Rhizosphere microbiota during invasion processes by exotic plants in semiarid ecosystems

Résumé

The relationships established between plant and rhizosphere microbiota are essential for plant growth and survival in a given habitat. However, the role of these interactions during plant invasion processes has been limitedly studied until recently. The application of new sequencing technologies, such as those used here (Roche's 454 pyrosequencing and Illumina MiSeq) have promoted the study of rhizosphere microbial communities. These platforms provide a more complete characterization of the targeted microbial communities as well as an increase in taxonomic resolution compared to the former techniques. Proceeding from the hypothesis that the rhizosphere microbial communities may be involved in plant invasion processes, the main objectives of this thesis were 1) to examine the response of soil microbial communities to plant invasion by exotic species at different Mediterranean semiarid ecosystems; and 2) to assess the impacts of these invasive plants on soil physicochemical and microbiological properties. The development of these objectives has resulted in the following summarized studies: The occurrence of Pennisetum setaceum - in five different sites under semiarid conditions - altered the soil bacterial community structure and function related to the N cycle, in comparison to the native and neighboring plant Hyparrhenia hirta. In particular, the invasive rhizosphere supported higher rates of protease activity, which was correlated closely with the bacterial community structure of the invasive plant. This suggest that the greater diversity in the P. setaceum rhizosphere bacterial communities and the prevalence of certain bacterial groups might be related to an adaptive superiority, with respect to these disrupted and changing environments. At the same time, P. setaceum and the native H. hirta, coexisting in different invaded locations, did not differ in their interactions with arbuscular mycorrhizal fungi (AMF), hosting similar AM fungal communities in their root systems. This might be due to the fact that both plant species belong to the same functional group and family. Despite the AMF communities varied between sampling locations, the flexibility of this mycotrophic grass species P. setaceum, in relation to its association with local species of AMF, might be linked to its ability to find a suitable niche in a variety of ecosystems outside its native range. The invasion of N. glauca in semiarid Mediterranean ecosystems exposed to different edaphic constraints has produced changes in rhizosphere bacterial and fungal communities, favoring the presence of specific bacterial and fungal indicator taxa in comparison to the native neighboring plant communities. It is noteworthy that the shifted microbial community in the rhizosphere of the invasive plant in the gypsum and saline soils displayed decreased rates of nutrient cycling and organic matter decomposition - which could have a detrimental impact on ecosystem functioning, leading to increased invasion. More research is needed to confirm whether potentially allelopathic compounds released into the environment by the invasive plant are affecting the soil microbial community and to examine which edaphic factors are regulating the involvement of allelopathy in the invasion processes. The results of the study of C. edulis invasion show its ability into coastal habitats for altering soil microbial community structure and harboring a specific microbiome in its rhizosphere was widespread and strong enough across the large invaded range. In particular, soil microbial communities followed a biogeographical pattern related to soil abiotic and biotic properties such as soil available nutrients and metabolic activity of soil microbial community but also by more global parameters such as climatic and geographic variables. Meanwhile, the response of soil microbial activity and soil available nutrients and soil C and N pools to plant invasion exhibited high variation by increasing or decreasing depending on edaphic features of invaded ecosystem.