Bacterial activity at the air/water interface.

By using substrate molecules of varying degrees of surface activity, we were able to measure some features of bacterial activity in the surface microlayers (SM) and in the subsurface (bulk) water. The fraction of active cells was determined by a combined microautoradiography-epifluorescence (ME) method. Measurements were made of(14)CO2 evolution to determine the rate of respiration. Results from in situ measurements showed no significant difference between fraction of active cells in the SM and in the bulk. This may be due to an exchange of bacteria between SM and bulk. This exchange was assessed by spreading a film of(3)H-palmitic acid on the surface and, after incubation, measuring the amount of labeled cells at the surface and in the bulk. Test bacteria showing a high accumulation at the surface also showed a low exchange between the 2 strata. When low concentrations of added(14)C-protein were used, the respiration measurements showed a lower value for bulk than for interface localized protein. At higher concentrations, the evolved(14)CO2 was the same whether the protein was mixed in the bulk or spread at the surface. When 2.4-12 ng·cm(-2) of(14)C-palmitic acid was spread on the surface, there was a linear relation between turnover time and amount of added substrate. At higher substrate concentrations there was a deviation from the straight line. Results are discussed in terms of the unique habitat found at an interface.