Prior exposure of microbial communities to seawater reduces resilience but increases compositional and functional resistance to flooding events

Storm surges, flooding, and the encroachment of seawater onto agricultural land are predicted to increase with climate change. These flooding events fundamentally alter many soil properties and have knock-on effects on the microbial community composition and its functioning. The hypotheses tested in this study were (1) that the extent of change (resistance) of microbial community functioning and structure during seawater flooding is a factor of pre-adaptation to the stress, and (2) if structure and function are altered, the pre-adaptation will result in communities returning to previous state prior to flooding (resilience) faster than unexposed communities. We chose a naturally occurring saltmarsh-terrestrial pasture gradient from which three elevations were selected to create mesocosms: Low and High elevation saltmarsh and agricultural pastureland. By selecting these sites, we were able to incorporate the legacy of differing levels of seawater ingress and exposure. Mesocosms were submerged in seawater for 0, 1, 96 and 192 hours, with half of the mesocosms sacrificed immediately after flooding, and the other half taken after a 14 day recovery period. Our results indicated that seawater inundation, for any length of time, significantly alters the physicochemical properties of the soils, although a greater change is observed in the pasture site compared to the saltmarsh sites, and these remained changed despite a recovery period. Interestingly, our results indicated that for community composition, there was a high degree of resistance for the Saltmarsh mesocosms, with the Pasture mesocosm displaying higher resilience. Further, we observed a functional shift in the enzyme activities with labile substrates being preferentially utilised over recalcitrant ones, with the effect increasing with longer floods. These results suggest that changing bacterial physiology is more critical to understanding the impact of storm surges on agricultural systems than bulk community change.