Fig. 1: Saddle chips at Marineholmen RASLab are maintained in large storage tanks with different salinities, thereby developing specific biofilters adapted to brackish water (15ppt NaCl), fresh water (0ppt NaCl) or sea water (34ppt NaCl). This allows a more efficient start-up of RAS units with active biofilters, compared to biofilter establishment on clean chips<\/figcaption><\/figure>\nBiofilter diversity at Marineholmen RASLab<\/h3>\n
At Marineholmen RASLab, the biofilters were originally inoculated with commercial enrichment culture in May 2020, but have developed to be specific biofilters adapted to three different salinities over the last two years: fresh water (0ppt NaCl), brackish water (15ppt NaCl), and sea water (34ppt NaCl) (Fig. 1). To obtain detailed information about the nitrifying bacteria in biofilms from all three types of biofilter, targeted 16S rRNA genes from the entre microbial communities were amplified and sequenced using Illumina sequencing technology.<\/p>\n
After two years of sustaining biofilters in the three storage tanks, the sequencing results revealed significant differences in microbial communities present in the specific types of biofilter. Overall, the diversity in the biofilter for brackish water was much lower compared to the biofilters for fresh water and sea water. Furthermore, the number of common bacterial taxa (bacteria present in all three biofilter types) was only 64, while the number of unique bacterial taxa were 238-335 (Fig. 2).<\/p>\n
This confirms that the microbial communities in the biofilters are affected by environmental factors, such as different salinity, and that various bacterial species will adapt to the different conditions and thereby form different biofilters.<\/p>\n
Within the complex microbial community in biofilters, the nitrifying bacteria usually establish slowly over time, and the colonisation period of new biofilters can vary from weeks to several months. In general, the ammonia oxidising step in nitrification is, in most cases, established before the nitrite oxidising step, and the nitrification process in RAS appear to be associated with a small group of bacteria, including the ammonia oxidisers Nitrosomonas, Nitrosospira, and Nitrosococcus; and the nitrite oxidiser Nitrospira.<\/p>\n