Microcosm experiments was done with water from the oxygen minimum zone (around 300-500 m depth) of the subtropical North Atlantic (17.71 ºN, -41.05ºE) to study the response of mesopelagic prokaryotic communities to pulses of nutrients. Three different treatments and a control were established after prefiltering the seawater through 0.8 µm polycarbonate filters. Microcosms were amended either with thiosulfate (Na2S2O3), ammonium (NH4Cl) or pyruvate plus acetate (C3H3NaO3 and C2H3NaO3) and the control was not supplemented with any organic or inorganic compound. The treatments and the control were established in duplicate in 20 l carboys and held in the dark at in situ temperature for 110-140 h. At the end of the incubation the remaining water was collected through 0.22 µm polycarbonate filters for metatranscriptomic analysis.