Trait-based analysis of subpolar North Atlantic phytoplankton and plastidic ciliate communities using automated flow cytometer

Plankton are an extremely diverse and polyphyletic group, exhibiting a large range in morphological and physiological traits. Here, we apply automated optical techniques, provided by the pulse-shape recording automated flow cytometer - CytoSense - to investigate trait variability of phytoplankton and plastidic ciliates in Arctic and Atlantic waters of the subpolar North Atlantic. We used the bio-optical descriptors derived from the CytoSense (light scattering (forward and sideward) and fluorescence (red, yellow and orange from chlorophyll a, degraded pigments and phycobiliproteins, respectively)) and translated them into functional traits to demonstrate ecological trait variability along an environmental gradient. Cell size was the master trait varying in this study, with large micro-plankton (> 20 µm in cell diameter), including diatoms as single cells and chains, as well as plastidic ciliates found in Arctic waters, whilst small-sized plankton groups, such as the pico-eukaryotes (<2 µm) and the cyanobacteria Synechococcus were dominant in Atlantic waters. Morphological traits, such as chain/colony formation and structural complexity (i.e. cellular processes, setae and internal vacuoles) appear to favour buoyancy in highly illuminated and stratified Arctic waters. In Atlantic waters, small cell size and spherical cell shape, in addition to photo-physiological traits, such as high internal pigmentation, offer chromatic adaptation for survival in the low nutrient and dynamic mixing waters of the Atlantic Ocean. The use of automated techniques that quantify ecological traits holds exciting new opportunities to unravel linkages between the structure and function of plankton communities and marine ecosystems.