Animal adaptation for an ever more acidic ocean

The level of carbon dioxide in the atmosphere has been increasing rapidly since industrialisation. One, often underreported, consequence of rising CO2 levels is that the ocean is absorbing much more of the stuff than it has for millions of years. As a result, our oceans and seas are becoming more acidic. Digging into the science reveals that we are now firmly in unchartered waters, and the consequences for marine life are likely to be severe.

In this piece we take a look at what ocean acidification actually is, and what consequences lay in store for a variety of key marine species as a result of it; from much loved lobsters and corals to lesser-known urchins and algae.

Photo by Brian Kinney / Shutterstock
Coral. Photo by Brian Kinney / Shutterstock

In the ocean, salt water has had an average pH of 8.2 for millions of years. However, since industrialisation, the oceans have absorbed 525 billion tons of CO2, the equivalent weight of 800 blue whales every day for over a century. When carbon is absorbed by the ocean, chemical reactions occur through CO2 molecules binding with water molecules to form carbonic acid. The carbonic acid is relatively weak but in large quantities releases substantial amounts of acidic hydrogen ions which reduce the pH of the ocean, making it more acidic. As a consequence ocean water acidity levels have risen by 25% from pre-industrialisation levels.

Hydrogen ions have a greater attraction to carbonate than calcium ions do and so these outcompete the calcium ions to bond with carbonate and this reduces the calcium carbonate ion concentration in the ocean. Consequently, shell building organisms are unable to extract as much of their needed shell building material—meaning their skeletons and homes are far weaker. If fossil fuels keep burning into the atmosphere unchecked then the ocean pH could drop substantially further and result in a huge increase in ocean acidity and up to a 30% reduction in calcium/shell building capability for marine organisms. The speed of this change puts the ocean into a zone of uncertainty, but how do we think marine organisms are adapting?

The losers: coral reefs are shell-shocked but can sea urchins cling on?

A black sea urchin. Photo by aquapix / Shutterstock
A black sea urchin. Photo by aquapix / Shutterstock

Calcifying species such as coral reefs and shelled organisms such as oysters and sea urchins will be drastically impacted by a more acidic ocean environment. Unfortunately, the speed at which the acidity of the ocean is changing does not give species much time in an evolutionary sense to keep up with the changing environmental pressure.

The last major acidification event, 55 million-years-ago, caused mass extinction. Some of the most vulnerable species are those with aragonite shells (a form of calcium carbonate) such as ‘sea butterflies’ in the Antarctic—shown to dissolve under acidic conditions. Weaker shells make species extremely vulnerable as they need to spend more energy on growing the shell, with less energy to spend on feeding. Mollusc shells such as oysters, snails and scallops have also been found to have stunted growth in acidic conditions and this will have ripple effects up the food chain. Elsewhere though, studies have shown some species are beginning to adapt. Purple sea urchins have shown a remarkable ability to up-regulate the genes involved in calcium transport to compensate for reduced availability of calcium carbonate, a trait which has not been found in corals.

The biggest losers of ocean acidification may be reef-building corals. Corals build their homes from calcium carbonate—forming reefs—and in doing so provide a rich biodiverse habitat for other organisms. The corals will end up in an arms race, with acidification limiting growth through corroding existing coral skeletons whilst simultaneously decelerating the growth of a new skeleton. Unfortunately, two other effects of climate change will combine to weaken the reefs substantially: erosion due to more frequent stormy weather, and rising sea temperatures exposing the reefs and leaving them more vulnerable to mass bleaching events. The crucial tipping point will be when reefs can no longer regenerate faster than they are being damaged. However, the effects of this will very much depend on the species of coral; luckily some are more resilient than others when it comes to tolerating a range of acidic conditions.

The winners: the grass is greener as crustaceans claw back

Seagrass. Photo by Ethan Daniels / Shutterstock
Seagrass. Photo by Ethan Daniels / Shutterstock

Bad news for some species can spell good fortune for others. Studies have shown that lobsters, crabs and shrimp actually fare better with higher acidity, building stronger shells more quickly and potentially benefiting from easier meals due to the acidity weakened shells of their molluscan prey, although the long term ecological effects of this change are uncertain.

The real winners will be the primary producers. Studies have shown sea grasses thrive with higher CO2 concentrations—they grow taller, quicker and reproduce better in more acidic waters and these habitats form crucial nursery grounds for fish and feeding grounds for turtles and sirenians. Seagrass habitats are under a lot of pressure globally due to being in vulnerable coastal waters, where pollution and fishing is particularly intensive, but more acidic waters could create more resilient seagrass habitats.

The fortune of algae is mixed; algae are the food source for a lot of zooplankton and so play a vital role in marine ecosystems. Whilst some algae grow better with more CO2 in the water, there are numerous species which build calcium carbonate shells and suffer similar problems to coral and molluscs. Studies have shown that coralline algae in particular contract in acidic conditions. However, because algae reproduce so quickly there is hope that they will be much quicker to adapt through building stronger shells. Another arms race will ensue between how quickly the algae can build resilience to more acidic conditions against the speed of ocean acidification.

The ecosystem as a whole

Sea turtle. Photo by Achimdiver / Shutterstock
Sea turtle. Photo by Achimdiver / Shutterstock

Ocean acidification combined with rising sea temperatures lead us into murky waters. There are genuine concerns for the future of habitats such as coral reefs which provide billions of dollars worth of ecosystem services and support an outstanding range of biodiversity. Elsewhere it appears that some species are going to struggle to adapt, whilst others have shown resilience and for the fortunate, a more acidic ocean may actually see a time of greater abundance. The full scale of these effects will be dictated by our ability to tackle climate change globally—the extreme scenario looks bleak as it will give all organisms precious little time to adapt. There will be winners and losers undoubtedly but the extent to which each is affected depends on our efforts to curb global carbon emissions as soon as possible.