Quantification of microbial cells and virus-like particles in sediments from Kermadec Trench

Sediment cores from sites K3, K4, K5, and K7 were recovered by subsampling a box corer (50 × 50 cm), while sediment cores from sites K4 and K6 were collected using an autonomous lander system. Microbes and virus-like particles were extracted from sediments in a 3°C room using a modified version of the washing protocol of Danovaro and Middelboe, 2010 (see Schauberger et al., 2021). After the washing procedure, the extracted microbial cells and virus-like particles were fixed with 25% glutaraldehyde (1% final concentration) and stored at −80°C prior to flow cytometry. These samples were measured in triplicates using a BD FACSCanto™ II flow cytometer, after staining with SYBR Green I. Sediment extracts were diluted 1 : 10 in 0.02 μm-filtered TE Buffer prior to all measurements. The flow rate was 5–7 μl/min, as determined by BD Trucount™ Beads. The laser settings and gating examples can be found in the Supporting Information of Schauberger et al. (2021).