3D modeling of reefs naturally exposed to acidification

© Jonathan Lancelot / Tara Expeditions Foundation

At age 24, Grace Klinges embarked aboard Tara for the first time and uses volcanic waters for a research to understand the future of the ocean. This young doctoral student is working alongside her professor, Rebecca Vega Thurber, a microbiologist at Oregon State University and the scientific leader of the transect between the Solomon Islands and Papua New Guinea. Over the past few weeks, the team has sampled 10 sites, one of which is the subject of a special study. Focus on the research conducted in the volcanic and acidified waters of Normanby Island, in the Milne Bay Province of Papua New Guinea.



What is the importance of such a site?

It’s a natural volcanic carbon dioxide seep that has been very well studied by other groups. We hope to add to the knowledge of the site by studying and mapping the microbiology of the acidified reef. Our study will measure how reduced pH of the seeps alters the biodiversity of microbes living on and in 3 species of corals residing in that location.


Why are we interested in coral microbiology?

The coral microbiome is the community of bacteria and viruses living with corals. It is hypothesized that this community influences how corals respond to external factors such as disease and environmental change (read Rebecca Vega Thurber’s interview on microbiome)


Where CO2 bubbles escape, the water is so acidified that coral cannot grow © Tara Expeditions Foundation


How is the coral microbial community different at this site?

We want to answer that question by making 3D models of the reef. These models will allow us to map the diversity of corals close to and far from the seeps. We will then place the microbial diversity data on top of the 3D model of reef structure, with different colours representing how much microbial diversity we find in each coral. For example, we could use red to represent more microbial diversity within a coral host and blue to represent less diversity. We expect to see ( ) that some coral species have more diverse microbial communities than others, and that there is greater microbial biodiversity moving away from the carbon dioxide seeps, just as there is greater coral diversity.


Which species did you observe the most in the acidified water?

Right around the seeps there is mostly Porites lobata, which are big non-branching corals. As you move away, even only 30 meters, there is much greater diversity; there are many more kinds of branching coral such as Acropora and Pocillopora. Normally when you see more coral diversity you expect to see greater microbial biodiversity. We think that at spots with the highest amount of seepage, only certain organisms, the bacterial extremophiles which tolerate harsh environments, can survive.

Can we assume that Porites lobata could be one of the coral species most resistant to acidification?

If the ocean continues to acidify, as we have seen due to global warming and increased carbon emissions, the organisms that can survive near those seeps might be what the future of coral reefs look like. The pH at the most acidic part is not even neutral pH, it’s around 7.3, but the corals prefer water with a pH around 8. So it’s an extreme environment for them. What we observed at the seeps was not nearly as pretty as the rest of the reef, but it’s kind of uplifting that there are organisms living at all in these conditions. We also saw that the effects of the carbon dioxide seep continued further away from the seeps. Even though 30 meters away the pH is lower than normal pH of coral reefs (at 7.7), there was much more diversity here. It’s not an ideal environment for the corals, but it’s kind of a middle ground that they can tolerate.


Test of the 3D mapping program, seen from above © Tara Expeditions Foundation.


How did you make the 3D maps?

I took 6 different rapid photo sequences (nearly 2,000 photos each) of the reef at intervals moving away from the seeps and will use them to make 3D models with the program PhotoScan. The level of resolution of this program is really impressive: you can see even the individual little polyps of the corals. The models generated near the seeps demonstrate very low coral coverage, and then 60 meters away there is a beautiful, vibrant reef again. This program has been used only a few times before to model corals, and to our knowledge has never been used in combination with microbial biodiversity data. It’s a novel project and Normanby Island is the perfect site to do this research because you see such a sharp visual change.

Ultimately, what this study will show us is kind of a theoretical timeline. The condition of our current ocean resembles conditions far from the seep in terms of pH, with very vibrant coral reefs still existing in many places. But if ocean acidification continues, we will be seeing coral reefs looking more like conditions near the seeps. If we don’t decrease our carbon emissions, the reefs could look like they do at the seeps: very sparse and damaged. But on the other hand, there is the potential for a high amount of diversity at slightly decreased pH levels, just as we saw at this site. This gives us hope that reefs can tolerate some of the damage they are dealt.


Noëlie Pansiot


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