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19.4.1 Picoeukaryotes Spatial Patterns Are
Not Linked to Environmental Factors
at a Regional Scale
According to the cosmopolitan view of the microbial world,
one might expect to fi nd similar microbial community struc-
ture (richness, diversity, and composition) in similar habitats
and differentiated microbial communities along an environ-
mental gradient (Green and Bohannan 2006 ). The use of
statistical analysis allows to estimate the impact of geo-
graphic distance vs. environmental conditions on assemblage
composition (Martiny et al. 2006 ), factors rarely taken into
account simultaneously. Partial correlations and Mantel test
results (Lepère et al. 2013 ) clearly showed that variations of
the alpha - and beta -diversity were signifi cantly infl uenced
by the geographic distance between lakes or areas rather than
the environmental factors analyzed, such as bottom-up fac-
tors (ie nutrients) or potential predators (ie heterotrophic
nano-fl agellates and metazooplankton). However, even
though the parameters explored are those commonly consid-
ered when attempting to explain the spatial partitioning of
aquatic microorganisms, the analysis did not include all
potential controlling factors. For example, Schiaffi no et al.
( 2011 ) showed that light penetration and dissolved organic
carbon had a structuring effect on microorganism popula-
tions in 45 lakes. Overall, our results contradict the hypoth-
esis of a general microbial cosmopolitanism. These patterns
have already been observed for bacterial communities
(Reche et al. 2005 ; Martiny et al. 2011 ). In addition, analysis
performed on monthly samples showed that temporal varia-
tions in the composition of the picoeukaryotes community
do not affect the importance of geographic distances.
19.4.2 Distribution of Dominant
Picoeukaryotes Is Linked to Lake Area
and Distance Between Lakes
The TAR is a fundamental pattern in ecology and an essen-
tial tool for biogeography studies. Most of what we know of
taxa–area curves is derived from analyses of terrestrial sys-
tems, but lakes and ponds can be considered as discrete habi-
tats with defi nable borders that are comparable in some ways
to oceanic islands (Dodson 1992 ). In Lepère et al. ( 2013 )
study, the linear relationships observed between lake area
and picoeukaryotes richness (determined by the fi ngerprint-
ing method (T-RFLP)) were signifi cant for total, dominant,
and rare T-RFs, whereas the same relationships determined
from pyrosequencing results involved mainly the dominant
species (ie OTUs). The TAR varied with the molecular
method used, as already highlighted (eg Zhou et al. 2008 ),
and therefore with the taxonomic resolution. However, all
methods suggested that the spatial distribution of dominant
picoeukaryotes follows a signifi cant TAR. Signifi cant TAR
was recently found for the richness of phytoplankton (Smith
et al. 2005 ) as well as bacteria (Reche et al. 2005 ). However,
as emphasized above, this theory seems restricted to the
dominant taxa (ie OTUs) and to a specifi c range of lake area.
Finally, the results contradict for dominant populations the
advocates of microorganism cosmopolitan distribution,
which suggests that microorganisms should be characterized
by a fl at TAR. Moreover, if the richness ( alpha -diversity)
seems to be constant for total OTUs (dominants and rares),
picoeukaryotes composition shows important changes.
Therefore, even if the pyrosequencing data highlighted that
the alpha -diversity did not vary with the habitat size, beta -
diversity was strongly associated with distance for total,
dominant, and rare OTUs. Hillebrand et al. ( 2001 ) showed
also a distance decay relationship for diatoms and ciliates,
but these authors did not consider the putative effects of envi-
ronmental parameters.19.4.3 Biogeographic Patterns of the Rare
PicoeukaryotesWhile the TAR does not work for rare OTUs ( pyrosequenc-
ing data), the composition ( beta -diversity) was linked to the
geographic distances for the rarest T-RFs and OTUs. Thus,
rare community composition varies between ecosystems but
not the richness. Similarly, Galand et al. ( 2009 ) reported that
the rare biosphere of the archaeal community followed pat-
terns similar to those of the most abundant members of the
community and has a biogeographic pattern. Based on the
assumption that rare OTUs are taxa with low abundance, we
assume that rare taxa could be more impacted by dispersal
limitation because the probability of immigrating and grow-
ing in a new ecosystem is limited compared to abundant taxa
(Green et al. 2004 ; Weisse 2008 ). The ‘rare biosphere’ has
traditionally been thought to indicate the presence of a seed
bank of potential new colonizers, according to the ‘every-
thing is everywhere’ hypothesis. However, statistical analy-
sis showed no correlation between environmental variables
and the rare T-RFs/OTUs. Galand et al. ( 2009 ) showed that
regardless of spatial or temporal scales, most of the rare phy-
lotypes are always rare within an ecosystem and the few rare
phylotypes that are sometimes detected as abundant repre-
sent traces of phylotypes that are highly abundant in some
habitats.19.4.4 What’s Happening at a Larger
Geographic Scale?To extend the discussion to a more global scale (across con-
tinent), we used sequences available on the same planktonicC. Lepère et al.