156 deux zones (naturelle et restaurée), indiquant que la diversité et l’activité microbienne sous-jacentes des deux zones sont différentes. Cette différence est notamment liée à une distinction entre la surface et la profondeur de la colonne de tourbe de la zone restaurée, qui suggère comme à Mukhrino qu’une perte de la continuité de la colonne de tourbe joue un rôle impacte l’activité et la diversité des procaryotes.
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Do viral abundance and diversity reflect the effects of human-
derived disturbances on peatland functioning?
Introduction
Many studies that focus on peatland functioning are recurrently justified by the importance of these ecosystems as a carbon-sink, accumulating organic matter since the beginning of the Holocene and currently storing around 25-30 % of the terrestrial C-stock (Mitra et al., 2005; Frolking et al., 2011). However, this functional process, recognized as a climate regulating ecosystem service (MEA, 2005) is threatened because of the increasing pressure of human activities such as land use change or mining, on both tropical and boreal peatlands (Hooijer et al., 2010 ; .Junk et al., 2012).
To-date most societal issues about the loss of ecosystem services surround our ability to model and predict ecosystems functioning in response to global change. The large panel of environmental changes expected in response to anthropic activity spans from increased mean annual temperatures, longer summer droughts or thaws, to disrupted biogeochemical cycles and trophic networks. Monitoring of climate indicators already suggested that these changes occur faster and stronger at higher latitudes where most of Sphagnum-dominated peatlands are situated (Serreze et al., 2000 ; IPCC, 2013). Whether peatlands can maintain their carbon storage capacity under such changes is a major issue (Yergeau et al., 2010). Increased temperatures, longer snow periods and longer summers affect the seasonality of northern peat soils, and lead to strong changes in C-fluxes, plant-microbial interactions, microbial activity, subsequent decomposition processes and net ecosystem exchange (Dorrepaal et al., 2009 ; Mackelprang et al., 2011; Jassey et al., 2013). For instance, longer and more intense permafrost soil thaws are expected to increase methane effluxes by activating initially unactive micro-organisms in deeper peat layers (Tveit et al., 2013). Direct warming effects on peatland functioning also induce changes inside the ecosystem that modulate such effects. Ward et al. (2013) showed that change of vegetation with warming concurrenced the direct effect and then exerted a significant feedback on the GHG (Greenhouse-Gaz) fluxes. Under natural peatland functioning, methane produced at depth can be oxidized at the surface by methanotrophic bacteria and the released carbon dioxide sustains Sphagnum growth and production (Vile et al., 2014). In addition to the disruption of the production/decomposition imbalance and the microbial communities implicated in the carbon cycle, longer thawing periods in Siberian peatlands have allowed the discovery of new viruses (Abergel and Claverie, 2014; Legendre et al., 2015)1, the
1 While viral ecologists were enthusiastic about these new giant viruses and their incredibly large genomes, virologists
were more concerned about sanitary consequences of the possible re-emergene of presently uninfecting viruses. But that issue is not addressed here.
Chapitre IV : Abondance et diversité virale dans les tourbières face aux perturbations d’origine anthropique
158 response of these ecosystems to upcoming changes is critical in regard of the evolution of Earth climate through the potential modification of their carbon cycle.
These concerns are not limited to the future functioning of pristine peatlands as these ecosystems have also long been exploited to sustain the production of human goods. Peat has traditionally been extracted for fuel or reclaimed for agriculture and forestry purposes (Francez, 2000; Mitsch and Gosselink, 2007). It is now acknowledged that these activities have strongly modified the structural, hydrological and biological peatland functioning and properties, turning exploited sites from carbon sinks to carbon sources (Francez and Vasander 1995; Strack, 2008).
Since 30 years, as awareness rose about the trade-offs between ecosystem exploitation, ecosystem services and human-derived global changes, restoration programs have been set-up in order to restore peatland structure and functioning. Restoration of ecosystems became a major strategy to enhance both the recovery of biodiversity losses and the provision of ecosystem services (Bullock et al., 2011), even if the concept of ecosystem services is not explicitly mentioned in peatland restoration manipulations (Kimmel and Mander, 2010). Once these restoration programs set up, peatland dynamic remains to be monitored in order to evaluate the success of the restoration practices. The success of peatland restoration programs is mainly assessed using vegetation records (Poulin et al., 2013), followed later by the recovery of carbon fluxes (Waddington et al., 2001) and microbial activity (Andersen et al., 2006 and 2010; Artz et al., 2008) but how the microbial functional diversity is recovered remains yet poorly addressed. In addition, how the viral loop can interact in the recovery of microbial functions is completely unknown. Given viruses specificity for their hosts, and their dependence on their host activity, their diversity represent a tool as it could be an indicator that mirror changes in the microbial compartment diversity and activity that has been shown to be crucial for peatlands functioning.
. The aim of the present chapter was to test whether viral ecology could give clues about the way peatland microbial compartment reacts in response to (i) experimental climatic changes (T°C and hydrology) induced with Open Top Chambers devices (OTC) and water-table level manipulations, and (ii) cutover peatland restoration. The questions were addressed using viral-particles and prokaryotic counts by cytometry in relation to the physico-chemistry (environmental changes). This approach was combined to the analysis of the viral diversity in the natural and restored area of a formerly exploited peatland in Québec.
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