Tri carb 2900 tr liquid scintillation analyzer

Heterotrophic carbon production was estimated by incorporation of [³H]-leucine (Kirchman et al., 1985 (link)). At each sampling time, duplicate 1.7-mL aliquots and one ‘killed’ control (by addition of 90 μL 100% trichloroacetic acid) were collected from each microcosm, and 20 nM of the radiotracer (specific activity, 52.9 Ci mmol^-1) was added, followed by incubation at 16.8–17.5°C in the dark. These incubations were stopped after 1 h by the addition of 5% trichloroacetic acid (final concentration). Extraction with 5% trichloroacetic acid and 80% ethanol was then carried out using the microcentrifugation method (Smith and Azam, 1992 ). The radioactivity of the [³H]-leucine incorporated into the samples was determined using a β-scintillation counter (Tri-Carb 2900 TR Liquid Scintillation Analyzer, Packard) after the addition of 1 mL scintillation cocktail (Ultima Gold MV; Packard). This incorporation of [³H]-leucine was converted into carbon produced via prokaryotic protein production, according to Simon and Azam (1989) (link), assuming twofold isotope dilution for leucine. The mean coefficient of variation among the replicates was 3%.