68 DECEMBER • WORLD AQUACULTURE • WWW.WAS.ORG the fish implanted with GH grew better than the control, but they also consumed more vitamin C and E (Figure 4). The protection against oxidation therefore became compromised and the fish were more exposed to oxidative stress as measured by malondialdehyde (MDA), GSH/GSSG, antioxidant enzymes and transcriptomics. Supplementing extra vitamin C and E in the diet counteracted the GH-induced oxidative stress and produced healthy fish with a high growth rate. GH also decreased bone mechanical strength and increased bone growth (Fjelldal, Saito et al. 2024). In a second study (Yin, Saito et al. 2023), we looked at how changes in photoperiod simulating spring, summer and autumn daylength affected growth and redox biology, also in Atlantic salmon postsmolts. The fish were kept at constant temperature (9ºC). Again, intracellular antioxidants were depleted during the period of increasing daylength and growth (Figure 5A). MDA, antioxidant enzyme activities and gene expression levels in the liver and muscle also responded, with a distinct upregulation of MDA and genes involved in maintaining redox homeostasis. Furthermore, a nuclear factor-erythroid 2-related factor 2 (Nrf2)-mediated oxidative stress response was detected in both muscle and liver, suggesting that fish integrate environmental signals through redox signaling pathways (Figure 5B). The results demonstrate that a change in photoperiod without the concomitant increase in temperature is sufficient to stimulate growth and change the tissue oxidative state in Atlantic salmon during spring and early summer. The third study investigated how simulating both daylength and temperature according to the changes that happen in spring, summer and autumn affected the redox biology of Atlantic salmon postsmolt (Yin, Saito et al. 2024). Daylength changes at constant temperature and simultaneous change in daylength and temperature both gave similar SGRs, also similar to an earlier experiment with salmon in sea cages with natural variations (Hamre, Micallef et al. 2022). The two simulations and the open cage study were all conducted according to the environmental conditions near Bodø, Norway (67ºN). Again, spring conditions lead to increased utilization of tissue antioxidants such as vitamin C and vitamin E, and reduced glutathione, which were restored in August. The more reduced glutathione-based redox potentials and the low levels of malondialdehyde (MDA) in liver and muscle in August compared to June/July, suggest lowered tissue oxidative activity at that time (Figure 6A). We further revealed that the expression profiles of genes involved in growth hormone signaling and cell cycle regulation correlated with oxidative stress patterns (Figure 6B). The main findings were therefore similar in study 2 and 3, but there were also differences which are difficult to interpret with our present knowledge. The three listed studies tested a range of biomarkers of redox biology, and the easiest to interpret were MDA and tissue concentrations of vitamin C and E, when the diets were supplemented with the two vitamins at constant levels. Astaxanthin is also a strong antioxidant, which can be consumed during oxidative stress in spring (Nordgarden, Ørnsrud et al. 2003, Hamre, Micallef et al. 2022). Further, our experience is that endogenous GSH production is stimulated by light oxidative stress, but the concentration declines when GSH-oxidation exceeds production. Often, the redox potential which is proportional to the ratio between GSH and GSSG, remains constant during these processes, since it is this ratio that regulates metabolism (Hamre, Zhang et al. 2024). With this background it is possible to understand variations in GSH/GSSG. Antioxidant enzyme activities (SOD, CAT, GPx and GR) are often used to measure changes in oxidative stress in response to various treatments, e.g. feed additives, dietary composition, handling, etc. However, interpretation of such data is extremely difficult. For example, it could be positive that activities increase due to improved protection against oxidation, or negative because it indicates higher oxidative stress. Lowered enzyme activities could mean that the enzymes are degraded by oxidative stress or that oxidative stress is low with limited need for the enzymes. In order to work comprehensibly with this system, one needs to have a much better understanding of the redox metabolism and perhaps include more biomarkers. We also used transcriptomics in the three studies with many interesting results. Again, we need more knowledge of salmon redox metabolism to better understand the outcomes of gene expression. Future approach to understand redox biology in fish is, of course, to first look into literature from other species. Because fish farming has special conditions and topics, a second direction is to validate biomarkers. This could be achieved through experiments with the candidate species, but since farmed species are big and usually have a long lifespan, such experiments will be costly. A good alternative to unravel the principles is to use model species like zebrafish or fish-cell FIGURE 3. Integrated redox regulatory network in cellular homeostasis. Figure by Peng Yin. FIGURE 4. Growth hormone (GH) implantation decreases vitamin C and E concentrations and increases MDA in Atlantic salmon. EP1 (Experimental period 1) included acclimatization and feeding diets with Low (L) and High (H) vitamin levels. At start of EP2, half of the fish in each vitamin group were given an intraperitoneal implant of growth hormone. (Yin, Bjornsson et al. 2022).
RkJQdWJsaXNoZXIy MjExNDY=