Nitrification in freshwater sediments as studied with microsensors and fluorescence in situ hybridization

Abstract

Nitrification is a central process of the benthic N cycle, which sets the preconditions for the removal of excess N from antropogenically impaired aquatic environments. The fact that nitrification usually occurs within a thin oxic surface layer necessitates techniques with sufficiently high spatial resolution for the in situ investigation of this important microbial process. In previous studies of nitrifying biofilms in wastewater systems microsensors and fluorescence in situ hybridization (FISH) served this purpose within even thinner oxic surface layers than in sediments. In the present thesis, the combination of these two in situ techniques was used to study some of the major ecological features of nitrifying bacteria in freshwater sediments. Microsensors and fluorescence in situ hybridization were successfully applied to investigate in situ activities and distribution of NH4-oxidizing bacteria (AOB) and NO2-oxidizing bacteria (NOB). While with FISH these two physiological groups of nitrifiers were identified and quantified, the microsensor measurements revealed the sediment layers in which the nitrifiers were actually active and what effect their activity had on the N exchange between the sediment and the overlying water. Findings of special interst were (i) the quantitative prevalence of the NO2-oxidizing genus :i:Nitrospira:/i: vs. the genus :i:Nitrobacter:/i: in both model and natural stream sediments, (ii) the detectability of AOB and NOB with FISH in anoxic sediment layers as a consequence of their high ribosome content even when not actively nitrifiying, and (iii) the supplementation of the complex array of factors influencing benthic nitrification in bioturbated sediments by the sediment depending plasticity of the larval behaviour.