Rainforests of the ocean. Jewels of the sea. Coral reefs are known by many names, and almost all of these hint at their incredible richness of life: more than a quarter of the world’s marine life can be found living in, on or around coral reefs, and these ecosystems take up less than one percent of the ocean’s area! Millions of people depend on coral reef ecosystems for their livelihoods, which provide us with a critical source of protein, a massive tourism market, coastal protection, and even medicines.
Unfortunately, our advancements in industry and globalization have had a devastating impact on our environment, and coral reefs around the globe are not doing very well. Overfishing, ocean acidification, global warming, and disease – among other phenomena that are caused by us – have decimated these ecosystems: an estimated 80% of Caribbean coral reefs alone have been destroyed beyond repair. There are very few, if any, reefs on earth that are not negatively affected in some way by our actions.
Among all these factors, the impacts of heavy metal pollution have been given the least attention, but may present more of a threat than most people think. Let’s talk about corals reefs and metals.
You are probably familiar in some way with a few or more of the ‘heavy metals’: things like lead, copper, nickel, iron, zinc, and mercury all fall into this category. Most of these metals are essential for life in very small amounts: we have iron in our blood, copper in our enzymes, and zinc helps regulate our immune system.
Above a certain threshold, though, these metals become extremely toxic, and thus, healthy environments usually are characterized by low metal concentrations. Unfortunately, our entry into the industrial revolution a century and a half ago has led to huge discharges of metals into global ecosystems, wet and dry. This form of pollution makes it harder for life to grow, thrive, and reproduce, and ecosystems with high biodiversity are under the greatest threat.
Heavy metals like copper and nickel make their way into the ocean through various sources, most of which have to do with run-off, sewage, and fertilizers from agriculture. Island nations bordered by coral reefs often have well-developed mining industries, and tend to release all of their heavy metal-rich wastes directly into the ocean instead of processing them, which is expensive. The manufacture of things like batteries, cosmetics, and pharmaceuticals all lead to huge amounts of heavy metal wastes being pumped into our oceans as well.
Among the most widespread and worst of these is copper, which is driving a little-known but serious crisis in the underwater world.
One of the largest sources of copper comes from antifouling paints: the toxic paint we cover the bottom of our boats with to make sure animals like oysters and barnacles don’t latch on. All kinds of growth on the bottom of a boat creates drag in the water, causing a boat to use more fuel, so antifouling paints do actually provide us with a critical service, and save millions of dollars in fuel every year in the maritime industry.
But this paint comes at a cost: over time, flecks of it leach off boat hulls and end up in the water column. Copper in this dissolved state is highly toxic, and because coral reef ecosystems experience more boat traffic than most marine environments (for tourism, fishing and research) some of the worst effects of it have been seen on coral reefs.
Copper causes toxicity to corals in a variety of ways. It creates oxidative stress, producing highly reactive molecules called free radicals that damage cells and degrade DNA. It’s been shown to inhibit the growth of various coral species at relatively low concentrations, and interferes with the coral’s ability to deposit its carbonate skeleton. Copper leads to a decrease in coral fertilization and inhibits the ability of coral larvae, the most sensitive stage of coral development, to settle onto the reef. In high enough concentrations, copper kills coral larvae outright, and even decreases the efficiency of photosynthesis of their symbionts. The list goes on.
It should not be overlooked or understated: copper, in the levels we are pumping it into the ocean, is a poison, and it is contributing to ecosystem degradation all over the globe.
The good news is that many organisms, including corals, seem to possess mechanisms for dealing with increasing copper levels. Some anemones have been shown to produce more mucus in the presence of heavy metals, and this mucus is able to bind the heavy metals in harmless complexes before they can do any damage. Certain coral symbionts (the little algae living within coral tissues that provide them with food) collect the heavy metals in their cells, allowing the coral to selectively expel algae containing high levels and rid themselves of the toxins. Bacterial communities in the coral have even been shown to have a protective function. But the important thing here is that these defensive measures are not foolproof: above a certain threshold, corals exposed to heavy metal contamination will die.
We are desperately in need of research that identifies thresholds of toxicity in these key organisms, so that we can determine and enforce water quality guidelines that will protect marine life in the long term.
I recently worked on a project with NOAA that aimed at doing just that. My research project had two goals: the first was to determine how a threatened Caribbean coral, Acropora cervicornis, responded to different levels of copper in order to determine at what levels the metal became toxic. The second goal of the research was to determine if different genotypes, or genetically differing variants of the same species, of this coral responded differently: Caribbean coral restoration efforts search for coral genotypes that are hardier and more resilient (using mechanisms like the ones listed above), so they can grow and plant more of these back out onto the reef.
We were able to accomplish both these aims, finding that some coral genotypes were more resistant to the copper exposure than others, and determined the copper toxicity threshold to be within the current US water quality guidelines – this is good news.
My project was able to confirm that at relatively low concentrations, copper inhibits the healing of coral tissue after being damaged, lowers the efficiency of photosynthesis of the algal symbionts, and causes bleaching. At higher concentrations, copper killed the corals outright. Unfortunately, all of the concentrations used in these experiments have been measured in highly polluted areas, so it is not unrealistic to see these effects in the oceanic environment.
The ultimate goal of research like this is to someday develop to a trait-based approach towards identifying resilient genotypes, such as finding those that can grow and heal faster (and could, for example, recover faster after a hurricane), or are resistant to things like bleaching and disease. Identifying and growing these genotypes of coral that are more capable of withstanding and adapting to changing ocean conditions could help increase the survival of outplanted corals in extremely degraded reef areas like the Caribbean, which we are now confident will not recover on their own.
While each successive research project tells us more about corals and their ability to adapt to and overcome stress, the battle is far from won. Ocean acidification, another major threat to the oceanic environment, is changing ocean chemistry, and is actually making copper and other heavy metals more bioavailable: copper is relatively harmless when it is bound up or attached to other molecules, but a more acidic ocean will keep it dissolved in the water column in a state where it can inflict the most damage.
In the next century, concentrations of copper are expected to increase significantly due to this phenomenon, and will only make the current problem worse. Nations all over the globe continue to ignore it, but the research and the effects are black and white: with increased copper input, the health of coral reefs will continue to decline, as will their ability to recover from natural fluctuations in the marine environment that will be more frequent due to climate change. There is simply no time left to do nothing.
Ultimately, while I do not expect that we will be stopping our use of copper-based antifouling paints any time soon, we are not powerless in this fight. We can encourage our legislators to make water quality guidelines stricter, minimizing the risk to tropical reef ecosystems, and to fund scientific research that studies the effects of our actions on coral reef environments. With enough scientific support, copper-based antifouling paints may someday be banned, similar to how other toxic substances have been in the past. But a good first step is simple awareness; it is our responsibility to at the very least understand how our actions are negatively impacting the world around us, and what we might be able to do to minimize that impact.