Biological drivers of bacterial communities in the Atlantic inflow to the Arctic Ocean revealed through manipulations of microbial food web interactions

To study microbial food web interactions grazer exclusion experiments were performed, where growth rates of different microbial groups in the absence of different grazers can be analysed. Surface water samples were gently reverse filtrated, using different mesh size filters (0.8, 3, 5, 10 and 90 µm), to retain organisms of different size fractions. After the filtration, water from every size fraction was transferred in 3.9L transparent polycarbonate bottles (Nalgene®) using silicone tubing and staggered filling. Experiment bottles were incubated at close to in situ temperatures and light conditions. During May and August incubations were placed in plexiglass tanks on deck with a continuously seawater flow through (May: 1.7 ± 1.6 °C and August: 1 ± 0.8 °C) and nylon wrapping around each bottle reduced PAR to 30% of surface irradiance. Incubations in January, March and November were placed in a dark cooling room at 2 °C, and only in March artificial light (5 µmol photons m-2 s-1) was supplied as in situ light cycle (16 h darkness and 8h light). Experiments were incubated for 5-10 days and samples for microbial abundance measurements taken daily and nutrient samples every second day. Samples for microbial community composition were taken at the start and end of the experiments in March, May, August and November.