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 i ncluding 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 t he introduced cells were mainly removed from the water column by grazi ng of heterotrophic nanoflagellates. The two types of inocula led to c omparable changes in the concentrations of ammonium, nitrate, dissolve d 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 inoculati ons stimulated the production of biomass, while a high grazing impact from heterotrophic nanoflagellates in combination with phosphorus limi tation seemed to limit the bacterioplankton population in the latter p art of the experiment. Taken together, these results indicate that the effects of the released bacterial cells per se were negliglible, wher eas the nutrients accompanying the cells cause more concern as they ma y alter the structure and function of the ecosystem. (C) 1997 Elsevier Science Ltd.