Analysis of 13C abundance
Around 1 mg of ground tissue material (same as used for the NSC
analysis) was weighed into tin capsules. Organic carbon was converted to
CO2 in an elemental analyzer Euro EA3000 (Hekatech GmbH,
Wegberg, Germany) connected to an isotope ratio mass spectrometer (IRMS;
Delta V Advantage, Thermo Fisher Scientific, Bremen, Germany) to
determine the total carbon and carbon isotopic composition. Laboratory
standards with known ð13C values were measured with a
precision of 0.1‰. The isotopic ratios in all samples were expressed in
δ notation (‰) relative to the international standard Vienna Pee Dee
Belemnite (VPDB). The carbon isotope ratio was corrected to account for
pre-labeling isotope ratios of bulk material to indicate the extent of13C-label incorporation in different tissues.
Δδ13C = δ13CL –
δ13CNA Eqn. 1
where δ13CL is the isotope ratio after
the start of the labeling and δ13CNAis the natural (pre-labeling) isotope abundance.