W. Tappe et al., CULTIVATION OF NITRIFYING BACTERIA IN THE RETENTOSTAT, A SIMPLE FERMENTER WITH INTERNAL BIOMASS RETENTION, FEMS microbiology, ecology, 19(1), 1996, pp. 47-52
The retentostat was developed for long-term continuous, axenic cultiva
tion of microorganisms at those low growth rates which prevail in most
natural habitats and which cannot be established properly in chemosta
ts. How a microbial population approaches 'zero-growth' was studied in
axenic cultures of Nitrosomonas europaea with complete biomass retent
ion at 25 degrees C and constant input of a nutrient solution containi
ng ammonium (0.57 mM) as energy source. Since only cell-free filtrate
left the reactor, biomass accumulated until a stable maximum of 2.7 X
10(9) cells ml(-1) (398 mg l(-1) dry matter) was reached after about 5
weeks. In this state, growth rate approached zero, and the ammonium i
nput just met the substrate demand required for maintenance energy (1.
43 mu mol NH3 - N mg dm(-1) h(-1)). The potential of the retentostat f
or studying interactions between different microorganisms was demonstr
ated with a cascade of cultures of Nitrosomonas, Nitrobacter, and a de
nitrifying Pseudomonas. Thereby the ammonia was completely eliminated
from artificial wastewater.