Macrophyte meadows mediate the response of the sediment microbial community to ultraviolet radiation

The decrease of the water level in Mediterranean wetlands due to global warming allows UVR to reach the sediment microbial community. Macrophyte meadows, through their structure and compounds contribution, exert influence on this community. Our goal was to establish how the sediment microbial community of a wetland is harmed by UVR, and how the macrophytes mitigate such effects. We performed a field factorial experiment (UVR and macrophytes as factors) in a Mediterranean wetland. The abundance and composition of the sediment microbial community (bacteria, Archaea, microalgae and cyanobacteria) and sediment stoichiometry in superficial and sub-superficial layers were analysed. The microbial community was altered even by the small doses of UVR of the Mediterranean coastal wetlands by decreasing the periphyton abundance, including bacteria involved in C and N metabolism. Submerged macrophytes favoured the periphytic biofilm and increased sub-superficial carbon and nitrogen amounts, by promoting bacteria involved in their cycles. The shade exerted by the meadows minimized the UVR effects, driving to a community similar to that of the UVR-filtered environment. Therefore, macrophytes, through different mechanisms, can mitigate the harmful effects of UVR in sediment communities. Thus, the conservation of macrophyte meadows in highly vulnerable wetlands becomes crucial in a global change context.