The biofilm ecology of proglacial floodplain streams

Glacial forelands figure among the most dynamic landscapes on Earth, and their formation is currently accelerating given glacier shrinkage. Draining these forelands are streams hosting unique microbial communities, which have the capacity to impact both the biogeochemistry and diversity of downstream ecosystems. Yet, this resident microbial life, and its impacts on the structure and functioning of glacier-fed streams, remains poorly studied despite the role of benthic biofilms in fulfilling critical functions. Thus, the main objective of this thesis was to assess how these benthic biofilms are assembled, and how these communities may change due to climate-related modifications in the proglacial stream environment. We quantified the effect of stream physico-chemical properties on benthic microbial communities along longitudinal and lateral chronosequences and anticipated the possible consequences of glacier retreat on proglacial stream biodiversity. First, we examined three glacial forelands in Switzerland and used high-resolution sampling to assess fine-scale benthic biofilm diversity patterns (a, ß diversity). We identified the dominant prokaryotic taxa in proglacial streams and investigated the role of phototrophic eukaryotes in structuring the biofilm community. We found that along the lateral chronosequence, benthic biofilms in non-glacial streams develop higher biomass and greater diversity, and that their community were distinct than those in glacial streams. Our results also suggest that photoautotrophic communities shape bacterial communities, presumably because algae act as the major source of organic matter in proglacial streams. Second, we used an analytical framework to assess the importance of stochastic versus deterministic processes governing bacterial community assembly processes. We found that extreme environmental conditions in proglacial streams led to homogenizing selection of biofilm-forming microorganisms, but environmental differences between proglacial streams seemed to impose different selective forces, resulting in nested-structure assembly processes. To elucidate the response of proglacial stream biofilms to climate-change induced stressors, we mixed streamwater from a glacial and non-glacial stream in streamside mesocosms and tracked changes in biomass and beta-diversity of bacterial and algal community structure. We observed that as the proportion of glacier-melt water decreased, bacterial abundance and algal biomass increased, further reinforcing the expected "greening" of proglacial habitats. We observed a rapid shift in prokaryotic and phototrophs community turnover immediately following the transition from incubation to the treatment phase, indicating that both bacteria and eukaryotic phototrophs rearranged with new conditions. Overall, we found that some bacteria are well adapted to the glacier-fed environment, and with decreases in the proportion of glacier meltwater, these communities are likely to transition to greener, more heterotrophic systems with fewer selected taxa. The reduction in glacier runoff will probably reduce environmental filtering allowing thus more generalist species not adapted to the harshness of glacial meltwater to colonize and establish throughout the entire floodplain. Therefore, glacier retreat may provoke a decrease in the abundance of specialized species adapted to glacier runoff and to a taxonomic homogenization of the aquatic microbial diversity within the floodplain.