Discrete water samples were collected using a rosette sampler equipped with 24×10 L Niskin bottles. Different depths were sampled ranging from 55 meters to 1500 meters. The sampling strategy specifically focused on targeting low oxygen zones within the water column. Water was dispensed into 2 mL microcentrifuge tubes in the laboratory. Microbial heterotrophic production was measured via the incorporation of [3H]-leucine into microbial biomass using a modified version of the microcentrifuge method (Baetge et. al., 2021). [3H]-leucine (specific activity 44.9 Ci mmol-1; Perkin Elmer) was added to 1.7 mL of sample at a final concentration of 20 nM and incubated for 2-3 h at in situ temperature. For each depth and station, triplicate live incubations and one killed control, to which 100 µL of trichloroacetic acid (TCA) was added immediately, were carried out. At one station, the effect of phenylacetylene and DMSO additions on heterotrophic production activities was tested over a time-span of 24h as described in detail in Bayer et al. 2024.

Incubations were terminated by addition of cold 100 µL 100% TCA and stored at 4°C until extraction following established procedures (Baetge et. al., 2021). DPM were measured on a scintillation counter (Perkin-Elmer Tri-Carb 2910 TR) for 2 min, and [3H]-leucine incorporation rates were converted to units of carbon using a conversion factor of 1.5 kg C (mol leucine incorporated)-1 (Simon and Azam 1989).

948411_v1_heterotrophic_production_rates.csv