Antarctica is a continent of extremes, and for species that dwell there, it presents a simple problem: Adapt to the harshness, or die out.
And over millennia, species have evolved to cope with low temperatures and other extreme conditions around the southernmost continent. However, now, in the face of warming waters brought on by climate change, species are confronted with new hurdles.
Buckley’s seminar, “Life in the Freezer: Cellular Responses to Temperature Stress in Antarctic Fishes,” covered his work with goby and rockfish species, as well as some of Antarctica’s natural history.
The following are various noteworthy points from Buckley’s presentation:
- The Antarctic Circumpolar Current is the largest ocean current on Earth. It became established around 22 to 25 million years ago, well after the continent of Antarctica drifted to the South Pole. The current now acts as a barrier to the influx of new species. Winter sea ice’s formation can double the size of the continent, and the seasonality of the ice helps structure the ecosystem there.
- Life has been associated with Antarctica for around 400 million years (minimum), from the period when it was more temperate. Of some 28,000 to 30,000 extant species found around Antarctica, only about 320 (possibly just the animal species) are found south of the Antarctic polar front. These are truly cold-adapted species and not just cold-tolerant ones; for them, 6º to 8º C is too hot. You find few animal types around Antartica but many variations on these forms, e.g. lots of rockfish. (These guys don’t move very far over the course of their lifetimes, and what little swimming they do is by sculling.)
- As far as Antarctic food chains go, krill attract fish, which in turn attract seals, penguins and whales.
- Of the fish, Buckley became interested not just in lethal levels of stress but also responses to stress that affect growth and can shut it down.
- Why should scientists be interested in studying temperature stress? Because animals living today will have to deal a changing climate. For example, rockfish live about 30 years, and some born or young today will see and have to cope with years of warming waters in their lifetimes.
- What happens with temperature stress from too much heating? From biology, we know cellular membranes can melt, proteins can deform, reactive oxygen species can form, and DNA can become damaged.
- In response to the threat of temperature stress, cells have evolved a number of mechanisms to cope with heat — up to a point. Called the heat-shock response, a cell under stress from rising temperatures will increase its output of certain proteins that chaperone other proteins in the cell that have become deformed/damaged. These chaperones (called heat-shock proteins, or hsps) can help stabilize, reform/reshape/refold or degrade damaged proteins. Doing so prevents the damaged proteins from becoming a sticky mess that would clog up the cell and hinder all the things it must do to stay alive.
- Antarctic fish make hsps constantly, and you find them in various tissues, but the fish don’t have classic heat-shock responses. So Buckley was interested in heat stress experiments, but not using lethal levels of temperature stress. Instead, he looked at smaller increases in temperatures, along the lines of what the fish will experience under climate change — temperatures of 0º, 2º and 4º C (32º, 35.6º and 39.2º F). (Southern Ocean water temperatures can range from -2º to 10º C, or 28.4º to 50º F.) During field seasons, fish were caught, sacrificed, organs harvested and tissues tested for hsps.
- An accidental experiment occurred during this field work regarding the impact of temperature stress on reproduction. Female fish were regularly separated from males in tanks to control any spawning during testing. Females in one tank were mistakenly exposed to warming water. The fish behaved normally until 2º C, when all the females released their eggs. More research is needed to understand whether 2º C is a significant threshold and, if yes, why. (In a question-and-answer session after the talk, an audience member suggested that for a long-lived fish, like rockfish, with a 30-year life expectancy, spawning at a certain level of heat stress may be a survival tactic of jettisoning eggs that are costly metabolically to maintain. Doing so may help the adult live to spawn another day. And that’s in contrast to other strategies in which organisms that are dying make one last attempt at reproduction before the end, e.g. as in some plants and other organisms that produce large numbers of offspring with little parental care.)
- In cellular stress responses to heating, you can see several coping strategies occur, including the protection of cellular proteins (as in the classic heat-shock response and hsps). In addition, there can be increased metabolic output from cells, temporary arrest of the cell cycle and apoptosis (or induced or programmed cell death). The response to the stress can vary in two phases, whether acute (lasting hours to days) or chronic (days to months).
- While warm-water fish would undergo classic heat-shock response, Antarctic fish may have adapted to cope with heat by using cellular stress responses that involve cell cycle arrest and apoptosis. That’s what Buckley hypothesized in his work.
- Buckley and colleagues looked at the genes expressed during heat stress in emerald rockcod (Trematomus bernacchii) and found 210 genes expressed differently than in a warm-water goby. Ninety-four genes associated with cellular homeostasis were upregulated (their output increased) during the recovery phase from stress; this represented a signal for genes involved in DNA repair and apoptosis to be active.
- In testing his hypothesis, Buckley’s team used -2º C as a control temperature for the water that T. bernacchii specimens were in, and treatments were small incremental steps up from there (recall the 0º to 4º C of climate change above). After 24 hours in 2º C water, a statistically significant rise in apoptosis in T. bernacchii was observed. This is the lowest upper level of heat-stress-induced apoptosis recorded in any species, Buckley said.
- The 2º C level is also noteworthy because of the accident that occurred earlier with female fish, which were also rockfish. It indicates some kind of temperature threshold may exist for these Antarctic fish.
- Eventually, Buckley and team zeroed in on a regulatory protein (and the gene that codes for it) that appears to be important in the fish’s the stress response: C/EBP-δ (or CCAAT/enhander binding protein delta). This protein acts as a brake on the cell cycle (inducing cell cycle arrest, which was part of Buckley’s hypothesis) and was found to be strongly expressed in the fish’s white muscle. During times of stress, the presence of C/EBP-δ was turned up, and during the recovery phase after stress ended, production of the protein dropped.
- C/EBP-δ also may be important in mammalian cancers. Cancer generally is the product of unchecked cellular growth and profileration that occurs when the normal mechanisms that keep cells from freely multiplying are turned down or off. Therefore, understanding how C/EBP-δ works in Antarctic fishes’ responses to stress could provide insights into how it could be used to put a brake on cancer cell cycles.
- And this leads to a question often asked about basic research: Why study this? Why study Antarctic fish? Because it can lead to an understanding of something that can benefit people. In this case, the gene for C/EBP-δ also is turned down in breast tumors, so seeing how it works in Antarctic fish could generate new ideas for cancer treatments, such as using heating or cooling therapies that trigger the gene to be turned back up in breast tumors.
- Buckley said he’d like to get someone from Oregon Health and Science University interested in pursuing research about such potential therapies.