Effects of glutamate and glucose on N cycling and the marine plankton community

Abstract

Phosphate-enriched natural coastal seawater was incubated with the addition of glutamate and glucose, and the effects on coastal bacteria were studied, as well as subsequent effects on phytoplankton and heterotrophic nanoflagellates (HNF). Based on the changes in bacterial and HNF density, chlorophyll a concentrations and 15N dilution, the addition of glucose resulted in bacteria out-competing phytoplankton for available nitrogen. Glutamate addition resulted in the enhancement of both bacterial and phytoplankton populations. However, the increased bacterial population returned to background level rapidly; this may have been caused by tight coupling between bacterial production and grazing on bacteria. In spite of adding the same amount of organic carbon in both treatments (about 90 μmol l-1), enhancement of the net particulate organic carbon production by the entire planktonic community was considerably higher when glutamate was added. These results suggest that the biochemical basis of the bacterial substrate controls not only bacteria but also phytoplankton and HNF, through trophic interactions such as competition for nitrogen and predator-prey relationships. © Inter-Research 2005.