Environmental sample collection
Sinking particles including eukaryotic microbial cells were sampled by modified automatic Kiel sediment traps with a sampling area of 0.5 m2 and coupled with 20 liquid-tight collector cups (Zeitzschel, Diekmann, & Uhlman, 1978; Kremling, Lentz, Zeitschel, Schulz-Bull, & Duinker, 1996). Here, we present results from the shallowest (~200-300 m below sea surface) sediment traps at the central station (HG-IV) of the LTER observatory HAUSGARTEN” at ~79° N, 4° E (water depth 2,550 m) (Figure 1). The trap samples analyzed in this study were collected during phases of maximum POC flux in spring and autumn. An electronic failure in the sediment trap resulted in data gap of data in 2003. The collector cups were filled with filtered sterile North Sea water. Salinity was adjusted with NaCl to 40 psu. The liquid in the collector cups (250 ml or 400 ml, depending on the sediment trap used) was spiked with mercury chloride (0.14% final concentration). After recovery of the moorings, about a year later after deployment, the trap samples were refrigerated until further processing in the laboratory. Samples were split by a wet splitting procedure after manual removal of zooplankton (swimmers) > 0.5 mm under a dissecting microscope at a magnification of 20 and 50. Subsequent molecular analyses were based on 1/32 splits of the original sediment trap sample. We collected aliquot samples for DNA extraction by filtration of a split fraction from the original sample onto a 0.2 µm Isopore GTTP membrane filter (Millipore, Schwalbach, Germany). Filters were washed with sterile North Sea water (~50 ml) to remove residual mercury chloride from the samples. The sterile sea water was applied and pumped over the filter while it was still kept in the filtration unit. Detailed information on the sediment trap collection, preservation, and sample preparation for DNA isolation have been reported previously (Metfies et al., 2017), while physico-chemical regimes during the sampling period have been reported elsewhere (Bauerfeind et al., 2009; Bauerfeind et al., 2015; Lalande, Bauerfeind, Nöthig, & Beszczynska-Möller, 2013).