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to succession with the age of the individual alga. On young meristems and cauloids
of
Saccharina latissima
microbial communities were more similar to each other
than aging blade sections of the same individual (Staufenberger et al.
2008
),
suggesting that tissue age might contribute to microbial community variability.
Conversely, no clear differences were found in microbial communities on meristem
and lamina of
Laminaria hyperborea
(Bengtsson et al.
2010
), indicating that
considerable variability exists even between closely related species.
Several lines of evidence suggest that microbe-macroalgae interactions might
not be host-specific when 16S rRNA gene phylogeny is the basis for microbial
community assessment (Burke et al.
2011b
). (1) Microbial communities can vary
considerably among individuals of the same algal species. The microbiota on
Ulva
australis
individuals from within the same and three different tidal pools, separated
by less than 20 m, were found highly variable (up to 40% based on DGGE analysis;
Tujula et al.
2010
). A large-scale sequencing analysis revealed that less than 20% of
sequences (at 97% sequence identity) were shared among six individuals of
Ulva
australis
(Burke et al.
2011b
)
.
Also, on
Laminaria hyperborea
variability in
community composition was detected between sampling locations (Bengtsson
et al.
2010
). (2) The algal microbiota varies considerably over seasons. Individuals
of
Ulva australis
varied as much as 40% in summer (Tujula et al.
2010
). Strong
seasonal shifts also occurred in the microbiota of
Fucus vesiculosus
,
Gracilaria
vermiculophylla
, and
Ulva intestinalis
; however, shifts were consistent at the
phylum level and reoccurred in two consecutive years (Lachnit et al.
2011
). The
considerable variability among individuals of the same species became especially
apparent in the study of the green algae
Ulva australis
based on the resolving power
of large-scale sequencing (Burke et al.
2011b
). It appears that at least in case of
Ulva australis
a core microbiota does not exist and that a large number of microbial
populations are capable of colonizing the algal surface (Burke et al.
2011b
).
10.4 A Model of Surface Colonization on Macroalgae
The high variability in microbial colonization of algal surfaces observed in a
number of studies has resulted in the proposal that a large functional redundancy
exists within the colonizing microbiota (Burke et al.
2011b
). If true, then phyloge-
netic aspects, e.g., which microbial populations (or taxa) are present on the alga,
would be inferior to the presence (and expression) of specific functions encoded by
their genes. A recent metagenomic analysis of the microbiota on
Ulva australis
supports the hypothesis that the algal surface provides a niche for microorganisms
carrying specific functional genes rather than belonging to a specific taxonomic
group (Burke et al.
2011a
). Despite the high taxonomic variability (~only 15-20%
similarity between individuals), the functional composition of genes was highly
similar (70%) and a core set of functional genes across all members of the microbial
community was detected. Core functions detected at higher rate than in the
surrounding seawater encompassed genes involved in detection and movement to
