Introduction
Fish employ a range of physiological mechanisms to acclimate to environmental salinity levels that differ from their internal osmotic conditions. Many of these adaptive responses are mediated by characteristic, teleost-specific gene expression patterns. Among the most studied are two isoforms of the atp1a1 gene, which encode the α-subunit of the Na⁺/K⁺-ATPase , an essential enzyme for osmoregulation. These isoforms have become established transcriptional markers in aquaculture, particularly for predicting the seawater adaptability of Atlantic salmon Salmo salar (McGowan et al., 2021).
Despite the growing body of research on osmoregulatory markers in salmonids, transcriptional indicators of saltwater tolerance in coregonids remain largely unexplored. To address this gap, we profiled gene expression in selected tissues of maraena whitefish Coregonus maraena reared either in the Baltic Sea with naturally fluctuating salinities or in a freshwater facility (see Figure a) . Acknowledging the influence of variable environmental conditions in these two open aquaculture systems, we also conducted a complementary salinity challenge under standardised conditions in a recirculating aquaculture system (RAS).
Material and methods
To investigate the potential effects of salinity on the expression of genes associated with salinity adaptation, we analysed transcript levels in the gills, skin, head kidney, and liver of maraena whitefish over a 10-week period. Fish were reared either in a freshwater pond system (BiMES, Friedrichsruhe/Frauenmark, Germany) or in a net-cage facility located within an artificial reef in the Baltic Sea with fluctuating salinity levels between 8.6 and 12.0 PSU (see Figure a). To complement field data and account for potential confounding environmental variables in the open systems, a controlled follow-up experiment was conducted at the experimental aquaculture facility of the FBN (Dummerstorf, Germany). During this 9-day experiment, two RASs were used: one maintained with freshwater, and the other adjusted to 8 PSU and later to 20 PSU for 48 h, using sea salt (Sale Marino, LP20259B, Fino Margherita, Milano, Italy) (see Figure b).
Gene expression profiling was performed using the BioMark HD system (Standard BioTools, San Francisco, CA, USA). A total of 23 candidate genes associated with salinity tolerance, along with two reference genes, were selected based on prior work by Houde et al. (2019).
Results and discussion
The atp1a1a isoform showed significantly higher expression in the gills of maraena whitefish reared in the freshwater pond compared to individuals from the brackish-water reef. In contrast, the seawater-associated isoform atp1a1b was significantly upregulated in the gills of whitefish from the reef relative to their pond-reared conspecifics (Figure c). Despite these differences, the overall expression profiles between the two groups were largely comparable. A PCA based on gene expression across all sampled tissues did not reveal clear separation between reef- and pond-reared individuals, indicating a high degree of similarity in their transcriptional profiles (Figure d). In the gills, as in the three other tissues analysed, only sporadic modulations in transcript abundances across time points were observed (Figures e and f).
In the follow-up experiment conducted in the RAS, an increase in salinity from 0 to 8 PSU led to significant decreased atp1a1a and increased fkbp5 transcript levels in the gills . E levation to 20 PSU was associated with further reduced levels of atp1a1a in the gills.
Overall, the data suggest that maraena whitefish tolerate salinities up to 20 PSU without showing signs of physiological stress. However, the limited transcriptional differentiation observed under natural rearing conditions indicates that the current set of candidate biomarkers may be less effective for evaluating open aquaculture systems. In contrast, gene expression responses in the RAS were more consistent and pronounced, highlighting the potential of these markers for controlled-environment applications.
References
Houde, A.L.S., Günther, O.P., Strohm, J., Ming, T.J., Li, S., Kaukinen, K.H., Patterson, D.A., Farrell, A.P., Hinch, S.G., Miller, K.M., 2019. Discovery and validation of candidate smoltification gene expression biomarkers across multiple species and ecotypes of Pacific salmonids. Conserv. Physiol. 7, coz051. https://doi.org/10.1093/conphys/coz051
McGowan, M., MacKenzie, S., Steiropoulos, N., Weidmann, M., 2021. Testing of NKA expression by mobile real time PCR is an efficient indicator of smoltification status of farmed Atlantic salmon. Aquaculture 544, 737085. https://doi.org/10.1016/J.AQUACULTURE.2021.737085