Ht flash

Stable isotope composition of organic carbon (δ¹³C) was measured on the bulk soil samples using isotope-ratio mass spectrometry (IRMS), following the USGS method⁷⁶ . Briefly, all samples were homogenized by grinding with a mortar and pestle. They were then decarbonated (soils) with HCl (3 N) and, after 24 h of equilibration, they were adjusted to neutral pH with ultrapure water. The samples were dried in an oven (50 °C) until constant weight. Ratios of δ¹³C and δ¹⁵N were measured in a MAT 253 IRMS (Thermo Fisher Scientific, Waltham, Massachusetts, USA) and reported in the standard per mil notation (‰). Three certified standards were used (USGS41, IAEA-600 and USGS40) with an analytical precision of 0.1 ‰. The content total organic carbon (TOC %) was measured with an elemental analyzer (HT Flash, Thermo Fisher Scientific, Waltham Massachusetts, USA) during measurement of the stable isotopes.

The stable carbon isotopic composition (δ¹³ C) of the biomass was measured on the bulk biofilm samples with isotope-ratio mass spectrometry (IRMS), following USGS methods (Révész et al., 2012 ). Briefly, subsamples of the freeze-dried biofilms (300 mg) were homogenized by grinding with a corundum mortar and pestle (except for sample CE4gr that had not enough material for all analyses). Subsequently, HCl was added to remove carbonates, and we let it equilibrate for 24 h, adjusting, then, the pH to neutral values with ultrapure water. Afterward, the residue was dried in an oven (50°C) for 72 h or until constant weight, and then it was analyzed by IRMS (MAT 253, Thermo Fisher Scientific, Waltham, MA, United States). The δ¹³C values were reported in the standard per mil notation using three certified standards (USGS41, IAEA-600, and USGS40), with an analytical precision of 0.1‰. The content of total organic carbon (TOC%) was measured with an elemental analyzer (HT Flash, Thermo Fisher Scientific, Waltham, MA, United States) during the stable isotope measurements.

Temperature (in situ), pH, and conductivity of the porewater of the sediment samples were measured with an Oakton waterproof pH/CON 450 Meter(Oakton Instruments, Vernon Hills, IL, United States). Inorganic anions and organic acids of low molecular weight were determined by ion chromatography (IC), according to the procedure previously described elsewhere (Parro et al., 2011 (link)). In short, 1 g of the sediment samples were sonicated (3 min × 1 min cycles), diluted in 10 mL of deionized water, filtered (22 μm GFF filter), the supernatants were analyzed in a Metrohm 861 Advanced compact ion chromatograph (Metrohm AG, Herisau, Switzerland) with a Metrosep A sup 7-250 column and 3.6 mM sodium carbonate (NaCO₃) as eluent.
The content of total nitrogen (TN%) and total organic carbon (TOC%) was measured with an elemental analyzer (HT Flash, Thermo Fisher Scientific, Waltham, MA, United States) as described previously (Carrizo et al., 2019 (link)).