Rising Ocean Temperatures Alter Their Environment
News — Oysters are in peril. In recent decades, these edible ectotherms have suffered unrelenting attacks from within and without. Parasitic diseases have decimated oyster populations off of the Carolina coast. Rising ocean temperatures have altered their environment and affected their physiology.
The oyster has been a staple in North Carolina's aquaculture economy and a lynchpin in the complex ecosystem of the state's estuaries.
According to the N.C. Division of Marine Fisheries, more than 260,000 pounds of oyster meat was harvested in 2003 at a value of more than $1.02 million compared to 724,000 pounds in 1983 valued at more than $1.12 million.
North Carolina oyster harvests peaked at 1.8 million bushels in 1902; in 2008, the harvest drew around 40,000 bushels.
In addition to their economic import, oysters are essential to North Carolina's water system. Oysters form elaborate reefs that house sea life. The oysters settle on top of one another to create habitats for more than 300 species of plants and animals. With their built-in biofiltration systems, the oysters cleanse the water, removing excess algae and vegetation.
Since the start of the 20th Century, oyster reefs have declined by more than 99 percent in North Carolina. The decline is a consequence of man-made conditions, including encroachment of development on oyster habitats, and pollution from industry, agriculture and human waste.
As a result, oyster restoration and protection has become a priority of state policy makers and universities.
In a lab at UNC Charlotte, professor of biological sciences Inna Sokolova studies these creatures that have managed for millennia to adapt and survive in extreme conditions. Fifteen-gallon salt water tanks containing hundreds of oysters rise from the laboratory floor. Sokolova plays the part of the not-so-benevolent dictator; she controls the water temperature, she introduces harmful pathogens to her subjects, charting the changes that result in their biochemistry.
Over millions of years, the mollusks have developed finely tuned systems to neutralize dangerous substances such as heavy metals in the seawater they filter every day, Sokolova explained.
Sokolova and colleagues made headlines last year when they demonstrated that marine organisms become more sensitive to pollution as ocean temperatures rise. The researchers studied the effects of increasing temperatures and heavy metals by measuring Eastern oysters' standard metabolic rates.
The heavy metal cadmium, found in paints, dyes, batteries and other waste, poses a threat to North Carolina water ways. The introduction of the metal into the oyster's system triggers a complex molecular defense mechanism that makes the metal unable to enter the oyster's cells.
To test the relationship between temperature and the oysters' defense system, the researchers monitored the oysters at different temperatures, both in clean sea water and in the presence of cadmium.
Sokolova found at elevated temperatures the oyster defense systems falter. Consequently, the oysters absorbed the cadmium and other toxins more quickly and the toxicity of the cadmium increased.
Though the oysters remain able to detoxify themselves enough to remain alive, the energy they expend doing so is energy they are unable to expend on other important processes. It takes all of their energy just to survive.
According to Sokolova, as they struggle to defend themselves against the toxic onslaught, the oysters are left with an energy deficit. Thus, they lack the extra energy they need to invest in growth and protect themselves against repeated exposure to toxins.
Dermo devastates oyster populations.
The stress brought on by increasing water temperatures could be responsible for recent disease outbreaks among Eastern oysters. Sokolova has turned her attention to the ways in which stress on the oyster's metabolic process affects how they cope with a common parasitic disease called Dermo.
The invasiveness and abundance of the parasite and the host's ability to ward the parasite off determine the outcome of the disease " namely, survival or death.
In the past, flare ups of Dermo have caused concern, but infected oyster populations have always adapted, defeating the disease. That's why scientists, fishermen and policymakers view the Dermo-related deaths of millions of oysters along the Eastern seaboard in recent years as cause for alarm.
Sokolova points to metabolic dysfunction as a contributing factor to the oyster's increased disease susceptibility. Simply put, a substantial amount of energy must be expended to maintain most immune functions. The oysters are expending energy to ward off the effects of increasing water temperatures, leaving less energy for immunity.
"On top of this, if the parasite can better and faster proliferate in the warmth, the balance can be tipped towards disease," Sokolova said.
When an infected oyster dies, more parasites are released into the ocean, facilitating the spread of the disease. Oysters employ two weapons in the fight against Dermo. Either their immune cells engulf the parasite and destroy it, or the cells commit suicide (a process known as apoptosis) to stop the disease from spreading.
For reasons that remain mysterious, unlike their counterparts in the Gulf of Mexico, Eastern oysters cannot defend themselves by virtue of apoptosis. Sokolova and her team are investigating ways to kick-start this crucial defense mechanism.
First, Sokolova infects the oyster cells with parasites. She then introduces metals that might induce apoptosis. Copper, zinc and cadmium are on the docket for review, but Sokolova cautions, "The problem with cadmium is that it's a very toxic metal. You don't want to cure your oysters with cadmium because then you cannot sell them and you cannot consume them."
And keep in mind that because a metal works " copper has shown the most promise " it would be impractical to release it into natural oyster habitats because it would upset the ecosystem. However, North Carolina's cultured oyster farmers stand to benefit from Sokolova's research.
It takes about 12 to 18 months for the oysters to reach market size. It only takes days for Dermo to devastate an oyster crop, killing the oysters before they are mature enough to be harvested.
If Sokolova's approach proves successful, the cultured oysters, which are grown in cages on top of the water, would be removed from their cages, given a copper bath and placed back into the water. The remaining copper would be washed out of their system by the time they are ready to be sold for consumption.
While Sokolova is hopeful her research will provide a viable solution for the aquaculture industry, she warns that the effects of rising temperatures and heavy metal contamination on metabolic chemistry will likely not be limited to oysters. After all, many cold-blooded marine animals, including clams, crabs and some fish have similar defense systems. Sokolova worries these creatures could be at risk as water temperatures rise.
Sokolova advocates for further research into the interactive effects between pollutant toxicity and elevated temperatures. Ultimately, she would like to see pollution levels reduced and a more stringent set of water quality criteria that takes into account the effects of climate change.
While it is unclear whether the oyster will succumb to the effects of climate change or somehow adapt, it is clear that this ectotherm that has been a staple in the economy and a lynchpin in the ecosystem has become a bellwether for sea life everywhere.
About UNC Charlotte
A public research university, UNC Charlotte is the fourth largest campus among the 17 institutions of The University of North Carolina system. It is the largest institution of higher education in the Charlotte region offering doctoral, master's and bachelor's programs. Fall 2008 enrollment numbered 23,300 students, including nearly 5,000 graduate students.
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