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Nitrification plays a crucial role in aquatic ecosystems and in the biofilters used in fish farms. Despite their importance, the role of canonical nitrifiers, comammox bacteria, and archaea has not yet been sufficiently investigated. The aim of this study was to characterize the microbiome of the external canister biofilter in a freshwater fish aquarium, with particular focus on the role of comammox Nitrospira and their competition with other nitrifiers. To achieve this, a comprehensive approach combining metagenome sequencing and co-occurrence network analysis was used to study the interactions between microorganisms in portable biofilter. The fish were subjected to a changing feeding regime that affected the ecological relationships and abundance of different microbial taxa. The results showed the presence of two types of nitrifiers in the biofilter: comammox Nitrospira and ammonia-oxidizing archaea (AOA). Five comammox Nitrospira genomes were reconstructed, with comammox clade B being the most abundant with an average abundance of 7.8 ± 0.4%. In addition, two families of archaea were identified: Nitrosopumilaceae and Nitrososphaeraceae, with an average abundance of 4.3 ± 0.4%. Heterotrophs were also abundant in the bacterial community, particularly in the genera Actinomycetota, Planctomycetota, and Pseudomonadota. Network analysis indicated competitive interactions between comammox and heterotrophs, whereas no competition was observed between comammox and AOA. The predominance of comammox Nitrospira, and AOA over canonical nitrifiers emphasizes their better adaptation to oligotrophic environments. This study highlights the importance of competition within the biofilter microbiome and the role of ecological interaction networks, which can contribute to the optimization of water purification systems in RASs. [ABSTRACT FROM AUTHOR]

Globally, fish are frequently introduced beyond their native range. Some, like Ponto-Caspian gobies, are becoming invasive, achieving high colonization rates and constituting frequent prey for native predators. However, little is known about the effectiveness of antipredator behaviors of the invaders, which may shape their role in the invaded community and contribute to the invasion success. We compared antipredator behaviors of invasive gobies and native fish species after their detection by the predator, when the danger becomes direct. We studied 2 fish pairs, each consisting of an invasive and native species co-occurring in the environment and belonging to the same prey guild: (1) the racer goby Babka gymnotrachelus versus European bullhead Cottus gobio , (2) the monkey goby Neogobius fluviatilis versus gudgeon Gobio gobio , facing a naïve predator (the Eurasian perch Perca fluviatilis). We analyzed behaviors of single prey individuals (escaping, staying in shelter, and activity) and single predators (activity, searching, following, capturing, and latency to prey consumption). In the predator presence, the bullhead was less active and more often managed to escape after capture than the racer goby. The gudgeon escaped before the capture more often than the monkey goby. The predator succeeded later with the bullhead compared to racer goby, whereas no differences in ingestion time occurred between the gudgeon and monkey goby. The results suggest that, in terms of hunting effort of native predators, the invasive gobies are equivalent to or more profitable prey than their native analogs, which can facilitate the integration of the gobies into local food webs. [ABSTRACT FROM AUTHOR]

Mercury belongs among the highly hazardous substances present in the environment and represents a great health risk for many animals including predatory and piscivorous birds. The aim of this study was to assess the total mercury content in the main detoxifying organs (liver and kidney) and in the muscles of adult great cormorants (Phalacrocorax carbo) caught at two localities (the Krogulna ponds and the Nysa Kłodzka River) in southern Poland. The aim was to compare the locality with the iron ore mining history (the Krogulna ponds) with a site without such load (the Nysa Kłodzka River). The total mercury content in the great cormorants decreases as follows: kidneys > liver > muscle in both monitored localities and significantly differs between the localities. The average mercury content varied from 0.58 ± 0.38 mg/kg (muscle) to 1.39 ± 1.42 mg/kg (kidney) in the cormorant from the Krogulna ponds and from 1.09 ± 0.40 mg/kg (muscle) to 3.12 ± 1.55 mg/kg (kidney) in the cormorant from the Nysa Kłodzka River. The accumulation of mercury does not correlate with the mining history as it was higher at the Nysa Kłodzka River, but it is probably influenced by the different fish stocks in these two localities (omnivorous fish in the Krogulna ponds versus predatory fish in the Nysa Kłodzka River). [ABSTRACT FROM AUTHOR]