Microbial loop in an oligotrophic pelagic marine ecosystem: possible roles of cyanobacteria and nanoflagellates in the organic fluxes

In an attempt to quantify the organic fluxes within the microbial loop of oligotrophic Mediterranean water, organic pools and production rates were monitored. The production of cyanobacteria and its dynamics dominated the overall productivity in the system. The largest standing stock was that of the bacterioplankton and its growth consumed 8.3 pg C 1-' d-', hence about 60 % of the primary production was required for bacterial growth. Using the MiniCap technique, we measured a predation on bacteria of 2 6 X 104 bacteria ml-' h-'. This was in good agreement with the bacterial production rate of 2.3 X 104 cells rnl-' h-' Thus, growth and predation were balanced for heterotrophic bacterioplankton. Almost all of this predation on bacteria was due to organisms passing a 12 vm Nuclepore filter. This raises the question of what mechanisms channel 60 % of primary production into bacteria. We therefore outlined a mass-balance model to illustrate routes that could explain this transfer. According to our model the main flux route is cyanobacteria and concomitantly consumed heterotrophic bacteria carbon into bacterivores. A substantial fraction of the bacterivore and the microplankton carbon is released by excretion and/or cell lysis, to be used by the heterotrophic bacterioplankton. About 86% of the autotrophic production is balanced by respiration due to heterotrophic bacteria and protozoa, leaving 6 % of the primary production to higher trophic levels. This scenario should apply to ecosystems where bacterial production rate is high and comparable to primary production, and the dominant primary producers are cyanobacteria. A significant fraction of the photosynthetically fixed carbon will be mineralized within a simple microbial loop, thus rendering it an energy sink in the foodweb.