Abstract
We have previously shown that K-selection and microbial stability in the rearing water increases survival and growth of Atlantic cod (Gadus morhua) larvae, and that recirculating aquaculture systems (RAS) are compatible with this. Here, we have assessed how water treatment influenced the larval microbiota and host responses at the gene expression level. Cod larvae were reared with two different rearing water systems: a RAS and a flow-through system (FTS). The water microbiota was examined using a 16S rDNA PCR/DGGE strategy. RNA extracted from larvae at 8, 13, and 17 days post hatching was used for microbiota and microarray gene expression analysis. Bacterial cDNA was synthesized and used for 16S rRNA amplicon 454 pyrosequencing of larval microbiota. Both water and larval microbiota differed significantly between the systems, and the larval microbiota appeared to become more dissimilar between systems with time. In total 4 phyla were identified for all larvae: Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria. The most profound difference in larval microbiota was a high abundance of Arcobacter (Epsilonproteobacteria) in FTS larvae (34 ± 9% of total reads). Arcobacter includes several species that are known pathogens for humans and animals. Cod larval transcriptome responses were investigated using an oligonucleotide gene expression microarray covering approximately 24,000 genes. Interestingly, FTS larvae transcriptional profiles revealed an overrepresentation of upregulated transcripts associated with responses to pathogens and infections, such as c1ql3-like, pglyrp-2-like and zg16, compared to RAS larvae. In conclusion, distinct water treatment systems induced differences in the larval microbiota. FTS larvae showed up-regulation of transcripts associated with responses to microbial stress. These results are consistent with the hypothesis that RAS promotes K-selection and microbial stability by maintaining a microbial load close to the carrying capacity of the system, and ensuring long retention times for both bacteria and water in the system.
Highlights
Aquaculture of marine fish often face problems with high mortality, infections and malformations/deformities in the production of juveniles
We investigated the effects that two different water treatment systems, one recirculating aquaculture system (RAS) and one flow through system (FTS), had on water microbiota, larval microbiota, and larval transcript profiles
Our hypothesis was that through differences in rearing technology we are able to manipulate the microbiota of the rearing water and that this will affect the microbiota associated with cod larvae
Summary
Aquaculture of marine fish often face problems with high mortality, infections and malformations/deformities in the production of juveniles. Most marine fish larvae in culture, like Atlantic cod (Gadus morhua), are immature and fragile upon hatching (Kjørsvik et al, 2004; Magnadóttir et al, 2004; Magnadóttir, 2006), and the larvae are reared in close proximity to high numbers of bacteria and other microorganisms. Toranzo et al (2005) suggested that most bacterial infections associated with marine fish larvae are caused by opportunistic bacteria that are usually present in the natural environment of the fish. The environmental conditions are less favorable for the opportunistic bacteria compared to that in an aquaculture system, and these bacteria rarely cause mortalities in natural settings (Toranzo et al, 2005). Due to high loads of organic matter, and uncontrolled bacterial recolonization after disinfection of the rearing water, the microbial community in the rearing water is more unstable, dominated by opportunists, and with higher and more variable bacterial numbers than in nature (Hess-Erga et al, 2010; Attramadal et al, 2014)
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