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The oxygen-dipping probe optode was also used for respiration toxicity tests of INT for unfiltered plankton communities, marine bacteria seawater cultures (0.8-μm filtered sea water fraction with no substrate addition), and V. harveyi cultures in the following way. Cultures were set up in a 125-ml glass bottle stoppered with a solid silicon stopper with the oxygen probe inserted and a port for INT addition. Incubations wereperformed in the dark, under agitation at 18 °C in a water bath. The cultures were allowed to respire for minutes to several hours after the oxygen equilibrium was established. Oxygen consumption was recorded continuously for less than 24 h with data acquisition periods between 30 s and 10 min. The effect of the addition of the tetrazolium salt was evaluated by quantifying the oxygen consumption rate of change after the different INT addition (0.05, 0.1, 0.25, and 0.5 mM).