The microbial loop has been shown to play an important role in marine and freshwater plankton ecosystems in all climatic zones. Earlier research in two contrasting habitats: an Antarctic coastal/ shelf/oceanic ecosystem and a subtropical oceanic ecosystem, have shown significant differences in the ecological role of the microbial loop and the physiology of bacterioplankton. These two habitats are isolated from input of terrestrially derived organic matter and so provide excellent opportunities to study bacterial metabolism of autochthonous organic matter of phytoplankton origin. Strong contrasts in plankton community parameters make these two sites ideal for this comparison. This study will be embedded within on-going programs at each of these two sites (US-JGOFS Hawaii Ocean Time-series in the North Pacific and Palmer LTER in Antarctica) that provide both logistical support and supporting data on a wide range of hydrographic, chemical and biological parameters, with extensive documentation of spatial and temporal variation. Preliminary field results show that bacterial communities in th ese two areas are physiologically distinct, that bacterial physiology is regulated by the structure of the microbial community and particularly by grazer recycling of nitrogen, that organic compounds rare in seawater, especially histidine, play a large role in regulation of bacterial ectoenzyme expression, and that photolysis of dissolved organic matter by solar ultraviolet radiation also plays a role in regulation of bacterial ectoenzymes. More thorough investigation of these phenomena in ecologically distinct microbial communities will facilitate understanding of the complex interactions between bacterioplankton and the rest of the microbial community.
