Integrating morphological, physiological, and molecular approaches to monitor photosynthetic biodiversity of the Venice Lagoon.

The Venice Lagoon is the largest transitional environment within the Mediterranean Sea (almost 550 km2). Being a transitional environment, it contains many different ecological niches, hence the biodiversity in this environment is very high. At the same time, this lagoon system is under constant pressure from global climate change and human activities, and it has been greatly modified by humans during the centuries. After a bibliographic revision of past records of photosynthetic species reported in this environment between the 1800s and 2023, we conducted 3 monitoring campaigns from 2023 to 2025. All collected samples were classified using an integrated morphological and molecular approach. Based on morphological observation, a specific gene marker was selected for species identification through DNA barcoding. Reliable sequences were obtained for 50% of the samples and aligned with reference databases. To further increase identification reliability, a phylogenetic tree for each major taxon was reconstructed to evaluate clustering patterns and sequence relationships, and two independent species delimitation approaches (ASAP and GMYC) were applied. In total, we could identify 73 species (13 Chlorophyta, 11 Heterokontophyta, 25 Rhodophyta, 24 Tracheophyta). Additionally, we performed a monitoring campaign focused on Sporobolus species, relevant ecosystem-engineers in the Venice Lagoon. We confirmed the presence of S. x townsendii in the Southern and Central lagoon and observed a higher presence of the NIS S. anglicus compared to the native S. maritimus. This pattern can be influenced by extreme climatic events, which will likely increase in the future. Thus, we exposed S. maritimus and S. anglicus organisms to a simulated 5-day heatwave in laboratory-controlled conditions. To clarify their response, morphological effects were evaluated, and physiological and biochemical responses were analysed. Moreover, we combined these information with gene expression analysis and metabolites variation. To identify differentially expressed genes, we additionally annotated a de novo transcriptome for Sporobolus species. We observed a decline in the survival rate of S. maritimus after the induced heat stress, while the NIS did not show an increase in mortality. Accordingly, physiological and biochemical assays indicated that the native species is more susceptible to a heatwave and could not recover from the stress. On the other hand, the NIS tolerated the stress better and was able to recover from it. Gene expression analysis suggest that S. maritimus’ response is slower compared to S. anglicus’, and that the induced effects are more pronounced in the native species compared to the NIS. Overall, the negative effects were more striking and deadly in S. maritimus, the native species, rising concerns for its survival under future environmental scenarios.