Vascular macrophytes versus charophytes: how the macrophyte type and warming affect the sediment microbial community and the production of greenhouse gases

In the global warming context, adaptive management strategies involve the conservation and restoration of inland waters by planning the reintroduction of macrophytes to substantially mitigate greenhouse gas emissions (GHG). The selection of appropriate species is crucial. We studied the influence of two submerged macrophyte species on GHG production, sediment-oxygen microprofiles, and microbial community composition under two thermal regimes by means of a microcosm experiment. We chose the phanerogam Myriophyllum spicatum, more typical of meso-eutrophic habitats, and the charophyte Chara hispida, more frequent in oligotrophic waters and belonging to a much less studied macrophyte group, which, moreover, exhibit contrasting functional traits (i.e., roots versus rhizoids). The presence of both macrophyte types considerably reduced the diurnal diffusive CO₂ emissions due to their photosynthetic activity compared to bare-sediment situations. The radial oxygen loss of M. spicatum roots provided aerobic-microaerobic conditions beyond the first sediment centimeter, in contrast to more anoxic sediments created by C. hispida. As a result of this, each type of macrophyte determined a particular sediment microbiome. Despite dissolved CH₄ being greater in the presence of plants, no statistical differences in CH₄ diffusive flux caused by the presence or absence of plants was found. We demonstrated how the presence, or not, of macrophytes is more important than warming (in our case with a temperature difference of 3 °C) in relation to GHG emissions. Both species, being perennial, provide ecosystem services year-round and are strong candidates for management strategies aimed at transforming new or restored aquatic ecosystems into carbon sinks.