Ornamental fish aquaria as a reservoir of unusual predatory bacteria

Aquaria that host ornamental fish may support a vast array of microorganisms, but to date, little information is available on these microbial ecosystems. In the present study, a polyphasic approach based on conventional and culture-independent strategies was used to evaluate the microbial community in the filter and in the gravel of a home aquarium harbouring Pristella maxillaris. The presence of predatory bacteria was also assessed, and four predatory bacteria could not be identified as belonging to any known species within the genus Bdellovibrio. Strains differed in terms of prey preference but shared the same RAPD profiles. A total of 62 cultures were isolated and identified by 16S rDNA sequencing. Aeromonas hydrophila accounted for 82% and 50% of the isolates in the filter and gravel, respectively. However, recently isolated Pseudomonas species such as Pseudomonas ullengensis, Pseudomonas lalkuanensis, and Aquipseudomonas alcaligenes were also retrieved. According to high-throughput sequencing, in gravel, eubacteria accounted for 85.42%, while Archaea accounted for 14.57%. In the filter, eubacteria dominated (98.25%). Archaea were almost entirely represented by ammonium oxidizers, Nitrosopumilaceae (14.30%). Conversely, in the filter, ammonium oxidation was apparently carried out by Nitrosomonadaceae (19.52%). Home aquaria appeared to be complex ecosystems and sources of unusual predatory bacteria.