Morphological traits of prokaryoplankton across an Atlantic subarctic marine transect

The aim of this study was to advance the understanding of prokaryotic biomass and phenotypic traits in marine environments. Water samples were collected along one transect at a subarctic Atlantic spatial scale, including the eastern Fram Strait and Greenland. We estimated such features along the water column (maximum depth of 3500 m) using image analysis. The main physical and chemical parameters (temperature, salinity, pH, dissolved oxygen and nutrients) were also measured. Prokaryotic abundance and biomass ranged from 104 to 106 cells mL−1 and from 14 to 39.5 µg C L−1, respectively. Picophytoplankton abundance and biomass determined by flow cytometry, were dominated by unicellular cyanobacteria within the genus Synechococcus, in the order of 101–104 cells mL−1 and 0.01–4.7 µg C L−1, respectively. A wide range in prokaryotic cell volumes was observed (between 0.0042 and 1.063 µm3). Increasing in cell size with depth was opposed to cell abundance and biomass distribution. Six cellular morphotypes were observed, among which cocci, coccobacilli, and curved rods were the most abundant. Although cocci prevailed numerically, they always showed the smallest volumes. Significant differences were observed between epi–meso- and bathypelagic layers and analyzed parameters, particularly prokaryotic abundance and biomass, NO3 and PO4. Our study, utilizing phenotypic fingerprinting approaches, has identified significant changes in prokaryoplankton structure influenced by the environmental gradient. Given the rapid response of microbial assemblages to natural and anthropogenic disturbances, they stand as particularly useful bioindicators of environmental changes in peculiar ecosystems as the Arctic.