Microbial decomposition of phyto- and zooplankton in seawater II. Changes in the bacterial community

Fluctuation of bacterial communities during decomposition of the diatom Skeletonema costaturn (Grev.)Cleve, and the copepod Tigriopus japonicus Mori was investigated under laboratory conditions.A 5.0Nuclepore filter was employed to separate the 'attached' community from a 'freeliving' one.During phytoplankton decomposition, the attached community changed as follows: Pseudomonas-Alcaligenes (PS) group + Acinetobacter-MoraxelJa (Ac) group -t Chromogenic (C) group + PS' (PS group with low growth rate).Dur~ng zooplankton decomposition, on the other hand, the Vibrio (V) group of bacteria predominated at first and the change was as follows: V + PS -t Ac -+ C + PS'.During the later stages of phytoplankton decomposition, fresh plankton cell material was added again to the system.The results showed that the PS fraction of bacteria appeared again as the dominant community and that a similar succession could be traced in the bacterial community.In addition, high per-cell specific activity (maximum assimilation velocity/total cell count number) of attached bacteria, observed during the earlier stages of decomposition, gradually decreased to as low as that of free-living bacteria as the decomposition proceeded.These results suggest that during the process of decomposltion of particulate organic matter the bacterial community involved in the degradation shows a successive change not only in generic composition but also in terms of the heterotrophic activity of the bacteria in the community.