The functioning of recirculation aquaculture systems (RAS) is essential to maintain water quality for fish health, and one crucial process here is nitrification. The investigated RAS was connected to a rainbow trout production system and operated at an average temperature of 13°C and pH 6.8. Community analyses of the nitrifying biofilm revealed a coexistence of Nitrospira and Nitrotoga, and it is hypothesized that a slightly acidic pH in combination with lower temperatures favors the growth of the latter. Modification of the standard cultivation approach toward lower pH values of 5.7 to 6.0 resulted in the successful enrichment (99% purity) of Nitrotoga sp. strain HW29, which had a 16S rRNA sequence similarity of 99.0% to Nitrotoga arctica. Reference cultures of Nitrospira defluvii and the novel Nitrotoga sp. HW29 were used to confirm differentiation of these nitrite oxidizers in distinct ecological niches. Nitrotoga sp. HW29 revealed pH and temperature optima of 6.8 and 22°C, respectively, whereas Nitrospira defluvii displayed the highest nitrite oxidation rate at pH 7.3 and 32°C. We report here the occurrence of Nitrotoga as one of the main nitrite-oxidizing bacteria in freshwater aquaculture systems and indicate that a slightly acidic pH, in addition to temperatures below 20°C, can be applied as a selective isolation criterion for this microorganism.
Nitrogen is one of the most important elements of life and nitrification is a key process in the global N cycle, since the biological conversion of ammonia to nitrate is required wherever organic matter accumulates. This oxidation of ammonia to nitrate via nitrite is catalyzed by chemolithoautotrophic ammonia-oxidizing bacteria (AOB) and archaea (AOA) and nitrite-oxidizing bacteria (NOB). The biotechnological application of nitrification in recirculating aquaculture systems (RAS) has gained increasing attention for the production of aquatic organisms, since it allows an efficient and ecological way of fish breeding. To remove toxic ammonia released by fish gills or mineralization of feces and feed offcut, biological filtration is required. Nitrification is promoted on the surfaces of carrier elements in well-aerated moving-bed reactors. The carrier elements (plastic beads) are colonized by nitrifying bacteria, which form a dense biofilm together with heterotrophic bacteria.In biofilters of most aquaculture systems (marine and freshwater), Nitrosomonas and Nitrospira were identified as the main ammonia and nitrite oxidizers, respectively (1-4), but members of the genera Nitrobacter and Nitrotoga are also potential candidates for nitrite removal (5, 6). Cold-adapted Nitrotoga spp. were initially found in permafrost-affected soils in Siberia (7), and the preference for low temperatures was confirmed by the detection of Nitrotoga-like 16S rRNA gene sequences in other cold soils such as recently deglaciated soils (8) or periglacial soils (9). Nitrotoga was also abundant in oligotrophic aquatic environments such as the Movile cave (10), a neutral groundwater seep (11) ...