126 | Vertical stratification shapes microbial community assembly in the meromictic Lake Faro water column

Erika Arcadi¹, Carmen Rizzo^2|3, Rosario Calogero¹, Marilena Sanfilippo¹, Francesco Fabiano^4|5, Vittoria Dias^2|6, Teresa Romeo^5|7|8 | ¹Stazione Zoologica Anton Dohrn, Sicily Marine Centre, Messina, Italy; ²Department of Ecosustainable Marine Biotechnology, Stazione Zoologica Anton Dohrn, Sicily Marine Centre, Messina, Italy; ³Institute of Polar Sciences, National Research Council CNR-ISP, Messina, Italy; ⁴Department of Earth and Marine Sciences, University of Palermo, Italy; ⁵National Biodiversity Future Centre NBFC, Italy; ⁶Earth and Environmental Science Department, University of Milano Bicocca, Milano, Italy; ⁷Department of Biology and Evolution of Marine Organisms, Stazione Zoologica Anton Dohrn, Sicily Marine Centre, Milazzo [ME], Italy; ⁸National Institute for Environmental Protection and Research, Milazzo [ME], Italy.

Lake Faro is a unique coastal meromictic ecosystem characterized by a permanent density stratification that prevents complete vertical mixing, resulting in a stable oxic-anoxic interface (chemocline). This study aims to characterize microbial diversity and community assembly along the water column, thanks also to the measure of the environmental parameters of the water nutrients which gave us great support to understand how steep environmental gradients shape niche partitioning. Microbial community composition was investigated using Illumina MiSeq sequencing of the 16S rRNA (using cDNA), revealing interesting insights on taxonomic and functional transitions from surface waters to the reduced bottom layers. The highlighted a pronounced vertical zonation of microbial communities, tightly coupled to sulfur and nitrogen biogeochemical cycles. The monimolimnion (0–12m) is dominated by typical marine aerobic groups, including oxygenic Cyanobacteria (mainly Synechococcus spp.), followed by Bacteroidota and Proteobacteria, which collectively sustain primary production in the nutrient-limited upper layers. Chemocline (12-15 m) and Ipolimnion (15–25 m), represent a metabolic hotspot where a shift in community composition occurs. Here, bacterial communities are dominated by Desulfobacterota, the second most abundant phylum after Bacteroidota, with Dissulfuribacteraceae, Desulfobulbaceae, Desulfuromonadaceae as most representative families, well adapted to low-light and high-sulfide conditions. Overall, the findings of this study suggests that below 15m depth a complex anaerobic food web develops, primarily structured by dissolved oxygen, sulfide concentration, and redox potential. This study provides a comprehensive baseline for understanding microbial dark matter in meromictic lagoons and highlights the chemocline as a critical ecological filter regulating carbon and sulfur cycling in coastal Mediterranean environments.