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the host, attachment and biofilm formation, response to the algal host environment,
regulatory mechanisms in response to environmental stimuli, defense, and lateral
gene transfer. All these functions were regarded as consistent with the ecological
role of the Ulva australis microbiota (Burke et al. 2011a ). Phylogenetic and
functional analysis of the metagenome dataset revealed that the core functions
are not restricted to a particular taxonomic group, suggesting that different taxa
are responsible for the core functions of the microbial community on Ulva
australis . Based on their results, Burke et al. ( 2011a , b ) proposed a competitive
lottery model for community assembly on the surface of U. australis . The competi-
tive lottery model, originally proposed for coral reef fish (Sale 1979 ), integrates
functional and random components of community assembly. According to the
model, different species but with similar (or identical) functional traits can occupy
the same niche in an ecosystem facilitated through the stochastic chance (“lottery”)
to get to the particular niche space first (Sale 1979 ). Translated to microbial
community structuring, Burke et al. ( 2011a , b ) argue that microbial populations
can colonize the surface of an alga provided they have the necessary function in the
form of a particular gene (or sets of genes). The assemblage of microbial population
might, however, be determined by stochastics (“lottery”) rather than controlled by
specific host enforced selection mechanisms that would result in a very specific
microbiota that is known from other host-microbe systems. Such a model of
microbial community structuring implies that genes (or gene clusters) are more
important than microbial taxonomic entities (“species”). In microbiology, the
species concept is still a matter of debate and especially lateral transfer of genes
renders it difficult to define how a microbial species is defined (Rossello-Mora and
Amann 2001 ). Support for functional genes as drivers of community assembly also
comes from the observation that the microbiota on Ulva australis abundantly
harbors genes necessary for lateral gene transfer (Burke et al. 2011a ). Analyzing
functional genes revealed biological patterns on Ulva australis (Burke et al. 2011a )
that were not detected by studying 16S rRNA gene-based phylogenetic community
composition (Burke et al. 2011b ). Similar observations were made in the human
microbiome (Turnbaugh et al. 2009 ). The repercussions of these findings could
have far reaching impacts on studies of microbial community ecology eventually
shedding light on differences in the ecology of micro- and macroorganisms.
10.5 Conclusions
The molecular microbial ecology of macroalgal hosts is a rather young field with only
small datasets available but with large potentials to influence theory building in
microbial ecology and host-microbe interactions. Recent, more in-depth molecular
sequence analyses have challenged the notion that host-specific microbial taxa
colonize macroalgal surfaces. Rather, a model of algal colonization has been pro-
posed in which specific microbial functional genes govern the structuring of micro-
bial communities and not
taxonomic affiliations of microbial populations
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