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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 pulex


Brian 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

.

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