Can Road Salt and Other Pollutants Disrupt Our Circadian Rhythms?
By Jennifer Marie Hurley
Every winter, local governments across the U.S. apply millions of tons of road salt to keep streets navigable during snow and ice storms. Runoff from melting snow carries road salt into streams and lakes, and causes many bodies of water to have extraordinarily high salinity.
At Rensselaer Polytechnic Institute, my colleague Rick Relyea and his lab are working to quantify how increases in salinity affect ecosystems. Not surprisingly, they have found that high salinity has negative impacts on many species. They have also discovered that some species have the ability to cope with these increases in salinity.
But this ability comes at a price. In a recent study, Rick and I analyzed how a common species of zooplankton, Daphnia pulex, adapts to increasing levels of road salt. We found that this exposure affected an important biological rhythm: The circadian clock, which may govern Daphnia's feeding and predation avoidance behaviors. Since many fish prey on Daphnia, this effect could have ripples throughout entire ecosystems. Our work also raises questions about whether salt, or other environmental pollutants, could have similar impacts on the human circadian clock.
Daphnia pulexBrian Mattes, CC BY-ND
Daily biological rhythms and the circadian clock
In studying how road salt affects aquatic ecosystems, the Relyea lab showed that Daphnia pulex can adapt to handle moderate exposures in as little as two and a half months. These levels ranged from 15 milligrams of chloride (a building block of salt) per liter of water to a high of 1,000 milligrams per liter—a level found in highly contaminated lakes in North America.
However, an organism's ability to adapt to something in its environment can also be accompanied by negative trade-offs. My lab's collaboration with Rick's began in an effort to identify these trade-offs in salt-adapted Daphnia.
In my lab, we study how our circadian rhythms allow us to keep track of time. We investigate how the molecules in our cells work together to tick like a clock. These circadian rhythms allow an organism to anticipate 24-hour oscillations in its environment, such as changes from light (daytime) to dark (nighttime), and are essential to an organism's fitness.
Rick and I hypothesized that adaptation to high salinity could disrupt Daphnia's circadian rhythms based on recent evidence showing that other environmental contaminants can disrupt circadian behavior. One important behavior in Daphnia that may be controlled by the circadian clock is the diel vertical migration—the largest daily biomass migration on Earth, which occurs in oceans, bays and lakes. Plankton and fish migrate down to deeper water during the day to avoid predators and sun damage, and back up toward the surface at night to feed.
Echogram illustrating the ascending and descending phases of diel vertical migration, in which organisms ascend and descend through the water column. The color scale reflects acoustic scattering by concentrations of organisms at different depths. DEEP SEARCH—BOEM, USGS, NOAA
Given what we know about circadian function, it would be logical to assume that exposure to pollution would not affect an organism's circadian rhythms. While circadian clocks can incorporate environmental information to tell the time of day, they are heavily buffered against most environmental effects.
To understand the importance of this buffering, imagine that the timing of an organism's day length responded to environmental temperature. Heat speeds up molecular reactions, so on hot days the organism's 24-hour rhythm could become 20 hours, and on cold days it might become 28 hours. In essence, the organism would have a thermometer, not a clock.
Adaptation to pollution affects key circadian genes
To determine whether clock disruption is a trade-off to pollutant adaptation, we first had to establish that Daphnia is governed by a circadian clock. To do this, we identified genes in Daphnia that are similar to two genes, known as period and clock, in an organism that serves as a circadian model system: Drosophila melanogaster, the common fruit fly.
We tracked the levels of period and clock in Daphnia, keeping the organisms in constant darkness to ensure that a light stimulus did not affect these levels. Our data showed that the levels of period and clock varied over time with a 24-hour rhythm—a clear indication that Daphnia have a functional circadian clock.
We also tracked the same genes in populations of Daphnia that had adapted to increased salinity. Much to my surprise, we discovered that the daily variation of period and clock levels deteriorated directly with the level of salinity the Daphnia were adapted to. In other words, as Daphnia adapted to higher salinity levels, they showed less variation in the levels of period and clock over the day. This demonstrated that Daphnia's clock is indeed affected by pollutant exposure.
We currently don't understand what causes this effect, but the relationship between salinity levels and decreased variation in the levels of period and clock offers a clue. We know that exposure to pollutants causes Daphnia to undergo epigenetic regulation—chemical changes that affect the function of their genes, without altering their DNA. And epigenetic changes often show a gradual response, becoming more pronounced as the causal factor increases. Therefore, it is likely that high salinity is inducing chemical changes through these epigenetic mechanisms in Daphnia to suppress the function of its circadian clock.
The broad effects of circadian clock disruptions
We know that environmental conditions can affect what the clock regulates in many species. For example, changing the sugar that the fungus Neurospora crassa grows on changes which behaviors the clock regulates. But to our knowledge, this study is the first to show that genes of an organism's core clock can be directly impacted by adapting to an environmental contaminant. Our finding suggests that just as the gears of a mechanical clock can rust over time, the circadian clock can be permanently impacted by environmental exposure.
This research has important implications. First, if Daphnia's circadian clock regulates its participation in the diel vertical migration, then disrupting the clock could mean that Daphnia do not migrate in the water column. Daphnia are key consumers of algae and a food source for many fish, so disrupting their circadian rhythms could affect entire ecosystems.
Second, our findings indicate that environmental pollution may have broader effects on humans than previously understood. The genes and processes in Daphnia's clock are very similar to those that regulate the clock in humans. Our circadian rhythms control genes that create cellular oscillations affecting cell function, division and growth, along with physiological parameters such as body temperature and immune responses.
The human circadian clock regulates the cycles of many bodily functions.NIH
When these rhythms are disrupted in humans, we see increased rates of cancer, diabetes, obesity, heart disease, depression and many other diseases. Our work suggests that exposure to environmental pollutants may be depressing the function of human clocks, which could lead to increased rates of disease.
We are continuing our work by studying how the disruption of Daphnia's clock affects its participation in the diel vertical migration. We are also working to determine the underlying causes of these changes, to establish whether and how this could happen in the human brain. The impacts we have found in Daphnia show that even a simple substance such as salt can have extremely complex effects on living organisms.
Reposted with permission from our media associate The Conversation.
By Dana M Bergstrom, Euan Ritchie, Lesley Hughes and Michael Depledge
In 1992, 1,700 scientists warned that human beings and the natural world were "on a collision course." Seventeen years later, scientists described planetary boundaries within which humans and other life could have a "safe space to operate." These are environmental thresholds, such as the amount of carbon dioxide in the atmosphere and changes in land use.
The Good and Bad News<p><span>Ecosystems consist of living and non-living components, and their interactions. They work like a super-complex engine: when some components are removed or stop working, knock-on consequences can lead to system failure.</span></p><p>Our study is based on measured data and observations, not modeling or predictions for the future. Encouragingly, not all ecosystems we examined have collapsed across their entire range. We still have, for instance, some intact reefs on the Great Barrier Reef, especially in deeper waters. And northern Australia has some of the most intact and least-modified stretches of savanna woodlands on Earth.</p><p><span>Still, collapses are happening, including in regions critical for growing food. This includes the </span><a href="https://www.mdba.gov.au/importance-murray-darling-basin/where-basin" target="_blank" rel="noopener noreferrer">Murray-Darling Basin</a><span>, which covers around 14% of Australia's landmass. Its rivers and other freshwater systems support more than </span><a href="https://www.abs.gov.au/ausstats/[email protected]/latestproducts/94F2007584736094CA2574A50014B1B6?opendocument" target="_blank" rel="noopener noreferrer">30% of Australia's food</a><span> production.</span></p><p><span></span><span>The effects of floods, fires, heatwaves and storms do not stop at farm gates; they're felt equally in agricultural areas and natural ecosystems. We shouldn't forget how towns ran out of </span><a href="https://www.mdba.gov.au/issues-murray-darling-basin/drought#effects" target="_blank" rel="noopener noreferrer">drinking water</a><span> during the recent drought.</span></p><p><span></span><span>Drinking water is also at risk when ecosystems collapse in our water catchments. In Victoria, for example, the degradation of giant </span><a href="https://theconversation.com/logging-must-stop-in-melbournes-biggest-water-supply-catchment-106922" target="_blank" rel="noopener noreferrer">Mountain Ash forests</a><span> greatly reduces the amount of water flowing through the Thompson catchment, threatening nearly five million people's drinking water in Melbourne.</span></p><p>This is a dire <em data-redactor-tag="em">wake-up</em> call — not just a <em data-redactor-tag="em">warning</em>. Put bluntly, current changes across the continent, and their potential outcomes, pose an existential threat to our survival, and other life we share environments with.</p><p><span>In investigating patterns of collapse, we found most ecosystems experience multiple, concurrent pressures from both global climate change and regional human impacts (such as land clearing). Pressures are often </span><a href="https://besjournals.onlinelibrary.wiley.com/doi/abs/10.1111/1365-2664.13427" target="_blank" rel="noopener noreferrer">additive and extreme</a><span>.</span></p><p>Take the last 11 years in Western Australia as an example.</p><p>In the summer of 2010 and 2011, a <a href="https://theconversation.com/marine-heatwaves-are-getting-hotter-lasting-longer-and-doing-more-damage-95637" target="_blank" rel="noopener noreferrer">heatwave</a> spanning more than 300,000 square kilometers ravaged both marine and land ecosystems. The extreme heat devastated forests and woodlands, kelp forests, seagrass meadows and coral reefs. This catastrophe was followed by two cyclones.</p><p>A record-breaking, marine heatwave in late 2019 dealt a further blow. And another marine heatwave is predicted for <a href="https://www.theguardian.com/environment/2020/dec/24/wa-coastline-facing-marine-heatwave-in-early-2021-csiro-predicts" target="_blank" rel="noopener noreferrer">this April</a>.</p>
What to Do About It?<p><span>Our brains trust comprises 38 experts from 21 universities, CSIRO and the federal Department of Agriculture Water and Environment. Beyond quantifying and reporting more doom and gloom, we asked the question: what can be done?</span></p><p>We devised a simple but tractable scheme called the 3As:</p><ul><li>Awareness of what is important</li><li>Anticipation of what is coming down the line</li><li>Action to stop the pressures or deal with impacts.</li></ul><p>In our paper, we identify positive actions to help protect or restore ecosystems. Many are already happening. In some cases, ecosystems might be better left to recover by themselves, such as coral after a cyclone.</p><p>In other cases, active human intervention will be required – for example, placing artificial nesting boxes for Carnaby's black cockatoos in areas where old trees have been <a href="https://www.environment.gov.au/biodiversity/threatened/publications/factsheet-carnabys-black-cockatoo-calyptorhynchus-latirostris" target="_blank" rel="noopener noreferrer">removed</a>.</p><p><span>"Future-ready" actions are also vital. This includes reinstating </span><a href="https://www.abc.net.au/gardening/factsheets/a-burning-question-fire/12395700" target="_blank" rel="noopener noreferrer">cultural burning practices</a><span>, which have </span><a href="https://theconversation.com/australia-you-have-unfinished-business-its-time-to-let-our-fire-people-care-for-this-land-135196" target="_blank" rel="noopener noreferrer">multiple values and benefits for Aboriginal communities</a><span> and can help minimize the risk and strength of bushfires.</span></p><p>It might also include replanting banks along the Murray River with species better suited to <a href="https://www.abc.net.au/gardening/factsheets/my-garden-path---matt-hansen/12322978" target="_blank" rel="noopener noreferrer">warmer conditions</a>.</p><p>Some actions may be small and localized, but have substantial positive benefits.</p><p>For example, billions of migrating Bogong moths, the main summer food for critically endangered mountain pygmy possums, have not arrived in their typical numbers in Australian alpine regions in recent years. This was further exacerbated by the <a href="https://theconversation.com/six-million-hectares-of-threatened-species-habitat-up-in-smoke-129438" target="_blank" rel="noopener noreferrer">2019-20</a> fires. Brilliantly, <a href="https://www.zoo.org.au/" target="_blank" rel="noopener noreferrer">Zoos Victoria</a> anticipated this pressure and developed supplementary food — <a href="https://theconversation.com/looks-like-an-anzac-biscuit-tastes-like-a-protein-bar-bogong-bikkies-help-mountain-pygmy-possums-after-fire-131045" target="_blank" rel="noopener noreferrer">Bogong bikkies</a>.</p><p><span>Other more challenging, global or large-scale actions must address the </span><a href="https://www.youtube.com/watch?v=iICpI9H0GkU&t=34s" target="_blank" rel="noopener noreferrer">root cause of environmental threats</a><span>, such as </span><a href="https://www.nature.com/articles/s41559-018-0504-8" target="_blank" rel="noopener noreferrer">human population growth and per-capita consumption</a><span> of environmental resources.</span><br></p><p>We must rapidly reduce greenhouse gas emissions to net-zero, remove or suppress invasive species such as <a href="https://onlinelibrary.wiley.com/doi/abs/10.1111/mam.12080" target="_blank" rel="noopener noreferrer">feral cats</a> and <a href="https://theconversation.com/the-buffel-kerfuffle-how-one-species-quietly-destroys-native-wildlife-and-cultural-sites-in-arid-australia-149456" target="_blank" rel="noopener noreferrer">buffel grass</a>, and stop widespread <a href="https://theconversation.com/to-reduce-fire-risk-and-meet-climate-targets-over-300-scientists-call-for-stronger-land-clearing-laws-113172" target="_blank" rel="noopener noreferrer">land clearing</a> and other forms of habitat destruction.</p>
Our Lives Depend On It<p>The multiple ecosystem collapses we have documented in Australia are a harbinger for <a href="https://www.iucn.org/news/protected-areas/202102/natures-future-our-future-world-speaks" target="_blank">environments globally</a>.</p><p>The simplicity of the 3As is to show people <em>can</em> do something positive, either at the local level of a landcare group, or at the level of government departments and conservation agencies.</p><p>Our lives and those of our <a href="https://theconversation.com/children-are-our-future-and-the-planets-heres-how-you-can-teach-them-to-take-care-of-it-113759" target="_blank">children</a>, as well as our <a href="https://theconversation.com/taking-care-of-business-the-private-sector-is-waking-up-to-natures-value-153786" target="_blank" rel="noopener noreferrer">economies</a>, societies and <a href="https://theconversation.com/to-address-the-ecological-crisis-aboriginal-peoples-must-be-restored-as-custodians-of-country-108594" target="_blank" rel="noopener noreferrer">cultures</a>, depend on it.</p><p>We simply cannot afford any further delay.</p><p><em><a rel="noopener noreferrer" href="https://theconversation.com/profiles/dana-m-bergstrom-1008495" target="_blank" style="">Dana M Bergstrom</a> is a principal research scientist at the University of Wollongong. <a rel="noopener noreferrer" href="https://theconversation.com/profiles/euan-ritchie-735" target="_blank" style="">Euan Ritchie</a> is a professor in Wildlife Ecology and Conservation, Centre for Integrative Ecology, School of Life & Environmental Sciences at Deakin University. <a rel="noopener noreferrer" href="https://theconversation.com/profiles/lesley-hughes-5823" target="_blank">Lesley Hughes</a> is a professor at the Department of Biological Sciences at Macquarie University. <a rel="noopener noreferrer" href="https://theconversation.com/profiles/michael-depledge-114659" target="_blank">Michael Depledge</a> is a professor and chair, Environment and Human Health, at the University of Exeter. </em></p><p><em>Disclosure statements: Dana Bergstrom works for the Australian Antarctic Division and is a Visiting Fellow at the University of Wollongong. Her research including fieldwork on Macquarie Island and in Antarctica was supported by the Australian Antarctic Division.</em></p><p><em>Euan Ritchie receives funding from the Australian Research Council, The Australia and Pacific Science Foundation, Australian Geographic, Parks Victoria, Department of Environment, Land, Water and Planning, and the Bushfire and Natural Hazards CRC. Euan Ritchie is a Director (Media Working Group) of the Ecological Society of Australia, and a member of the Australian Mammal Society.</em></p><p><em>Lesley Hughes receives funding from the Australian Research Council. She is a Councillor with the Climate Council of Australia, a member of the Wentworth Group of Concerned Scientists and a Director of WWF-Australia.</em></p><p><em>Michael Depledge does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</em></p><p><em>Reposted with permission from <a href="https://theconversation.com/existential-threat-to-our-survival-see-the-19-australian-ecosystems-already-collapsing-154077" target="_blank" style="">The Conversation</a>. </em></p>
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