Comparison of active biofilm carriers and commercially available inoculum for activation of biofilters in marine recirculating aquaculture systems (RAS)

Abstract

A recirculating aquaculture system (RAS) for rearing Atlantic salmon requires nitrifying microorganisms in the biofilter in order to be fully functional. These microorganisms play a crucial role in water quality improvement by removing ammonia, but the nitrifying population is time-consuming to establish in the biofilter due to their slow growth rates. An efficient biofilter activation strategy is therefore necessary for an improved and short maturation period of biofilters in new RAS. In this study, microbial growth cultures added two different inoculums for RAS were compared. The observed biofilm establishment on clean biofilm carriers and quantified nitrification in the cultures simulated activation of new RAS biofilters. The inoculums used in the experiments included commercial inoculum for marine RAS and transferred biofilm carriers from a marine RAS biofilter operated 1.5 years after activation. The growth experiments revealed that the enrichment cultures inoculated with biofilm carriers from RAS initiated nitrite and nitrate production from ammonia on day 4, which is much earlier than in enrichment cultures with commercial inoculum where nitrite and nitrate production were detectable on day 15 and 18, respectively. The concentration of nitrite and nitrate measured at the end of the experimental period was also substantially higher in enrichment cultures inoculated with biofilm carriers. These results were supported by microbial community analyses of the enrichment cultures, showing that the populations in cultures added biofilm carriers had the highest relative abundance of Nitrosococcus and Nitrospira. Overall, our study shows that biofilm carriers from an established RAS were applicable as inoculum for biofilter activation. However, more knowledge on optimized growth conditions for nitrifying bacteria in RAS could benefit the colonization efficiency and the enrichment of these microorganisms during biofilter activation. In addition, methodical screening for pathogenic bacteria or viruses is needed before biofilter material can be successfully transferred between RAS without risk for disease transfer.