Detecting food limitation of bacterial growth during dilution experiments

The dilution method is typically applied to estimate the growth and mortality rates of phytoplankton, but it is also used to study bacterioplankton. The method comprises creating a gradient of dilutions to reduce the encounter rates between bacterivores and bacteria, thus allowing for estimations of bacterial growth and grazing pressure exerted on the bacteria. However, the manipulations involved in the method can lead to biased results. In this study, 12 dilution experiments performed in the coastal zone of the Baltic Sea were accompanied by additional measurements capable of detecting possible artefacts. Only six measurements performed during spring and summer (March–August) produced results that were free of artefacts and were statistically significant. During fall and winter (October–February) measurements were unsuccessful because of food limitation of bacterial growth during experimental incubation. Twice (in September and October) bacterial growth and grazing mortality rates were underestimated because grazing pressure was not successfully removed. The study demonstrated that 24-hour and five-day oxygen consumption measurements incorporated into dilution experiments permitted estimating the fraction of biodegradable organic matter used during incubation, and, thus, detecting the food limitation of bacterial growth.