Integrating automated cytometry and microbial surveillance in a public marine aquarium: Insights from a one-year study in Monaco

This study presents a one-year microbiological monitoring campaign conducted at the Oceanographic Institute of Monaco to evaluate microbial dynamics within a large public marine aquarium. The approach combined automated near-real-time flow cytometry, culture-based analyses, species identification, and antibiotic susceptibility testing. Continuous bacterial enumeration using the BactoSense® cytometer enabled high-resolution tracking of intact bacterial counts (ICC) across five hydraulic sectors, from seawater intake to exhibition tanks. The results revealed pronounced spatial gradients, with mean ICC values decreasing from 1.4 × 10⁵ cells mL⁻¹ at intake to below 1.3 × 10⁴ cells mL⁻¹ in internal tanks, confirming the high efficiency of UV-C disinfection and filtration systems. Seasonal analysis showed clear fluctuations at the intake, with peaks in spring and early summer associated with increased biological activity in the surrounding Mediterranean environment. Sector-specific variations indicated that hydraulic configuration and residence time influenced microbial stabilization, with localized differences in UV efficiency detected through near-real-time monitoring. Complementary bacterial identification revealed the presence of several opportunistic taxa, including V. fortis , V. harveyi , and Shewanella spp., typical of recirculating seawater systems. Antibiotic susceptibility testing revealed heterogeneous resistance profiles, with β-lactam resistance observed in several Vibrio isolates and multidrug resistance detected in S. algae and V. jasicida . The integration of biosensing, microbial identification, and resistance profiling provides an operational framework for evidence-based microbial surveillance in marine aquaria. These findings highlight the potential of automated near-real-time cytometry to enhance preventive management, support targeted interventions, and ensure microbiological stability in closed aquatic ecosystems. • First long-term marine use of the BactoSense® automated near-real-time flow cytometry system. • One-year monitoring revealed spatial and seasonal bacterial dynamics in a public aquarium. • UV-C disinfection and filtration reduced bacterial loads by more than one order of magnitude. • Hydraulic design and residence time shaped microbial stability across aquarium sectors. • Detection of opportunistic and resistant bacteria supports evidence-based aquarium management.