Release of Alcaligenes eutrophus JMP134 and/or nutrients into seawater mesocosms: Fate of the introduced cells and effects on the ecosystem

In this study, inocula of the bacterium Alcaligenes eutrophus JMP134 including nutrients, or of nutrients alone, were released into seawater mesocosms every third day during a 12-day period. The purpose was to compare the effects of the two types of inocula on important ecosystem parameters. A. eutrophus was not able to grow in the mesocosms, and the introduced cells were mainly removed from the water column by grazing of heterotrophic nanoflagellates. The two types of inocula led to comparable changes in the concentrations of ammonium, nitrate, dissolved organic carbon and chlorophyll a in the mesocosms. Also, the effects on the abundance and production of the natural bacterioplankton were comparable: During the initial 6 days of the experiment, the inoculations stimulated the production of biomass, while a high grazing impact from heterotrophic nanoflagellates in combination with phosphorus limitation seemed to limit the bacterioplankton population in the latter part of the experiment. Taken together, these results indicate that the effects of the released bacterial cells per se were negliglible, whereas the nutrients accompanying the cells cause more concern as they may alter the structure and function of the ecosystem.