Microbial community analysis by FISH for mathematical modelling of selective enrichment of gel-entrapped nitrifiers obtained from domestic wastewater

Nitrifying activated sludge from natural domestic sewage was entrapped in hydrogel beads, which were subsequently enriched for nitrifiers in a continuous stirred tank reactor (CSTR). Fluorescently labelled, 16S rRNA-targeted oligonucleotide probes specific for ammonia and nitrite oxidisers were used in combination with DAPI staining to monitor the selectivity of the enrichment process. The growth of both nitrifying and heterotrophic bacteria was more pronounced in the periphery of the beads, leading to a biofilm-like stratification of the biomass during the enrichment. Quantitatively, the relative number of nitrifiers increased from 20% immediately after immobilisation up to 64% after 30 days, but decreased again due to extensive heterotrophic growth. These changes were accompanied by an increase in nitrifying activity for about 30 days, whereupon it reached a stable level. This selective enrichment was mathematically modelled by applying finite difference techniques to the diffusion-reaction mass balances of all soluble substrates relevant in the nitrification process. To model biomass growth and spreading, balanced by both decay and detachment at the surface of the beads, the differential methods were combined with a descrete cellular automaton approach. The spatially two-dimensional model was used to calculate radial concentration profiles within a gel bead, as well as to estimate the corresponding total activity of the reactor. Qualitatively, this model could simulate all essential aspects observed experimentally. However, more and better population data as well as independent estimates of decay and hydrolysis rates are needed to refine and verify the quantitative model. In conclusion, even in the absence of an external carbon source and with excess ammonium, it was only possible to obtain a moderate enrichment of nitrifying cells compared to heterotrophs. Under long-term cultivation, the biofilm-like structure developed in the outer gel layers led to a vigorous competition between auto- and heterotrophs for space, and thereby, access to oxygen. FISH analysis in combination with mathematical modelling constitute a suitable toolbox for analysing the population dynamics and biocatalytic performance of such an ecosystem based on lithoautotrophic primary production.