ANOXIC AGGREGATES - AN EPHEMERAL PHENOMENON IN THE PELAGIC ENVIRONMENT

Radial microscale distributions of oxygen and pH were studied in ca 1. 5 mm large laboratory-made aggregates composed of phytoplankton detrit us and fecal pellets. Microsensor measurements were done at spatial in crements down to 0.05 mm in a vertical flow system in which the indivi dual aggregates stabilized their position in the water phase according to the upward flow velocity. The aggregates were surrounded by a diff usive boundary layer with steep gradients of oxygen and pH. They were highly heterotrophic communities both under natural light conditions a nd 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) a mplification using archaebacterial-specific primers. The oxygen respir ation rate decreased exponentially over time with a T-1/2 of 2.3 d. Th eoretical calculations of the volumetric oxygen respiration rate neede d to deplete oxygen inside aggregates was compared to the density of o rganic matter in natural marine aggregates. These calculations showed that carbon limitation of heterotrophic processes would limit anoxic c onditions to occurring only over a few hours, depending on the size of the aggregates. Therefore slow-growing obligate anaerobic microorgani sms such as sulfate reducing bacteria and methanogenic bacteria may be limited by the relatively short persistence of anoxia in marine aggre gates.