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Research good news for coralline algae, but not necessarily for reefs

As Doha disappoints on delivering any real progress on reducing global CO2 emissions, new research demonstrates that a key component of coral reef structures may be more resilient in the face of increasing…

Everyone wants good news about coral, but we shouldn’t misinterpret the latest findings. Phil Camill

As Doha disappoints on delivering any real progress on reducing global CO2 emissions, new research demonstrates that a key component of coral reef structures may be more resilient in the face of increasing CO2 levels, and consequent declining seawater pH, than was previously thought.

However, while this is a good news story for the reefs, it does not mean that the entire reef is going to survive the negative impacts of ocean acidification associated with rising atmospheric CO2.

While investigating the mineral structure of a common coral reef alga, Porolithon onkodes, we found that there was an extra mineral, dolomite, present in many of the algae collected from the high-energy reef front environment.

These algae are known as coralline algae because, similarly to corals, they produce a carbonate skeleton. Corals produce a carbonate mineral CaCO3 called aragonite. The coralline algae form magnesium calcite, a mineral that is mostly CaCO3 but with 10-20% substitution by magnesium for calcium.

In many reefs the upper-most reef front structure is predominantly built by coralline algae, as delicate branching corals cannot develop in this wave break zone. Therefore, discovering that this algae looks to be a survivor under higher CO2 scenarios is most definitely good news, particularly for tropical island communities that are protected from high energy waves by these algal ridges.

The discovery that these algae produce dolomite, which is 50% magnesium instead of calcium and chemically very stable, was in itself an exciting discovery. Dolomite is most familiar to people as the mineral that gives the Dolomite Alps of Italy their name: dolomitized fossil coral reefs dominate the alps.

This dolomitization process was thought to be a chemical alteration of the reef limestone that took place long after a coral reef had died. Discovering that living algae in modern coral reefs can form this dolomite prolifically meant reconsidering what we thought we knew about the chemical stability of these algae.

Our experiments showed that dolomite corallines had 6-10 times less dissolution of the skeleton compared to the coralline algae without dolomite. Although dissolution increased in the high CO2 water, the total rate was still minimal. We found that dolomite was common in algae from many tropical reefs, but it seems to be restricted to the shallow, highest energy parts of the reef.

What does this finding mean for the Great Barrier Reef and other tropical reefs? First, I must make clear that our research related to coralline algae, not corals.

The Canberra Times and The Age ran the story with the headings “Coral may be climate change’s silver lining” and “Resistant algae good news for coral”. Considering our research was actually on algae, not coral, the Age has the most accurate headline. We are sadly accustomed to the critical role of coralline algae being overlooked in favour of the more visually appealing corals.

Coralline algae are not the same as the symbiotic zooxanthellae algae that live within the coral branch. Coralline algae are a pink encrusting algae that grow over dead and living corals and other reef substrate. The reef front below the exposed coralline algae surface is typically built of overlapping layers of coral and coralline algae. Our results demonstrate that the structural role provided by this coralline algae looks set to continue under higher CO2 levels than we previously thought.

The Australian reported our research with two other positive coral news stories with the headline “Forget the doom: coral reefs will bloom”. The coral stories related to corals thriving in conditions thought to be inhospitable to their survival.

On the face of it this all points to a positive future for the coral reefs. However, the key point that is often missed is that a coral and a coral reef are not the same thing. Just because a coral grows, it does not necessarily follow that a reef will form, just as a few trees growing in harsh conditions does not indicate a forest can form.

The recent CSIRO marine report card for ocean acidification identified the need for research at an ecosystem scale to understand the complex and sometimes inter-related responses of various reef organisms to ocean acidification.

It is possible to have a reef made of coralline algae without corals, as happened ~16 million years ago during the Mid-Miocene climactic optimum when CO2 and temperatures were higher than today. However, a reef made only of coralline algae will not support the biodiversity presently found on our tropical reefs.

With the Doha talks failing, maybe world leaders are counting on our world being more resilient to climate changes than we thought. While this research shows that coralline algae may be more resilient than we thought, unfortunately, we still can’t rely on this to save our reefs.

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11 Comments sorted by

  1. Comment removed by moderator.

  2. Ian L. McQueen

    Retired

    We should always keep in mind that "ocean acidification" and its cousins is misleading and wrong. The oceans are alkaline and always will be thanks to the chemical concept known as buffering. The very word "acidification" should be banned from such discussions because of the misleading notion that it gives that sea water could actually have a pH of 7.0 or lower. It'll never happen, so put all thoughts of the effects of acidity out of your mind.
    Now, on to the effects of CO2. First, there is no scientifically…

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    1. Mike Hansen

      Mr.

      In reply to Ian L. McQueen

      The lack of links to any peer reviewed science is the first indication that this is the usual copy and paste junk from a climate science denier.

      You claim "There are lots of people involved with climate studies who do not accept the fable that CO2 is warming our atmosphere"

      You should have no trouble linking to their publications in the peer reviewed science journals then.

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    2. Tim Scanlon

      Debunker

      In reply to Ian L. McQueen

      Ian, you don't seem to understand the difference between acidifying and acid. Just because something is acidifying, doesn't mean that the solution (or other substrate) has become or will become acidic. It means that there has been a change in the ionic potential of the solution.

      Also, pH is far more complicated than what you are making it out to be. pH is essentially a measure of relative reactivity of the solution, whether it will donate or accept electrons and how forcefully. Buffering capacity…

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  3. Alvin Stone

    logged in via Facebook

    Hi Merinda, This is a great article. It has clarified the news reporting around this, which I now realise I badly misinterpreted. Aside from many of the fascinating new stories on The Conversation, it is these clarifying articles around research papers that get a lot of publicity that keep bringing me back to the website. I'll be retweeting this shortly.

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  4. Comment removed by moderator.

  5. Ian L. McQueen

    Retired

    Mike Hansen wrote (to an unknown person, but presumably me): "The lack of links to any peer reviewed science is the first indication that this is the usual copy and paste junk from a climate science denier."

    Mike, you seem unaware of the concept of "pal review" and of how editors of journals can and do select or reject articles so that the erroneous concept of AGW is perpetuated. "Peer review" is no guarantee that the article in question has been evaluated accurately, and experience shows that…

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    1. Alvin Stone

      logged in via Facebook

      In reply to Ian L. McQueen

      Ian, you could have saved so much space by just saying, "Yes Mike, you are exactly right".

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