A bacterium 5N-3 possessing a remarkable inhibitory effect on the growth of Gymnodinium nagasakiense was isolated from Uranouchi Inlet, Kochi. This bacterium was tentatively identified as Flavobacterium sp. The growth inhibiting effect of 5N-3 on G. nagasakiense was drastic in particular when the alga was in the logarithmic growth phase, and cell density decreased to less than 1% of the initial concentration within 4 days after inoculating 5N-3, indicating that the effect was algicidal.The effect was obtained when the density of the bacterium was more than 106 cells/ml. However, they grew very rapidly up to 108 cells/ml by using extracellular released organic carbon from various phytoplankton species. On the other hand, the algicidal effect of 5N-3 was only observed on G. nagasakiense but not on Chattonella antiqua, Heterosigma akashiwo, or Skeletonema costatum.These results indicate that the effect of 5N-3 was G. nagasakiense-specific and suggest that it grows to a level of cell density effective in inhibiting the alga in the field by using naturally occurring organic carbon from phytoplankton.
To test the hypothesis that different phytoplankton cellular components undergo distinct decomposition processes, we examined bacterial decomposition of extracted dissolved organic matter (EX-DOM) and cell debris from the diatom (Skeletonema costatum) using long-term incubation experiments. The decay rates indicated that EX-DOM is extremely bioreactive (1.2-1.7 d -1 ), and that particulate organic carbon (POC) from cell debris is resistant to rapid microbial degradation (0.047-0.055 d -1 ). In the experiment with grazers, the percentage of accumulated dissolved organic carbon (DOC) in the EX-DOM and cell debris experiments was 5% and 64%, respectively, of the initial DOC concentrations derived from phytoplankton cells. The initial C/N ratio of DOM in the cell debris experiments was 19.2-19.7, higher than those in the EX-DOM experiments (5.5-6.0). The percentage of accumulated DOC in the cell debris experiment with grazers was 35% at day 13. Thereafter DOC accumulated until it reached 64%. Our data indicate that these two distinct phytoplankton-derived organic materials (EX-DOM and cell debris) decay and accumulate differently. These results suggest that structural components of phytoplankton cells may persist as semi-labile POC, and that DOC produced by solubilization of structural components of cell and phytoplankton-derived DOM with high C/N ratios may accumulate as semilabile DOC in seawater.
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