. Diverse
thermophilic sulfate-reducing Desulfotomaculum are present as dormant
endospores in marine sediments worldwide where temperatures are too low
to support their growth. Therefore, they are dispersed to here from
elsewhere, presumably a hot, anoxic habitat. While dispersal through
ocean currents must influence their distribution in cold marine
sediments, it is not clear whether even earlier historical factors,
related to the source habitat where these organisms were once active,
also have an effect. We investigated whether these historical factors
may have influenced the diversity and distribution of thermophilic
endospores by comparing their diversity in 10 Arctic fjord surface
sediments. Although community composition varied spatially, clear
biogeographic patterns were only evident at a high level of taxonomic
resolution (> 97% sequence similarity of the 16S rRNA gene) achieved
with oligotyping. In particular, the diversity and distribution of
oligotypes differed for the two most prominent OTUs (defined using a
standard 97% similarity cutoff). One OTU was dominated by a single
ubiquitous oligotype, while the other OTU consisted of ten more
spatially localized oligotypes that decreased in compositional
similarity with geographic distance. These patterns are consistent with
differences in historical factors that occurred when and where the taxa
were once active, prior to sporulation. Further, the influence of
history on biogeographic patterns was only revealed by analyzing
microdiversity within OTUs, suggesting that populations within standard
OTU-level groupings do not necessarily share a common ecological and
evolutionary history.