Oxygen consumption during in vitro dark bottle incubations is the most common method to estimate plank- tonic respiration. This method is time consuming and labor instensive and, consequently, the database of planktonic respiration rates is scarce. Electron Transport System (ETS) activity measurement has gained accept- ance as a routine technique to estimate respiration due to its high sensitivity. However, the in vitro ETS assay commonly used yields potential rates. Hence, the empirically determined ratio between in situ respiration and potential ETS activity is not constant, varying over two orders of magnitude, highly limiting the routine appli- cation of this technique to estimate microbial respiration. We hypothesized that in vivo ETS activity should be a reliable estimator of in situ respiration in marine plankton communities. The aim of this study was to devel- op a methodology to estimate in vivo ETS activity rates by using tetrazolium salt 2-para (iodo-phenyl)-3(nitro- phenyl)-5(phenyl)tetrazolium chloride (INT) as electron acceptor. We established a procedure to apply the in vivo INT reduction method to natural marine microplankton samples. Optimum incubation times were found to be lower than 2-6 h, even for oligotrophic waters. The method was tested in natural waters, covering a very wide range of respiration rates and trophic conditions. A significant linear relationship was found between in situ respiration and in vivo INT reduction. Moreover, the ratio between oxygen consumption and INT reduc- tion was constant across contrasting environments, which reveals the INT reduction method as a simple, quick, sensitive, and robust technique suitable to substantially improve the microbial plankton respiration database.
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