Ea 1112 ht

DNA or protein samples from the different cultures were placed in tin capsules and were combusted at 1020 °C using an elemental analyzer EA 1112-HT (Thermo Scientific). The elemental analyzer was coupled in continuous flow mode, via the Conflo III interface, to a spectrometer of Isotopic Ratio Masses (IRMS) Delta V-Advantage (Thermo Scientific). As carrier gas, He was used at a flow rate of 95 ml/min. As ¹³C and ¹⁵N are naturally present in the samples, the basal concentration of these stable isotopes was quantified in a negative control in which S. aureus WT cells were grown in the absence of labeled DNA.

Determination of δ¹⁵N_AIR was carried out through sample combustion at ~1020 °C in an elementary analyser EA 1112-HT (Thermo, Massachusetts, USA). The generated combustion gases were purified and transferred to a Delta VAdvantage (Thermo, Massachusetts, EEUU) isotope ratio mass spectrometer (IRMS) through a Conflo III gas distribution system. International standards used were IAEA N2, IAEA 305-Be IAEA 311. JGOFS [42 ] protocols based on Dugdale and Goering [43 (link)] were used to measure ¹⁵N isotope tracer levels and estimate N uptake (pmol N cell⁻¹ h⁻¹) according to:
where t is the incubation time (hours), ¹⁵N_xs is the exceeding ¹⁵N (measured ¹⁵N minus ¹⁵N ambient level, 0.366 atom %) in the post-incubation particle sample, PN_t the particulate nitrogen content of the sample after incubation in units of nmol·L⁻¹, and ¹⁵N_enr the ¹⁵N enrichment in the dissolved fraction, estimated according to:
where ¹⁵N is the concentration of labeled N, nmol·L⁻¹, ¹⁴N the concentration (same units) of unlabeled N and ¹⁵N in the ambient level of ¹⁵N. Cell counts were used in the transformation from nmol·L⁻¹·t⁻¹ to pmol N·cell⁻¹·h⁻¹.