Combined autoradiography and immunofluorescence for estimation of single cell activity by ammonium-oxidizing bacteria

Immunofluorescence and 14CO2 autoradiography were used for simultaneously enumerating and assaying the autotrophic activity of ammonium-oxidizing bacteria in seawater. Relative activity (14CO2 assimilation as measured by autoradiography) and abundance were measured in simulated in situ incubations at seven stations in the primary NO2- maximum region of the Northeast Pacific Ocean. More than 104 cells-liter-1 were present; relative activity often showed a peak near the surface and an increase in the NO2- max region below the photic zone. The method permits assessment of individual cell activity; most cells at all depths were active in CO2 assimilation, usually at low and quite variable levels. Relative activity was positively correlated with the abundance of ammonium-oxidizing bacteria, temperature, total dark CO2 assimilation and phenopigment concentration