Prokaryotic viability and active metabolism across a Greenland Sea transect (75°N latitude)

In this study, we provided the first comprehensive assessment of prokaryotic viability and respiratory activity across a 75°N transect in the Greenland Sea. Seawater samples collected during the CASSANDRA cruise (early September 2021, Italian Arctic Research Program PRA) were analyzed using LIVE/DEAD BacLight viability staining (L/D) and 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) methods to quantify viable and metabolically active cells, respectively. Total prokaryotic abundance ranged between 0.13 and 8.8 × 10⁵ cells mL^-1, with metabolically active (CTC+) cells accounting for 0.1-12% of the total. Viable cells accounted for 7-48% of the bacterial community, showing a significant vertical variability that increased with depth (Coefficient of variability 44%), particularly in deeper, nutrient-rich water masses such as the Greenland Sea Deep Water and the Greenland Sea Arctic Intermediate Water, occupying the deep layer (below 2500 m depth) and the intermediate layer (500-2500 m depth), respectively. Significant correlations were found between microbial parameters and environmental variables associated with different water masses, notably nutrients (nitrates and phosphates), whereas temperature showed a more complex, indirect influence. These findings highlight that the prokaryotic community inhabiting the examined transect is well adapted to this extreme marine environment, emphasizing the complex interactions of multiple environmental factors in shaping microbial community structure and activity under low-temperature conditions.