Magnoni, L.J. et al. (2017). Dietary supplementation of heat-treated Gracilaria and Ulva seaweeds enhanced acute hypoxia tolerance in gilthead sea bream (Sparus aurata). Biology Open. 6: 897-908. doi:10.1242/bio.024299. Open access article available here.
What are antioxidants?
Claimed to do almost everything – to slow ageing, get rid of wrinkles, improve chronic disease like diabetes, and prevent diseases – antioxidants are the darling of health and beauty industries. Going by the ads, it seems like pomegranates, chocolate, and red wine are destined to reverse baldness, cure cancer, and give you superpowers. How true these claims are, especially at the doses offered by moisturizers and a typical daily intake of fruits and vegetables, is a point of contention. However, there’s no question that antioxidants have stimulated intense interest and research in the scientific community on topics as diverse as cancer prevention, nutrition, physiology, and aquaculture.
Free radicals & oxidative stress
Regardless of how many pomegranates you eat, your body’s antioxidant defense system plays a vital role in preventing and repairing cellular damage. This network of specialized proteins and antioxidant molecules (like vitamins C and E, glutathione, melatonin), react with free radicals before attack sensitive cellular structures and proteins. Oxidative stress and damage occurs when the antioxidant defence system can’t absorb all the free radicals, for example if there are too many being produced at once.
Animals routinely encounter situations where they are put at risk of oxidative stress. For example, low levels of free radicals are produced naturally as a by-product of metabolism because of how electrons flow through the mitochondria, the energy-producing powerhouse of the cell. Metal contaminants like copper, iron, and nickel can cause oxidative stress by trading electrons with oxygen molecules (creating a free radical) or depleting the antioxidant defense system. Exposure to warm temperatures increases the metabolic rate of cold-blooded ectotherms like fish and increases the normal metabolic production of free radicals, and this can causes issues for animals not accustomed to living at warm temperatures.
Low-oxygen conditions (hypoxia) can also cause increased free radicals and oxidative stress, though the physiological mechanism by which this occurs is not clear. Without a strong antioxidant defense system, these radicals can lead to damage to key proteins and cell structures, compromising a fish’s ability to survive, cope, and grow.
Can we protect farmed fish from hypoxia-induced oxidative stress?
Because a huge number of oxygen-consuming fish are crammed into a small space, fish farms often have to contend with bouts of hypoxia. Even brief periods of low oxygen can have huge economic implications due to increased mortality and/or decreased growth rate in the stock. Other than adding more oxygen to the water (which can be risky – too much oxygen can also cause oxidative stress), what can be done to protect fish from oxidative stress?
A team of researchers from Portugal, Spain, Argentina, and The Netherlands investigated the effects of dietary supplementation with heat-treated, antioxidant rich seaweed (Gracilaria vermiculophylla or Ulva lactuca) on the survival and antioxidant capacity of the gilthead sea bream (Sparus aurata) exposed to hypoxia.
The sea bream were divided into three groups: (1) fish fed a typical diet without seaweed, (2) fish fed a diet supplemented with 5% Gracilaria seaweed, and (3) fish fed a diet supplemented with 5% Ulva seaweed. After 34 days on their new diets, the fish were either exposed to hypoxia and recovered, or kept in well-oxygenated conditions (to act as a control group). The researchers then used several indexes to characterize how well the antioxidant system of these fish functioned and determine whether a seaweed-rich diet had any discernible benefit.
Fish fed seaweed do better under hypoxia
Fish on seaweed diets had a higher survival rate when exposed to low-oxygen conditions. They also looked better doing it, showing less liver peroxidation, a marker of oxidative damage. Other parts of the oxidative defense system differed between fish fed a normal diet and those supplemented with seaweed.
Fish fed Gracilaria seaweed had lower activities of two key antioxidant enzymes – catalase and glutathione peroxidase – in their livers after exposure to hypoxia. But why would these antioxidant defenses be lower in a situation when expect an elevated level of free radicals? The authors suggest that the old adage of “use it or lose it” applies. Perhaps the antioxidant compounds in the seaweed soaked up a lot of free radicals, leaving catalase and glutathione peroxidase with a reduced workload. Since enzymes are energetically expensive to maintain, reducing their activity may free up that energy to be put to better use, such as towards growth.
In addition to enzyme activities, the team also looked at changes in gene expression. Genes are special sections of DNA that tell the cellular machinery what proteins (including enzymes) to make and when. Tracking gene expression allows biologists to get a more complete picture of what is going on in a cell than they would get by just measuring a few key enzymes.
In the livers of fish fed seaweed diets, the expression of genes coding for antioxidant enzymes (including catalase and glutathione peroxidase) declined. Genes known to respond to stressful conditions (such as crowding) also showed reduced expression. This agrees with reduction in catalase and glutathione peroxidase activity, and suggests that these proteins are produced at a decrease rate in hypoxia, perhaps because they are not relied upon as heavily as in the control diet condition.
Interesting, this molecular signature was maintained in Gracilaria diet group but not the Ulva group after hypoxia exposure. This suggests that Ulva fish had a reduced capacity to deal with free radical production during hypoxia when compared to the Gracilaria group, since they had to resort to making more antioxidant enzymes to counteract free radicals produced during hypoxia.
Overall, the diet had an important influence on the oxidative stress levels of fish exposed to low-oxygen conditions. Fish fed diets supplemented with seaweed, particularly Gracilaria vermiculophylla, showed better survival, and fewer signs of oxidative stress and damage than control animals. It appears that the compounds with antioxidant properties present in the supplemented dietary extract reduced the requirement for antioxidant enzymes, which in turn may leave more energy available for growth and other important biological functions.
Applications for aquaculture
Where do we go next? This study did not did not evaluate antioxidant content in the seaweed extracts, so further work needs to done to identify compounds contained in heat-treated Gracilaria, and to determine exactly how they react with and protect tissues from free radicals.
This work also has some interesting potential applications for animal welfare and improving the conditions in fish farms. Perhaps by supplementing fish with relatively little seaweed (5% of diet), we can protect fish from oxidative stress. This could improve the health, well-being, and food production from these animals by making them less vulnerable to bouts of hypoxia, high temperature, pollution, and environmental challenges.
Brittney is a PhD candidate at McMaster University in Hamilton, ON, Canada, and joined Oceanbites in September 2015. Her research focuses on the physiological mechanisms and evolution of the respiratory and metabolic responses of Fundulus killifish to intermittent (diurnal) patterns of hypoxia.