Effects of submarine groundwater discharge on coastal bacterioplankton communities from the NW Mediterranean.

Supervisor: Clara Ruiz-González (Instituto de Cièncias del Mar (ICM-CSIC).
Bacteria play major roles in biogeochemical cycles across all kinds of terrestrial and aquatic ecosystems. Although there has been plenty of research on the ecology and diversity of bacterial communities from surface terrestrial and aquatic ecosystems, we know very little about bacteria inhabiting subsurface environments like groundwater aquifers. In particular, coastal groundwater aquifers are gaining increasing attention because submarine groundwater discharge (SGD) has recently been shown to be extremely important for coastal ecosystems, delivering large amounts of nutrients. Due to our limited knowledge of groundwater microbial diversity, however, whether these coastal aquifers also represent reservoirs of bacterial diversity for coastal bacterial communities remains completely unknown. Here, we first aimed at characterizing the taxonomic composition and activity of bacterial communities in various coastal groundwater aquifers and their adjacent marine coastal sites along the NW Mediterranean coast, where SGD has been shown to be much more important than previously believed. Second, we experimentally explored the responses of marine bacterial communities to groundwater additions, as well as the capacity of groundwater taxa to grow under marine conditions. Using catalyzed reporter depositionfluorescence in situ hybridization (CARD-FISH) coupled with bioorthogonal non-canonical aminoacid tagging (BONCAT), we could quantify the abundances of major bacterial groups and their single-cell activity across aquifers, or in response to the experimental mixing of seawater and groundwater. Our results show that Mediterranean groundwater aquifers show much lower bacterial abundances and activity than surface sea- or river water, and that are much more heterogeneous in terms of taxonomic composition at the spatial scale, probably due to higher isolation and more variable physico-chemical conditions between aquifers. Interestingly, our mixing experiment showed fast but transient increases in activity of marine opportunistic bacterial groups after groundwater addition. Moreover, despite the low in situ abundance and activity of groundwater bacteria, we observed a large potential of some groundwater bacterial groups such as Gammaproteobacteria, Roseobacter and Bacteroidetes to grow when exposed to marine conditions. This work represents the first attempt to describe spatial variation in bacterioplankton communities in the Mediterranean aquifers. Moreover, our results provide evidence that groundwater discharge to the ocean could also transport some viable bacterial taxa able to grow in the ocean. All this will enlarge our knowledge on the influence of SGD on marine bacterioplankton communities, as well as on the bacterial potential to persist under unfavourable conditions and grow in different environments.