AME 41:125-130 (2005)  -  doi:10.3354/ame041125

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

Toru Hasegawa1,2,*, Hideki Fukuda1, Isao Koike1

1Ocean Research Institute, University of Tokyo, 1-15-1, Minamidai, Nakano, Tokyo 164-8639, Japan
2Present address: Seikai National Fisheries Research Institute, Fisheries Research Agency, 1551-8, Taira-machi, Nagasaki 851-2213, Japan

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.

KEY WORDS: Bacteria · Labile DOM · Phytoplankton · Heterotrophic nanoflagellates

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