Experimental assessment of marine bacterial respiration

We present a systematic experimental assessment of the effect of pre-incubation filtration procedures on marine bacterial respiration (BR) measurements. The in vivo electron transport system (ETS) method, which enables measurements of BR using short incubation times (similar to 1 h) and without the requirement of pre-incubation size-fractionation procedures, was employed in 20 experiments from 2 different ecosystems: the NW Iberian Peninsula shelf and shelf-break, and the North Pacific Subtropical Gyre. BR was determined in both pre-incubation size-fractionated filtered (PF) and pre-incubation unfiltered (PU) treatments. Additionally, the effect of incubation time (up to 24 h) on BR, community respiration (CR), bacterial production (BP), and picoplankton community composition was assessed in 6 of the experiments; the standard oxygen consumption method (i.e. Winkler) was also applied in PF treatments. The mean contribution of BR to total CR (%BR) obtained with the in vivo ETS method in PU and short-time incubated samples was 31 +/- 4% (mean +/- SE; n = 20). PF procedures increased BR by 264 +/- 46% (n = 20). This overestimation increased with incubation time. The %BR in PF 24 h-incubated samples was >100% using either the in vivo ETS method (%BR = 109 +/- 31%, n = 6) or the Winkler method (%BR = 185 +/- 34%, n = 6). By contrast, incubation time did not significantly affect BR or CR rates in the PU experiments. Metabolic changes during extended incubations following PF coincided with a significant in crease in the proportion of very high nucleic acid content bacteria to total heterotrophic bacteria. In this study, PF combined with extended incubation times resulted in an overestimation of BR and %BR of similar to 300% and an underestimation of bacterial growth efficiency of similar to 50% compared to PU, short-incubated samples. These results may partially reconcile bacterial carbon consumption assessments and estimates of organic carbon flow in oligotrophic waters.