34120.8 Conclusions and Perspectives
Chytrid and their trophic modes, primarily parasitism, are
ubiquitous in aquatic ecosystems, including marine habitats
(Gleason et al. 2011 ). Pioneering ecological and molecular
research in Lakes Pavin and Aydat have helped demonstrat-
ing that chytrid parasites of phytoplankton are extremely
diversifi ed in freshwater habitats, with different taxa charac-
terized by different biological characteristics and require-
ments that determine their distributions in response to
environmental parameters, but primarily to the seasonal
dynamics of their hosts. Host abundance, size and biomass
establish the threshold for the critical prevalence of infection
and the related decline in host communities. Associated eco-
logical implications are considerable, because chytrid para-
sites can kill their hosts, release substrates for microbial
processes, and provide nutrient-rich particles as zoospores
and short fragments of fi lamentous cyanobacterial hosts for
the grazer food chain. This implies that cyanobacterial
blooms, and other large-size inedible phytoplankton blooms
as well, may not necessarily represent trophic bottlenecks.
Based on the observation that phytoplankton fungal parasit-
ism preferentially impacts larger species (i.e. characteristics
of climax populations), chytrid epidemics represent an
important driving factor in phytoplankton successions and
maturation, in addition to seasonal forcing. The activity of
chytrid parasites of phytoplankton thus represents an impor-
tant but yet overlooked ecological driving force in aquatic
food web dynamics. In addition to their capabilities to resist
adverse conditions and use different sources of carbon and
nutrients, chytrid parasites can affect the plankton food web
functions, ecosystem properties and topology, such as stabil-
ity and trophic transfer effi ciency. We are perhaps approach-
ing paradigm-shift in the development of aquatic microbial
ecology.
However, studies on phytoplankton chytridiomycosis
remain restricted to a few temperate lakes, and extensive
studies in the world’s aquatic ecosystems, at wider geo-
graphical and time scales, are needed. Besides, the identifi -
cation of chytrid species based on phenotypic features
requires time and experience, and the chytrid diversity cur-
rently described is probably underestimated. In this context,
the increasing development of molecular tools is instrumen-
tal and has already been improving linkage between cell
identity and function, which is critical for the consideration
of microbial parasites in the pelagic food webs in terms of
carbon fl ows, and the impact on biogeochemical cycling in
aquatic ecosystems. Furthermore, the parasitic lifestyle is
generally highly subtle and can, for example, control compe-
tition by dominant species for resources, thereby promoting
species coexistence and diversity. Parasites can also form
long-lived associations with hosts, reducing their survival, or
allowing infected hosts to remain strong competitors,
although few models exist for microbial fungus-host
pairings.Acknowledgements This study includes results from PhD and postdoc
researches supported by various instances: Région Auvergne, CNRS,
Université Blaise Pascal, French Ministère de la Recherche et de la
Technologie, and the Ministry of Culture, High School and Research of
Grand-Duché de Luxembourg. Thanks are owed to my colleagues, par-
ticularly G Boutheina, FH Gleason, and M Kagami for fruitful discus-
sions and inspiring collaborative papers. We thank Prof. Felix Bärlocher,
Mt Allison University, Sackville NB, Canada, for critical and proof-
reading of the manuscript. This is a contribution to the French ANR
Programme Blanc ROME – Rare and overlooked microbial diversity -
Coordinator TSN, PIs: U Christaki, C Gachon, S Monchy, N Niquil,
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