Acidic oceans are corroding the tooth-like scales of shark skin



CT scan of a catshark hatchling head. Note the ridged scales.
Rory Cooper, Kyle Martin & Amin Garbout/Natural History Museum London, Author provided

Rory Cooper, University of Sheffield

Shark skin might look perfectly smooth, but inspect it under a microscope and you’ll notice something strange. The entire outer surface of a shark’s body is actually covered in sharp, little scales known as denticles. More remarkable still, these denticles are incredibly similar to human teeth, as they’re also comprised of dentine and enamel-like materials.

Your dentist will no doubt have warned you that acidic drinks like fizzy cola damage your teeth. This is because acid can dissolve the calcium and phosphate in the enamel tooth covering. For the first time, scientists have discovered a similar process acting on the tooth-like scales of sharks in the ocean.




Read more:
How we uncovered the feeding habits of sharks, thanks to plankton ‘post codes’


The carbon dioxide (CO₂) that humans release into the atmosphere doesn’t just heat the planet. As more of it dissolves in the ocean, it’s gradually increasing the acidity of seawater. In the past 200 years, the ocean has absorbed 525 billion tonnes of CO₂ and become 30% more acidic as a result. Now scientists worry that the lower pH is affecting one of the ocean’s top predators.

Denticles have sharp ridges and are arranged in an overlapping pattern, similar to chainmail.
Rory Cooper, Author provided

An unwelcome sea change

Over hundreds of millions of years, the denticles that make up shark skin have evolved to allow sharks to thrive in different environments. Different species have distinct denticle shapes and patterns that enable a range of remarkable functions. I’ve spent the last four years attempting to understand how the development of these scales is genetically controlled in shark embryos, and how their intricate details give each species an edge.

Denticles have highly specialised ridges which help reduce drag by up to 10%, allowing sharks to swim further and faster while using less energy. This works in a similar fashion to the ridges in the hulls of speed boats, which help the vessel move more efficiently through the water. In fact, these scales are so effective at reducing drag that scientists and engineers have long tried to create shark skin-inspired materials for boats and aircraft that can help them travel further on less fuel.

A catshark embryo about 80 days after fertilisation.
Rory Cooper, Kyle Martin & Amin Garbout/Natural History Museum London, Author provided

The patterning of denticles also works as a defensive armour, which protects sharks from their environment and from other predators. Some female sharks – such as the small-spotted catshark – have even developed a region of enlarged denticles which provide protection from a male shark’s bites during mating.

The changing chemistry of the ocean has been linked to coral bleaching, but its effect on other marine animals is less clear. To address this, researchers exposed puffadder shysharks – a species found off the coast of South Africa – to different levels of acidity in aquariums, and used a high-resolution imaging technique to examine the effect of acid exposure on their skin. After just nine weeks, they found that increased water acidity had weakened the surfaces of their denticles.

The puffadder shyshark (Haploblepharus edwardsii) is a slow moving species that lives on the sea floor.
Derekkeats/Wikipedia, CC BY-SA

Corrosion and weakening of the denticle surface could degrade the highly specialised drag-reducing ridges, affecting the ability of these sharks to swim and hunt. Many shark species are top-level predators, so if they’re not able to hunt as effectively, this might have an unpredictable impact on the population size of their prey and other animals in the complex marine environment. Some species of shark need to swim constantly to keep oxygen-rich water flowing over their gills and to expel CO₂ – another process which might be hindered by increased drag.




Read more:
Sharks: one in four habitats in remote open ocean threatened by longline fishing


Sharks belong to an ancient group of vertebrates known as the cartilaginous fishes, which split from the bony fishes – a lineage that later gave rise to humans – roughly 450 million years ago. Sharks, and other cartilaginous fish like rays, arose long before the dinosaurs, and have outlived multiple mass extinction events. But the rate of change in the marine environment over the last few centuries is an unprecedented challenge. These ancient predators may struggle to adapt to the fastest known change in ocean chemistry in the last 50 million years.The Conversation

Rory Cooper, PhD Researcher in Evolutionary Developmental Biology, University of Sheffield

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Meet the super corals that can handle acid, heat and suffocation


Resilient corals are offering hope for bleached reefs.
Emma Camp

Emma F Camp, University of Technology Sydney and David Suggett, University of Technology Sydney

Climate change is rapidly changing the oceans, driving coral reefs around the world to breaking point. Widely publicised marine heatwaves aren’t the only threat corals are facing: the seas are increasingly acidic, have less oxygen in them, and are gradually warming as a whole.

Each of these problems reduces coral growth and fitness, making it harder for reefs to recover from sudden events such as massive heatwaves.




Read more:
Acid oceans are shrinking plankton, fuelling faster climate change


Our research, published today in Marine Ecology Progress Series, investigates corals on the Great Barrier Reef that are surprisingly good at surviving in increasingly hostile waters. Finding out how these “super corals” can live in extreme environments may help us unlock the secret of coral resilience helping to save our iconic reefs.

Bleached coral in the Seychelles.
Emma Camp, Author provided

Coral conservation under climate change

The central cause of these problems is climate change, so the central solution is reducing carbon emissions. Unfortunately, this is not happening rapidly enough to help coral reefs, so scientists also need to explore more immediate conservation options.

To that end, many researchers have been looking at coral that manages to grow in typically hostile conditions, such as around tide pools and intertidal reef zones, trying to unlock how they become so resilient.

These extreme coral habitats are not only natural laboratories, they house a stockpile of extremely tolerant “super corals”.

What exactly is a super coral?

“Super coral” generally refers to species that can survive both extreme conditions and rapid changes in their environment. But “super” is not a very precise term!

Our previous research quantified these traits so other ecologists can more easily use super coral in conservation. There are a few things that need to be established to determine whether a coral is “super”:

  1. What hazard can the coral survive? For example, can it deal with high temperature, or acidic water?

  2. How long did the hazard last? Was it a short heatwave, or a long-term stressor such as ocean warming?

  3. Did the coral survive because of a quality such as genetic adaption, or was it tucked away in a particularly safe spot?

  4. How much area does the coral cover? Is it a small pocket of resilience, or a whole reef?

  5. Is the coral trading off other important qualities to survive in hazardous conditions?

  6. Is the coral super enough to survive the changes coming down the line? Is it likely to cope with future climate change?

If a coral ticks multiple boxes in this list, it’s a very robust species. Not only will it cope well in our changing oceans, we can also potentially distribute these super corals along vulnerable reefs.

Some corals cope surprisingly well in different conditions.
Emma Camp, Author provided

Mangroves are surprise reservoirs

We discovered mangrove lagoons near coral reefs can often house corals living in very extreme conditions – specifically, warm, more acidic and low oxygen seawater.

Previously we have reported corals living in extreme mangroves of the Seychelles, Indonesia, New Caledonia – and in our current study living on the Great Barrier Reef. We report diverse coral populations surviving in conditions more hostile than is predicted over the next 100 years of climate change.

Importantly, while some of these sites only have isolated populations, other areas have actively building reef frameworks.

Particularly significant were the two mangrove lagoons on the Great Barrier Reef. They housed 34 coral species, living in more acidic water with very little oxygen. Temperatures varied widely, over 7℃ in the period we studied – and included periods of very high temperatures that are known to cause stress in other corals.

Mangrove lagoons can contain coral that survives in extremely hostile environments, while nearby coral reefs bleach in marine heatwaves.
Emma Camp, Author provided

While coral cover was often low and the rate at which they build their skeleton was reduced, there were established coral colonies capable of surviving in these conditions.

The success of these corals reflect their ability to adapt to daily or weekly conditions, and also their flexible relationship with various symbiotic micro-algae that provide the coral with essential resources.

While we are still in the early phases of understanding exactly how these corals can aid conservation, extreme mangrove coral populations hold a reservoir of stress-hardened corals. Notably the geographic size of these mangrove locations are small, but they have a disproportionately high conservation value for reef systems.




Read more:
Heat-tolerant corals can create nurseries that are resistant to bleaching


However, identification of these pockets of extremely tolerant corals also challenge our understanding of coral resilience, and of the rate and extent with which coral species can resist stress.The Conversation

Emma F Camp, DECRA & UTS Chancellor’s Research Fellow, Climate Change Cluster, Future Reefs Research Programe, University of Technology Sydney and David Suggett, Associate Professor in Marine Biology, University of Technology Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Acid oceans are shrinking plankton, fuelling faster climate change



Researchers investigated how acidic oceans affect plankton in Prydz Bay, East Antarctica.
Daniel A. Nielsen, Author provided

Katherina Petrou, University of Technology Sydney and Daniel Nielsen, University of Technology Sydney

Increasingly acidic oceans are putting algae at risk, threatening the foundation of the entire marine food web.

Our research into the effects of CO₂-induced changes to microscopic ocean algae – called phytoplankton – was published today in Nature Climate Change. It has uncovered a previously unrecognised threat from ocean acidification.

In our study we discovered increased seawater acidity reduced Antarctic phytoplanktons’ ability to build strong cell walls, making them smaller and less effective at storing carbon. At current rates of seawater acidification, we could see this effect before the end of the century.




Read more:
Ocean acidification is already harming the Great Barrier Reef’s growth


What is ocean acidification?

Carbon dioxide emissions are not just altering our atmosphere. More than 40% of CO₂ emitted by people is absorbed by our oceans.

While reducing the CO₂ in our atmosphere is generally a good thing, the ugly consequence is this process makes seawater more acidic. Just as placing a tooth in a jar of cola will (eventually) dissolve it, increasingly acidic seawater has a devastating effect on organisms that build their bodies out of calcium, like corals and shellfish.

Many studies to date have therefore taken the perfectly logical step of studying the effects of seawater acidification on these “calcifying” creatures. However, we wanted to know if other, non-calcifying, species are at risk.

Diatoms in our oceans

Phytoplankton use photosynthesis to turn carbon in the atmosphere into carbon in their bodies. We looked at diatoms, a key group of phytoplankton responsible for 40% of this process in the ocean. Not only do they remove huge amounts of carbon, they also fuel entire marine food webs.

Diatoms use dissolved silica to build the walls of their cells. These dense, glass-like structures mean diatoms sink more quickly than other phytoplankton and therefore increase the transfer of carbon to the sea floor where it may be stored for millennia.

Diatoms are microscopic plant plankton that collectively remove huge amounts of carbon from the atmosphere.
Alyce M. Hancock, Author provided

This makes diatoms major players in the global carbon cycle. That’s why our team decided to look at how climate-change-driven ocean acidification might affect this process.

We exposed a natural Antarctic phytoplankton community to increasing levels of acidity. We then measured the rate at which the whole community used dissolved silica to build their cells, as well as the rates of individual species within the community.

More acid means less silicone

The more acidic the seawater, the more the diatom communities were made up of smaller species, reducing the total amount of silica they produced. Less silica means the diatoms aren’t heavy enough to sink quickly, reducing the rate at which they float down to the sea bed, safely storing carbon away from the atmosphere.

On examining individual cells, we found many of the species were highly sensitive to increased acidity, reducing their individual silicification rates by 35-80%. These results revealed not only are communities changing, but species that remain in the community are building less-dense cell walls.

Most alarming, many of the species were affected at ocean pH levels predicted for the end of this century, adding to a growing body of evidence showing significant ecological implications of climate change will take effect much sooner than previously anticipated.

Marine diversity is in decline

These losses in silica production could have far reaching consequences for the biology and chemistry of our oceans.

Many species affected are also an important component of the diet of the Antarctic krill, which is central to the Antarctic marine food web.

Fewer diatoms sinking to the ocean floor mean significant changes in silicon cycling and carbon burial. In a time when carbon drawn down by our ocean is crucial to helping sustain our atmospheric systems, any loss from this process will exacerbate CO₂ pollution.

Our new research adds yet another group of organisms to the list of climate change casualties. It emphasises the urgent need to reduce our dependency on fossil fuels.




Read more:
Our acid oceans will dissolve coral reef sands within decades


The only course of action to prevent catastrophic climate change is to stop emitting CO₂. We need to cut our emissions soon, if we hope to keep our oceans from becoming too acidic to sustain healthy marine ecosystems.The Conversation

Katherina Petrou, Senior Lecturer in Phytoplankton Ecophysiology, University of Technology Sydney and Daniel Nielsen, Casual Academic, University of Technology Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Our acid oceans will dissolve coral reef sands within decades



File 20180222 132650 xoo39k.jpg?ixlib=rb 1.1
Researchers studied reef sands at Heron Island, Hawaii, Bermuda and Tetiaroa. In this photo, white areas show the predominance of sand on reefs.
Southern Cross University

Bradley Eyre, Southern Cross University

Carbonate sands on coral reefs will start dissolving within about 30 years, on average, as oceans become more acidic, new research published today in Science shows.

Carbonate sands, which accumulate over thousands of years from the breakdown of coral and other reef organisms, are the building material for the frameworks of coral reefs and shallow reef environments like lagoons, reef flats and coral sand cays.

But these sands are sensitive to the chemical make-up of sea water. As oceans absorb carbon dioxide, they acidify – and at a certain point, carbonate sands simply start to dissolve.

The world’s oceans have absorbed around one-third of human-emitted carbon dioxide.

Carbonate sand is vulnerable

For a coral reef to grow or be maintained, the rate of carbonate production (plus any external sediment supply) must be greater than the loss through physical, chemical and biological erosion, transport and dissolution.

It is well known that ocean acidification reduces the amount of carbonate material produced by corals. Our work shows that reefs face a double-whammy: the amount of carbonate material produced will decrease, and the newly produced and stored carbonate sands will also dissolve.

Researchers used benthic chambers (pictured) to test how different levels of seawater acidity affect reef sediments.
Steve Dalton/Southern Cross University

We measured the impact of acidity on carbonate sands by placing underwater chambers over coral reefs sands at Heron Island, Hawaii, Bermuda and Tetiaroa in the Pacific and Atlantic Oceans. Some of the chambers were then acidified to represent future ocean conditions.

The rate at which the sands dissolve was strongly related to the acidity of the overlying seawater, and was ten times more sensitive than coral growth to ocean acidification. In other words, ocean acidification will impact the dissolution of coral reef sands more than the growth of corals.

This probably reflects the corals’ ability to modify their environment and partially adjust to ocean acidification, whereas the dissolution of sands is a geochemical process that cannot adapt.

Sands on all four reefs showed the same response to future ocean acidification, but the impact of ocean acidification on each reef is different due to different starting conditions. Carbonate sands in Hawaii are already dissolving due to ocean acidification, because this coral reef site is already disturbed by pollution from nutrients and organic matter from the land. The input of nutrients stimulates algal growth on the reef.

In contrast, carbonate sands in Tetiaroa are not dissolving under current ocean acidification because this site is almost pristine.

What will this mean for coral reefs?

Our modelling at 22 locations shows that net sand dissolution will vary for each reef. However, by the end of the century all but two reefs across the three ocean basins would on average experience net dissolution of the sands.

A transition to net sand dissolution will result in loss of material for building shallow reef habitats such as reef flats and lagoons and associated coral cays. What we don’t know is whether an entire reef will slowly erode or simply collapse, once the sediments become net dissolving, as the corals will still grow and create reef framework. Although they will most likely just slowly erode.

It may be possible to reduce the impact of ocean acidification on the dissolution of reef sands, by managing the impact of organic matter like algae at local and regional scales. This may provide some hope for some already disturbed reefs, but much more research on this topic is required.

The ConversationUltimately, the only way we can stop the oceans acidifying and the dissolving of coral reefs is concerted action to lower CO₂ emissions.

Bradley Eyre, Professor of Biogeochemistry, Director of the Centre for Coastal Biogeochemistry, Southern Cross University

This article was originally published on The Conversation. Read the original article.

Ocean acidification is already harming the Great Barrier Reef’s growth


Kennedy Wolfe, University of Sydney and Maria Byrne, University of Sydney

A new experiment on the Great Barrier Reef has shown, for the first time, that ocean acidification is already harming the growth of coral reefs in their natural setting.

As our research published in Nature today shows, the reduction in seawater pH – caused by carbon dioxide from human activities such as burning fossil fuels – is making it more difficult for corals to build and maintain their skeletons.

We and our colleagues, led by Rebecca Albright and Ken Calderia from the Carnegie Institution for Science in Stanford, California, carried out the first experimental manipulation of seawater chemistry in a natural coral reef ecosystem. Previous climate change studies on coral reefs have been done either in the laboratory or in closed-system tanks on the reef.

One Tree Island forms a naturally isolated lagoon where pH levels can be manipulated.
One Tree Island Research Station/University of Sydney/Nature

Coral reefs are particularly vulnerable to ocean acidification because calcium carbonate, the mineral building blocks of their skeletons, dissolves easily in acid. Below a certain pH, this dissolution is predicted to outweigh the accumulation of new calcium carbonate that allows reefs to grow and to recover from erosion processes such as storms.

Previous studies have shown large-scale declines in coral reefs over recent decades. Rates of reef calcification were 40% lower in 2008-09 than in 1975-76.

However, it was hard to pinpoint exactly how much of the decline was due to acidification, and how much was caused by other human-induced stresses such as ocean warming, pollution and overfishing. Understanding this is essential to predicting how coral reefs may fare in the face of continued global climate change.

The study used pink dye to track the movement of the experimental seawater.
Rebecca Albright/Nature

To answer this question, we manipulated the pH of seawater flowing over a reef flat at One Tree Island in the southern Great Barrier Reef. By adding sodium hydroxide (an alkali), we brought the reef’s pH closer to levels estimated for pre-industrial times, based on estimates of atmospheric carbon dioxide from that era. In doing so, we pushed the reef “back in time”, to find out how fast it would have been growing before human-induced acidification began.

It was clear from our results that reef calcification was around 7% higher under pre-industrial conditions than those experienced today.

Most other ocean acidification experiments manipulate seawater conditions based on the low pH levels predicted for coming decades, to understand the potential effects of future ocean conditions. But we have shown that present-day conditions are already taking their toll on corals.

As Albright explains:

Our work provides the first strong evidence from experiments on a natural ecosystem that ocean acidification is already causing reefs to grow more slowly than they did 100 years ago. Ocean acidification is already taking its toll on coral reef communities. This is no longer a fear for the future; it is the reality of today.

With greenhouse gas emissions continuing to rise, our results suggest a bleak future for coral reefs over the coming decades, with reduced calcification and increased dissolution. This is particularly concerning in light of the major coral bleaching events observed globally over the past few years amid prolonged high sea surface temperatures. The mixed effects of ocean warming and acidification, as well as other human-induced and natural stressors, pose serious threats to the ecosystems we know today.

Increasing the alkalinity of ocean water around coral reefs has been proposed as a geoengineering measure to save shallow marine ecosystems. Our results suggest that this could be effective in isolated areas, but implementing such measures at large scales would be almost impossible.

As our colleague Ken Caldeira has pointed out, the only real and lasting solution is to make deep, rapid cuts in our carbon dioxide emissions. Otherwise the next century could be one without coral reefs.

Kennedy Wolfe will be online to answer questions about this research from 11.30 am to 12.30 pm AEDT on Thursday February 25. Leave your comments below.

The Conversation

Kennedy Wolfe, PhD Candidate, University of Sydney and Maria Byrne, Professor of Developmental & Marine Biology, University of Sydney

This article was originally published on The Conversation. Read the original article.

The Great Barrier Reef faces a mixed future in acidifying oceans


Mathieu Mongin, CSIRO; Andrew Lenton, CSIRO; Jennifer Skerratt, CSIRO, and Mark Baird, CSIRO

Those of us who have been fortunate enough to have travelled to spectacular coral reefs marvel at their colour and biodiversity.

At around 2,000 km long, the Great Barrier Reef is the largest coral reef system in the world. It includes 3,581 individual reefs and an immense lagoon. But the likelihood of future generations being able to enjoy the beauty of the Great Barrier Reef is dwindling, as it comes under increasing pressure from the degradation of water quality and climate change.

Warming water is one of the greatest threats facing the reef in the long term. But what about another consequence of rising carbon dioxide, ocean acidification?

When carbon dioxide dissolves in water it (slightly) increases the water’s acidity, or lowers its pH. This affects the ability of marine creatures such crustaceans, corals and coralline algae to build their skeletons. But exactly how it will affect the whole reef ecosystem is unknown.

In research published in Nature Communications, we mapped parts of the reef that are most exposed to ocean acidification. As you’d expect, there will be some regions more strongly affected than others, indicating where we might focus our efforts to preserve the reef.

Building skeletons

Conditions in the marine tropics are becoming less friendly for coral. Coral bleaching, cyclones, outbreaks of pest species and nutrient-impacted river run-off are now regular events that impact coral reef health.

What’s more, and perhaps more ominously, as the world’s oceans take up more carbon dioxide, it becomes harder for corals to secrete and maintain their calcium carbonate skeletons. While the exact response remains unknown, at some point thresholds will be reached at which dissolution exceeds calcification, leading to overall coral loss.

But ocean acidification doesn’t affect the whole reef equally. Corals change the chemistry of the seawater around them. In fact, corals are constantly building and dissolving their skeletons, taking up and releasing calcium carbonate into the water, thus increasing or lowering the pH.

The fine balance between these processes changes over the course of the day. Ocean circulation, as well as photosynthesis and respiration of other non-calcifying marine organisms, also determine the overall variability in pH of water above reefs, and therefore a coral’s ability to produce and maintain their structure.

While scientists have researched these effects on individual reefs, how do they play out on the thousands of reefs that make up the entire Great Barrier Reef?

To find the answer we used a new information system developed for the Great Barrier Reef. We found that some inshore reefs experience a lower pH now than is projected for offshore reefs in the future.

Which reefs are most threatened?

On the Great Barrier Reef, the ability for coral to build skeletons tends to decrease towards the coast. This is a consequence of the lower pH, and more nutrients, fresh water and sediment coming from the land.

GBR Coral reef’s exposure to global ocean acidification, green reefs have some protection, white are neutral and red are already exposed.
CSIRO

But details of a more complex picture emerged from the study, highlighting the interaction between the thousands of reefs.

The outer reefs generally have Coral Sea water flowing over them, and for a thin band, especially in the north, their ability to build skeletons is actually driven by large scale oceanographic processes. But as the outer reef corals build their skeletons, the water flowing off them has lowered pH (more acidic). Circulation carries this water onto parts of the inner reefs, changing the average pH above their corals.

In other words, good coral health in the outer reefs, especially in the northern and southern regions, creates less favourable conditions for the mid lagoon central reefs.

What can we do?

While atmospheric carbon dioxide concentrations are increasing, focus should shift to conserve parts of the Great Barrier Reef and its corals which can be achieved through changes in the way we manage the reef. The new map of pH on the Great Barrier Reef presents the exposure to ocean acidification on each of the 3,581 reefs, providing managers with the information they need to tailor management to individual reefs.

Thus we see the Great Barrier Reef is not a singular reef nor a physical barrier that prevents exchange between reefs; it is a mixture of thousands of productive reefs and shallow areas lying on a continental shelf with complex oceanic circulation.

We cannot treat the Great Barrier Reef as one entity. We cannot summarise the impact of global ocean acidification as one number, and we cannot have one management strategy (aside from cutting global carbon emissions) to protect it.

The Conversation

Mathieu Mongin, Biogeochemical Modeller, CSIRO; Andrew Lenton, Senior Research Scientist, CSIRO Oceans and Atmosphere Flagship, CSIRO; Jennifer Skerratt, Coastal and enivronmental modeller, CSIRO, and Mark Baird, Team leader, Coastal and Environmental Modelling, CSIRO

This article was originally published on The Conversation. Read the original article.

Ocean acidification causes young corals to develop deformed skeletons


Taryn Foster, University of Western Australia and Peta Clode, University of Western Australia

Coral reefs around the world are facing a whole spectrum of human-induced disturbances that are affecting their ability to grow, reproduce and survive. These range from local pressures such as overfishing and sedimentation, to global ones such as ocean acidification and warming. With the third global coral bleaching event underway, we now more than ever, need to understand how coral responds to these stressors.

Our new research, published in Science Advances, now shows that young corals develop deformed and porous skeletons when they grow in more acidified waters, potentially making it more difficult for them to establish themselves on the reef and survive to adulthood.

Juvenile corals

Corals vary in their responses to stress, not only between species and location, but also among different stages of their life cycle. Juvenile corals are extremely important to the health of a reef, as they help to replenish the reef’s coral population and also help it recover from severe disturbances such as bleaching and storms.

However, newly settled young corals are small (typically about 1 mm across) and therefore very vulnerable to things like overgrowth and predation. To survive into adulthood they need to grow quickly out of this vulnerable size class. To do that they need to build a robust skeleton that can maintain its structural integrity during growth.

Two major factors that affect coral skeletal growth are ocean temperature and carbon dioxide concentration. Both are on the rise as we continue to emit huge amounts of CO₂ into the atmosphere. Generally with adult corals, increased temperature and CO₂ both reduce growth rates. But this varies considerably depending on the species and the environmental conditions to which the coral has been exposed.

Much less is known about the impacts of these factors on juvenile corals. This is mainly because their small size makes them more difficult to study, and they are only usually around once a year during the annual coral spawn. The corals we studied spawn for just a couple of hours, on one night of the year, meaning that our study hinged on taking samples during a crucial one-hour window.

When collecting the samples, at Western Australia’s Basile Island in the Houtman Abrolhos archipelago in March 2013, we watched the adult spawners each night waiting to see if they would spawn and, when they did, we worked all night fertilising the eggs to collect our juvenile samples.

Having collected our elusive coral samples, we cultured and grew newly settled coral recruits under temperature and CO₂ conditions that are expected to occur by the end of the century if no action is taken to curb the current trajectory of CO₂ emissions.

We then used three-dimensional X-ray microscopy to look at how these conditions affect the structure of the skeleton. This technique involves taking many X-ray projection images of the sample (in this case around 3,200) and then reconstructing them into a 3D image.

A 3D X-ray microscopy image of a one-month-old coral skeleton.
Taryn Foster/Science Advances, Author provided

Deformed and porous skeletons

Corals grown under high-CO₂ conditions not only showed reduced skeletal growth overall, but developed a range of skeletal deformities.

These included reduced overall size, gaps, over- and under-sized structures, and in some cases, large sections of skeleton completely missing. We also saw deep pitting and fractures in the skeletons of corals grown under high CO₂, typical of skeletal dissolution and structural fragility.

Surprisingly, increased temperature did not have a negative impact on skeletal growth and for some measures even appeared to help to offset the negative impacts of high CO₂ – a response we think may be unique to sub-tropical juveniles.

Nevertheless, our study highlights the vulnerability of juvenile corals to ocean acidification.

Under the current CO₂ emissions trajectory, our findings indicate that young corals will not be able to effectively build their skeletons. This could have wider implications for coral reef health, because without healthy new recruits, reefs will not replenish and will be less able to bounce back from disturbances.

The effect of temperature in this study however, was both a surprising and welcome finding. There is a lot of variation even between species, but it is possible that subtropical organisms have more plasticity due to their natural exposure to a wider range of conditions. This could indicate that subtropical juveniles may have an unexpected edge when it comes to ocean warming.

The Conversation

Taryn Foster, PhD Candidate, School of Earth and Environment, University of Western Australia and Peta Clode, Associate Professor, University of Western Australia

This article was originally published on The Conversation. Read the original article.