Tricarb 3100

The instruments used are described elsewhere21 (link)33 (link)34 . Briefly, for strontium recovery calculation, stable strontium (⁸⁸Sr) was analysed applying inductively coupled plasma mass spectrometer Agilent 8800 (Agilent Technologies, USA) at NIRS and ⁸⁵Sr spike solution was used at IFJ-PAN. Gamma-spectrometer ORTEC GEM-100210 (ORTEC, USA) was used mounted with high-purity germanium detector and multichannel analyser for radiocesium determination. After the separation procedure, liquid scintillation counter TriCarb-3100 and Wallac 1414-003 (Perkin Elmer, USA) was used with Ultima Gold AB scintillation cocktail for ⁹⁰Sr determination. The full analytical procedure was verified using soil reference sample produced by the International Atomic Energy Agency (IAEA 375), and tested using water samples in a world-wide open proficiency test, the IAEA-TEL-2014-03. The accuracy and precision of the applied procedure received accepted status.

The ambient bioavailable concentrations of amino acids, leucine and lysine, and microbial uptake rates of these amino acids were estimated using the radiotracer dilution bioassay^{33 (link),34 (link)}. Subsamples of 1.6 ml from the surface mixed layer (25 m) were incubated with either 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0 nmol L-[3,5-³H] leucine (103 Ci mmol⁻¹, Hartmann Analytics) or L-[3,5-³H] lysine (32 Ci mmol⁻¹, Hartmann Analytics) at RT for 15, 30, 45 and 60 min. To increase sensitivity of measurements in deep waters (>900 m) 1 litre samples were amended with ³H-leucine or ³H-lysine to final concentrations 0.01, 0.025, 0.05, 0.075, 0.1 nmol l⁻¹ and incubated at in situ temperature (Supplementary Fig. S1). Subsamples of 250 ml were withdrawn after approximately 6, 14, 22 and 30 h. Subsamples were fixed with 2% PFA at RT for 1 h. The particulate material was collected onto 0.2 μm pore size polycarbonate filters (Nucleopore, Whatman) and rinsed twice with deionised water. Filters were placed in plastic 8 ml scintillation vials. Vials were filled with 5 ml of Gold Star scintillation cocktail (Meridian Biotechnologies, Tadworth). Radioactivity retained on the filters was radio-assayed using a liquid scintillation counter (Tri-Carb 3100, Perkin Elmer). Leucine and lysine uptake rates of bacterioplankton were determined using a linear regression model (Supplementary Fig. S1).