In too deep?

30 September 2015 - By: Caroline Wood

In too deep?

Stickleback-In txt

Female Stickleback. Photo: Dario Fiorentino


By Caroline Wood

Rising temperatures, brought on by global warming, represent possibly the most daunting challenge for marine organisms, particularly those adapted to conditions in the Polar Regions. But ocean communities face many other challenges on top of this which could test their resilience to breaking point – including acidification, pollution and overfishing. In this article we hear from selected scientists, who presented their latest research at the SEB Meeting, Prague 2015.

Down-sizing

“Because of about 30 million years of isolation, the Antarctic harbours unique biota, which are physiologically adapted to these conditions, but have a very low thermal tolerance range”, explained Tina Sandersfeld (Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research and University of Bremen, Germany).

Previous research into how rising temperatures will affect polar fish species has tended to focus at molecular and cellular levels. Tina and her colleagues, however, have been studying the impacts on whole organisms, using Emerald Rockcod (Trematomus bernacchii)collected from the Ross Sea. “The Emerald Rockcod preys on a lot of bottom living species such as bivalves, but it is also eaten by predators such as penguins and large squid”, said Tina. “Thus, any impacts on this model species are likely to influence the whole ecosystem”.

Groups of fish were held at either 0, 1, 2 or 4 °C for 9 weeks, during which their standard metabolic rate, individual food intake and excretion rates were measured. “We then calculated how efficiently each fish converted food into energy and how much of this they could allocate to growth”, said Tina. The results showed that higher temperatures had a significant effect on body mass, with a reduction of 84% at 2 °C compared to 0°C. It is thought that this was due to the food conversion becoming less efficient. “Despite the fish ingesting food at a similar rate, at higher temperatures they could not convert it so efficiently and so excreted more energy, possibly via the faeces”, explained Tina.

This could have severe consequences beyond individual organisms as Tina concluded: “Antarctic fish do not reproduce before reaching a certain size. A significant growth reduction is likely to affect population structures with severe impacts on Antarctic fish communities and ecosystems”. 

Mum’s the word


Temperatures may be rising too quickly for species to adapt through traditional Darwinian evolution. Could transgenerational inheritance – where parental experiences can shape offspring performance – be an answer?

To date, most transgenerational-related research has focused on the nuclear genome, but animals also have another source of genetic material within the mitochondria – the energy generating powerhouses within each cell, which are inherited maternally. “Acclimation to a changing environment is energetically very costly so we could expect mitochondria to be part of that process”, said Felix Mark (Alfred Wegener Institute for Polar and Marine Research, Germany). To investigate this, Felix and his colleagues tested how the mitochondrial capabilities of marine sticklebacks were affected when the parents were kept at different acclimation temperatures. Crucially, the fish were divided into different groups to test the specific effect of the mother’s acclimation temperature, as this would indicate transgenerational effects that were inherited via the mitochondria.

Male and female stickleback parents were kept at either 17 or 21 °C and the offspring divided into four groups: those with a “cold-reared” mother and father, those with a cold mother and warm father, those with a warm mother and cold father and those who parents were both “warm-reared”. The offspring were then themselves divided into two groups and reared at either 17 or 21°C until they grew to adult fish. During this time, various fitness parameters were tested, including hatching success, growth rates, growth efficiency and mitochondrial function.

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Sockeye Salmon Alevin. Photo:Natalie Sopinka

 

“Our main finding was that, although all fish held at 17°C performed better than those kept at 21°C, under the warmer conditions fish performed better and grew larger when their mothers had been reared under the warm temperatures as well”, explained Felix. The increase in growth size was found to correlate with improved mitochondrial function, indicating that more efficient energy conversion allowed more energy to be invested in growth.

“This suggests that the mitochondria are a primary target for non-genetic transgenerational thermal compensation”, said Felix. However, he cautioned that the same scope for adaptation may not apply to Polar species. “Polar fish usually have very long generation times so climate change may be too fast for transgenerational plasticity to keep pace”. 

Stressing factors


Although transgenerational inheritance could help certain species adapt to future conditions, in some cases it could compound the problem, putting offspring at a further disadvantage. Amanda Banet (University of British Columbia, Canada), found that this appeared to be the case with Pacific salmon that had undergone particularly stressful migrations.

“Migration in itself is already a stressful event”, said Amanda, “but in recent years, many fish are also encountering stressors that historically have not been present, including increased water temperatures due to climate change, greater angling pressures and changes in water flows due to human-created disturbances.” Previous research has found that Pacific salmon females arriving at their spawning grounds in a physiologically poor state produce offspring with impaired swimming abilities. This could suggest that the females have genetic differences that make them less capable of meeting the physical demands of migration. However, another explanation is that more difficult migrations induce higher levels of stress in the mothers which is transmitted to the offspring.

To investigate this, Amanda and her colleagues collected eggs from female Pacific salmon at the end of migration and exposed them to cortisol baths to simulate high levels of maternal stress. The eggs were then fertilized, reared to fry and put through a series of swim performance trials. Aerobic capacity was assessed by placing the fish in a “swim treadmill” equipped with a probe to measure oxygen uptake. After taking a baseline reading when the fish were resting, the researchers gradually increased the current until the fish became too fatigued to keep swimming. “By subtracting the maximal rate of oxygen consumption just before fatigue occurs, we can calculate the aerobic scope”, Amanda exlpained. “This can be thought of as the aerobic capacity a fish can devote to additional activities beyond baseline metabolism.”

The results showed that exposure to cortisol during early development significantly reduced aerobic scope by up to 25% in the juvenile fish. It remains unknown what mechanisms caused these effects or whether epigenetic modifications were involved but this could dramatically impact fish populations. “We would hypothesize that juveniles with a higher aerobic scope would have a higher survival as they are better able to meet the demands of migration, foraging, and escaping predators”, concluded Amanda. 

Phishing for clues


As Jodie Rummer alluded during the “Science with Impact” session (see pages 48-49), a critical challenge facing marine life is rapid ocean acidification. “Current projections suggest we can expect to see a pH drop of 0.3-0.4 by the year 2100”, she said. To put this in context, a 0.1 pH drop in a human being’s blood system could cause potentially lethal acidosis. Although fish have well developed short-term acid-base compensation mechanisms, the effects of chronic exposure to high CO2 are unknown, as reported by Dr Amélie Crespel (IFREMER Laboratoire Adaptation Reproduction et Nutrition des poissons, France), using European Sea Bass as a model.

Two day old fish were reared for over a year under one of three CO2 conditions: current levels (500 µatm ) and two of the more severe IPCC predictions for 2100 (1000 and 1500 µatm). The researchers then tested two different fitness parameters– maximum swimming performance and hypoxia tolerance. “We used a flow chamber to measure swimming performance”, said Amélie. “We gradually increased the velocity until the fish could no longer swim against the flow.” This was combined with other measurements, such as blood haemoglobin concentration and aerobic scope, to build a comprehensive picture of energetic capacity. The results demonstrated that chronic acidification had a negative impact on swimming performance, but tolerance to hypoxia was improved.

But are these effects of early acidification exposure due to genetic imprinting or compensation mechanisms? To investigate this, fish from the different treatments were pooled together under common current CO2 level conditions for three months before being tested. Intriguingly, this removed the effects of acidification exposure on swimming performance, but the fish still showed greater tolerance to hypoxia. This suggests that ocean acidity alters different traits through separate mechanisms. “The processes underlying hypoxia tolerance seem to have been imprinted (probably through epigenetic changes) but not the processes underlying swimming performance”, said Amélie. “We will need further investigations to understand this result.” 

Bad Chemistry


Besides acidifcation, marine life has to cope with the cocktail of chemicals we release into the oceans. Of particular concern are estrogenic endocrine disruptive chemicals (EEDCs) which are present in a wide array of products such as plastics, pesticides and hormone based medicines, and can disrupt sexual differentiation and reproduction in wildlife. Most treatment plants lack equipment capable of removing these contaminants and worrying new research suggests that EEDCs can have impacts beyond direct exposure, even affecting future generations that do not contact them.

“EEDCs have direct effects on developing fish embryos but there is also evidence that they can impact the epigenome through DNA methylation and histone modification”, said Drew Peterson (City University of Hong Kong). “If this happens in sperm or egg cells, these effects could be passed on to the next generation”. To investigate this, Drew and his colleagues tested the multigenerational effect of EEDCs on Marine Medaka (Oryzias melastigma). They chose the compound 17α-Ethinylestradiol (EE2), a component of many oral contraceptive medicines. “The human body can only absorb a fraction of this hormone with the rest excreted through the urine”, explained Drew. “Consequently, EE2 can reach high concentrations in waste water”. Adult fish were directly exposed to either high or low EE2 concentrations, for either a short (7-day) or long (21-day) period. The eggs were collected daily and reared in clean seawater.

The results demonstrated that, for all treatments, parental exposure to EE2 had cross-generational effects. Hatching was significantly delayed in the offspring and fewer fish survived this process. In addition, offspring of parents exposed to high EE2 concentrations were more vulnerable to infections caused by pathogenic bacteria – an effect which even persisted to the following generation.

According to Drew, these studies should prompt national governments to clean up their waste water discharges. “It is critical that toxicological risk assessments consider the effects on multiple generations to assess the full impacts on marine populations” said Drew. 

Gasping for breath


Carbon dioxide may be one matter, but in some areas it is oxygen – or rather a lack of it – that is the critical problem. “Globally, hypoxia is one of the most pressing problems in aquatic environments, with more than 400 so-called “dead zones” worldwide”, said Doris Au (City University of Hong Kong). “It is equally problematic for oceans and freshwater systems with over 77% of the freshwater ecosystems in China now considered under serious threat by hypoxia”. These dead zones are typically caused by excess nutrients (often released by human activities) that lead to cyanobacteria and algae blooms. When they decompose, the process uses up considerable oxygen, suffocating other marine life. “This has already been shown to have severe repercussions, such as decreasing biodiversity, altering ecosystem and population structures and eradicating more sensitive species from certain areas”, added her collaborator, Rudolf Wu (The University of Hong Kong, HKU). It is largely unknown how early exposure to hypoxia affects lifelong health of individual fish.

Doris’s team investigated this by exposing Medaka fish to hypoxic conditions during embryonic development. The results demonstrated that early exposure to hypoxia significantly reduced growth and immunity to the bacterial pathogen Edwardsiella tarda. To see if a genetic mechanism was responsible, epigenetic analysis was carried out. “In the “pre-hypoxia” adults, there was a good correlation between repressive epigenetic marks, such as DNA methylation and histone modification, and genes involved in growth hormone regulation and anti-microbial peptide production in the liver”, Doris explained.

Worryingly, Doris and her collaborators (Rudolf Wu and Simon Wang, HKU) also showed that these epigenetic repressions are also passed on to the future generation, particularly in genes relevant for reproductive function. This could explain why the offspring of hypoxia-exposed adults showed reduced sperm counts and disrupted gonad development. “Our results suggest that current assessments on the risk of hypoxia on aquatic environments might have been grossly under-estimated and that hypoxia will pose a much more serious and long lasting threat to fish populations than previously thought”, Doris concluded.

Category: Animal Biology
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Caroline Wood

Caroline Wood was the SEB’s 2014 science communication intern. Since then, Caroline has been a regular contributor the SEB, reporting on events and writing insightful features for our members.
Caroline has an undergraduate degree from Durham University in Cell Biology and is currently a PHD student at Sheffield University studying parasitic Striga weeds that infect food crops. You can read her blog here.