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Radial microscale distributions of oxygen and pH were studied in ca 1.5 mm large laboratory-made aggregates composed of phytoplankton detritus and fecal pellets. Microsensor measurements were done at spatial increments down to 0.05 mm in a vertical flow system in which the individual aggregates stabilized their position in the water phase according to the upward flow velocity. The aggregates were surrounded by a diffusive boundary layer with steep gradients of oxygen and pH. They were highly heterotrophic communities both under natural light conditions and in darkness. pH was lowered from 8.2 in the surrounding water to 7.4 in the center of an anoxic aggregate. Sulfide was not detectable by use of sulfide microelectrodes in anoxic aggregates, and methanogenic bacteria could not be detected after PCR (polymerase chain reaction) amplification using archaebacterialspecific primers. The oxygen …