Microbial Diversity, Structure and Predicted Carbon-cycling Pathways Show Partial Convergence in Restored and Natural Salt Marshes in Venice Lagoon

Salt marshes are increasingly restored following widespread degradation, yet the responses of sediment microbial communities to restoration remain poorly understood, despite their central role in ecosystem functioning. We compared microbial diversity, composition, structure and predicted function between natural (N) and two types of restored salt marshes in Venice lagoon, Italy: marshes restored with tidal creeks (RC), favouring a more natural tidal exchange and creek formation at the low marsh, and marshes restored with barriers (RB), which constrain tidal flow. We analysed sediment microbial communities in the three dominant habitat types of each marsh (unvegetated low marsh, vegetated low marsh, vegetated mid marsh) by amplicon sequencing of the V4 and V5 hypervariable regions of the 16 S rRNA gene. Microbial diversity was similar across natural and restored salt marshes, but community composition and structure were different. Natural marshes were characterised by higher abundances of taxa associated with sulphate reduction and organic matter degradation (e.g. Syntrophobacterales, Bacteroidales and Desulfatiglandales), whereas restored marshes showed greater representation of Desulfobacterales, Desulfobulbales and Chromatiales. In vegetated habitats, microbial communities also differed between low- and mid-marsh elevations, with low marshes enriched in Syntrophobacterales and Desulfobulbales and mid marshes characterised by Ignavibacteriales and Defluviicoccales. Despite these taxonomic differences, predicted carbon-cycling pathways were remarkably similar across restoration types and elevations. This suggests that key microbial functions can recover even when community composition remains distinct, indicating rapid re-establishment of microbial functional potential following restoration.