Combined autoradiography and immunofluorescence for estimation of single cell activity by ammonium‐oxidizing bacteria1,2

Abstract

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 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. There were no differences in relative activity between samples incubated under simulated in situ conditions and in the dark.Relative activity was positively correlated with the abundance of ammonium‐oxidizing bacteria, temperature, total dark CO2 assimilation (as measured by liquid scintillation counting of replicate samples), and pheopigment concentration, and negatively correlated with oxygen concentration.