Coral reef conservation is a steppingstone to protect marine biodiversity and life in the ocean as we know it. The health of coral also has huge societal implications: reef ecosystems provide sustenance and livelihoods for millions of people around the world. Conserving biodiversity in reef areas is both a social issue and a marine biodiversity priority.

In the face of climate change, Annalisa Bracco, professor in the School of Earth and Atmospheric Sciences at Georgia Institute of Technology, and Lyuba Novi, a postdoctoral researcher, offer a new methodology that could revolutionize how conservationists monitor coral. The researchers applied machine learning tools to study how climate impacts connectivity and biodiversity in the Pacific Ocean’s Coral Triangle — the most diverse and biologically complex marine ecosystem on the planet. Their research, recently published in Nature Communications Biology, overcomes time and resource barriers to contextualize the biodiversity of the Coral Triangle, while offering hope for better monitoring and protection in the future.

“We saw that the biodiversity of the Coral Triangle is incredibly dynamic,” Bracco said. “For a long time, it has been postulated that this is due to sea level change and distribution of land masses, but we are now starting to understand that there is more to the story.”

Connectivity refers to the conditions that allow different ecosystems to exchange genetic material such as eggs, larvae, or the young. Ocean currents spread genetic material and also create the dynamics that allow a body of water — and thus ecosystems — to maintain consistent chemical, biological, and physical properties. If coral larvae are spread to an ecoregion where the conditions are very similar to the original location, the larvae can start a new coral.

Bracco wanted to see how climate, and specifically the El Niño Southern Oscillation (ENSO) in its phases — El Niño, La Niña, and neutral conditions — impacts connectivity in the Coral Triangle. Climate events that move large masses of warm water in the Pacific Ocean bring enormous changes and have been known to exacerbate coral bleaching, in which corals turn white due to environmental stressors and become vulnerable to disease.

“Biologists collect data in situ, which is extremely important,” Bracco said. “But it’s not possible to monitor enormous regions in situ for many years — that would require a constant presence of scuba divers. So, figuring out how different ocean regions and large marine ecosystems are connected over time, especially in terms of foundational species like coral, becomes important.”

Machine Learning for Discovering Connectivity

Years ago, Bracco and collaborators developed a tool, Delta Maps, that uses machine learning to identify “domains,” or regions within any kind of system that share the same dynamic. Bracco initially used it to analyze domains of climate variability in models but also suspected it could be used to study ecoregions in the ocean.

For this study, they used the tool to map out domains of connectivity in the Coral Triangle using 30 years of sea surface temperature data. Sea surface temperatures change in response to ocean currents over scales of weeks and months and across distances of tens of kilometers. These changes are relevant to coral connectivity, so the researchers built their machine learning tool based on this observation, using changes in surface ocean temperature to identify regions connected by currents. They also separated the time periods that they were considering into three categories: El Niño events, La Niña events, and neutral or “normal” times, painting a picture of how connectivity was impacted during major climate events in particular ecoregions.

Novi then applied a ranking system to the different ecoregions they identified. She used rank page centrality, a machine learning tool that was invented to rank webpages on the internet, on top of Delta Maps to identify which coral ecoregions were most strongly connected and able to receive the most coral larvae from other regions. Those regions would be the ones most likely sustain and survive through a bleaching event.

Climate Dynamics and Biodiversity

Bracco and Novi found that climate dynamics have contributed to biodiversity because of the way climate introduces variability to the currents in the equatorial Pacific Ocean. The researchers realized that alternation of El Niño and La Niña events has allowed for enormous genetic exchanges between the Indian and Pacific Oceans and enabled the ecosystems to survive through a variety of different climate situations.

“There is never an identical connection between ecoregions in all ENSO phases,” Bracco said. “In other parts of the world ocean, coral reefs are connected through a fixed, often small, number of ecoregions, and if you eliminate this fixed number of connections by bleaching all connected reefs, you will not be able to rebuild the corals in any of them. But in the Pacific the connections are changing all the time and are so dynamic that soon enough the bleached reef will receive larvae from completely different ecoregions in a different ENSO phase.”

They also concluded that, because of the Coral Triangle’s dynamic climate component, there is more possibility for rebuilding biodiversity there than anywhere else on the planet. And that the evolution of biodiversity in the Coral Triangle is not only linked to landmasses or sea levels but also to the evolution of ENSO through geological times. The researchers found that though ENSO causes coral bleaching, it has helped the Coral Triangle become so rich in biodiversity.

Better Monitoring Opportunities

Because coral reef survival has been designated a priority by the United Nations Sustainable Development Goals, Bracco and Novi’s research is poised to have broad applications. The researchers’ method identified which ecoregions conservationists should try hardest to protect and also the regions that conservationists could expect to have the most luck with protection measures. Their methodology can also help to identify which regions should be monitored more and the ones that could be considered lower priority for now due to the ways they are currently thriving.

“This research opens a lot of possibilities for better monitoring strategies, and especially how to monitor given a limited amount of resources and money,” Bracco said. “As of now, coral monitoring often happens when groups have a limited amount of funding to apply to a very specific localized region. We hope our method can be used to create a better monitoring over larger scales of time and space.”

CITATION: Novi, L., Bracco, A. “Machine learning prediction of connectivity, biodiversity and resilience in the Coral Triangle.” Commun Biol 5, 1359 (2022). 

DOI: https://doi.org/10.1038/s42003-022-04330-8

A new type of solar technology has seemed promising in recent years. Halide perovskite solar cells are both high performing and low cost for producing electrical energy – two necessary ingredients for any successful solar technology of the future. But new solar cell materials should also match the stability of silicon-based solar cells, which boast more than 25 years of reliability. 

In newly published research, a team led by Juan-Pablo Correa-Baena, assistant professor in the School of Materials Sciences and Engineering at Georgia Tech, shows that halide perovskite solar cells are less stable than previously thought. Their work reveals the thermal instability that happens within the cells’ interface layers, but also offers a path forward towards reliability and efficiency for halide perovskite solar technology. Their research, published as the cover story for the journal Advanced Materials in December 2022, has immediate implications for both academics and industry professionals working with perovskites in photovoltaics, a field concerned with electric currents generated by sunlight.

Lead halide perovskite solar cells promise superior conversion of sunlight into electrical power. Currently, the most common strategy for coaxing high conversion efficiency out of these cells is to treat their surfaces with large positively charged ions known as cations.

These cations are too big to fit into the perovskite atomic-scale lattice, and, upon landing on the perovskite crystal, change the material’s structure at the interface where they are deposited. The resulting atomic-scale defects limit the efficacy of current extraction from the solar cell. Despite awareness of these structural changes, research on whether the cations are stable after deposition is limited, leaving a gap in understanding of a process that could impact the long-term viability of halide perovskite solar cells. 

“Our concern was that during long periods of solar cell operation the reconstruction of the interfaces would continue,” said Correa-Baena. “So, we sought to understand and demonstrate how this process happens over time.”

To carry out the experiment, the team created a sample solar device using typical perovskite films. The device features eight independent solar cells, which enables the researchers to experiment and generate data based on each cell’s performance. They investigated how the cells would perform, both with and without the cation surface treatment, and studied the cation-modified interfaces of each cell before and after prolonged thermal stress using synchrotron-based X-ray characterization techniques.

First, the researchers exposed the pre-treated samples to 100 degrees Celsius for 40 minutes, and then measured their changes in chemical composition using X-ray photoelectron spectroscopy. They also used another type of X-ray technology to investigate precisely what type of crystal structures form on the film’s surface. Combining the information from the two tools, the researchers could visualize how the cations diffuse into the lattice and how the interface structure changes when exposed to heat. 

Next, to understand how the cation-induced structural changes impact solar cell performance, the researchers employed excitation correlation spectroscopy in collaboration with Carlos Silva, professor of physics and chemistry at Georgia Tech.  The technique exposes the solar cell samples to very fast pulses of light and detects the intensity of light emitted from the film after each pulse to understand how energy from light is lost. The measurements allow the researchers to understand what kinds of surface defects are detrimental to performance.

Finally, the team correlated the changes in structure and optoelectronic properties with the differences in the solar cells’ efficiencies. They also studied the changes induced by high temperatures in two of the most used cations and observed the differences in dynamics at their interfaces.

“Our work revealed that there is concerning instability introduced by treatment with certain cations,” said Carlo Perini, a research scientist in Correa-Baena’s lab and the first author of the paper. “But the good news is that, with proper engineering of the interface layer, we will see enhanced stability of this technology in the future.”

The researchers learned that the surfaces of metal halide perovskite films treated with organic cations keep evolving in structure and composition under thermal stress. They saw that the resulting atomic-scale changes at the interface can cause a meaningful loss in power conversion efficiency in solar cells. In addition, they found that the speed of these changes depends on the type of cations used, suggesting that stable interfaces might be within reach with adequate engineering of the molecules.

“We hope this work will compel researchers to test these interfaces at high temperatures and seek solutions to the problem of instability,” Correa-Baena said. “This work should point scientists in the right direction, to an area where they can focus in order to build more efficient and stable solar technologies.”

CITATION: Perini, C. A. R., Rojas-Gatjens, E., Ravello, M., Castro-Mendez, A., Hidalgo, J., An, Y., Kim, S., Lai, B., Li, R., Silva-Acuña, C., Correa-Baena, J.-P., Interface Reconstruction from Ruddlesden–Popper Structures Impacts Stability in Lead Halide Perovskite Solar Cells. Adv. Mater. 2022, 34, 2204726.

DOI: https://doi.org/10.1002/adma.202204726

Ten projects have been chosen for the Brook Byers Institute for Sustainable Systems (BBISS) Initiative Leads program. Project themes include climate adaptation and mitigation solutions, innovation and social impact, computation and design approaches to sustainability, sustainable development, and conservation. BBISS Initiative Leads receive $10,000 in discretionary funds to advance their project.

The projects chosen involve 15 faculty members hailing from all 6 of the colleges at Georgia Tech. Several of the projects are also joint initiatives with other Georgia Tech Interdisciplinary Research Institutes (IDEAS, IPAT, and SEI), the Ray C. Anderson Center for Sustainable Business, or the Office of Sustainability.

The Initiative Leads and projects are:

• Michael Helms - ME, “Nature’s Voice: Amplifying the Narrative of Biologically Inspired Sustainable Design at Georgia Tech”
• Josiah Hester - Interactive Computing, “Computational Sustainability”
• Co-Leads Xiaoming Huo - ISYE, and Yi Deng – EAS, “Microclimate Monitoring and Prediction at Georgia Tech”
• Jian Luo - CEE, “Coastal Urban Flooding in a Changing Climate”
• Brigitte Stepanov - Modern Languages, “Energy Today, Tomorrow: Illuminating the Effect of Energy Power Dynamics on the Environment”
• Co-Leads Anjali Thomas – INTA, and Shatakshee Dhongde - ECON, “SEEDS (Southeast Exchange of Development Studies) 2023 Conference at Georgia Tech”
• Co-Leads Danielle Willkens - Arch, and Junshan Liu – Auburn University, “Sustainable Tourism, Petra”
• Co-Leads Yuanzhi Tang - EAS, and Hailong Chen – ME, “Sustainable Resources for Clean Energy”
• Co-Leads Dori Pap - Institute for Leadership and Social Impact, and Neha Kumar – Interactive Computing/INTA, “Collaborative Social Impact”
• Alex Oettl - COB, “A Sustainability-Focused Stream of the Creative Destruction Lab”

The Initiative Leads program has several overarching goals. BBISS aims to cultivate promising topics for future large-scale collaborative sustainability research, research translation, and/or high-impact outreach; to provide (mostly mid-career) faculty with leadership and community building opportunities; and to broaden and strengthen the BBISS sustainability community as a whole.

Plants, like animals and people, seek refuge from climate change. And when they move, they take entire ecosystems with them. To understand why and how plants have trekked across landscapes throughout time, researchers at the forefront of conservation are calling for a new framework. The key to protecting biodiversity in the future may be through understanding the past.

Jenny McGuire, assistant professor in the Schools of Biological Sciences and Earth and Atmospheric Sciences at Georgia Tech, spearheaded a special feature on the topic of biodiversity in The Proceedings of the National Academy of Sciences along with colleagues in Texas, Norway, and Argentina. In the special feature, “The Past as a Lens for Biodiversity Conservation on a Dynamically Changing Planet,” McGuire and her collaborators highlight the outstanding questions that must be addressed for successful future conservation efforts. The feature brings together conservation research that illuminates the complex and constantly evolving dynamics brought on by climate change and the ever-shifting ways humans use land. These factors, McGuire said, interact over time to create dynamic changes and illustrate the need to incorporate temporal perspectives into conservation strategies by looking deep into the past.

One example of this work highlighted in the journal is McGuire’s research about plants in North America, which investigates how and why they’ve moved across geography over time, where they’re heading, and why it’s important.

“Plants are shifting their geographic ranges, and this is happening whether we realize it or not,” McGuire said. “As seeds fall or are transported to distant places, the likelihood that the plant’s seed is going to be able to survive and grow is changing as climates are changing. Studying plants’ niche dynamics over thousands of years can help us understand how species adapt to climate change and can teach us how to protect and maintain biodiversity in the face of rapid climate change to come.”

Climate Fidelity: A New Metric for Understanding Vulnerability

The first step is to understand which type of plants exhibit what McGuire terms “climate fidelity,” and which do not. If a plant has climate fidelity, it means that the plant stays loyal to its preferred climatic niche, often migrating across geographies over thousands of years to keep up with its ideal habitat. Plants that don’t exhibit climate fidelity tend to adapt locally in the face of climate change. Being loyal to one’s climate, it turns out, doesn’t necessarily mean being loyal to a particular place.

To investigate the case of trees, McGuire and former Georgia Tech postdoctoral scholar Yue Wang (associate professor in the School of Ecology at Sun Yat-sen University in China) studied pollen data from the Neotoma Paleoecology Database, which contains pollen fossil data from sediment cores across North America. Each sediment core is sampled, layer by layer, producing a series of pollen data from different times throughout history. The data also contains breakdowns of the relative abundance of different types of plants represented by the pollen types – pine versus oak versus grass, for example – painting a picture of what types of plants were present in that location and when.

McGuire and Wang looked at data from 13,240 fossil pollen samples taken from 337 locations across the entirety of North America. For each of the 16 major plant taxa in North America, they divided the pollen data into six distinct chunks or “bins” of time of 4,000 years, starting from 18,000 years ago up to the present day. Wang used the data to identify all climate sites containing fossil pollen for any individual type of tree – such as oak, for example – for each period. Then, Wang looked at how each tree’s climate changed from one period to the next. Wang did this by comparing the locations of pollen types between adjacent time periods, which enabled the team to identify how and why each type of tree’s climate changed over time.

“This process allowed us to see the climate fidelity of these different plant taxa, showing that certain plants maintain very consistent climatic niches, even when climate is changing rapidly,” Wang said.

For example, their findings showed that when North American glaciers were retreating 18,000 years ago, spruce and alder trees moved northward to maintain the cool temperatures of their habitats.

Crucially, McGuire and Wang found that most plant species in North America have exhibited long-term climate fidelity over the past 18,000 years. They also found that plants that migrated farther did a better job of tracking climate during periods of change.

But some plants fared better than others. For example, the small seeds of willow trees can fly over long distances – enabling them to track their preferred climates very effectively. But the large seeds of ash trees, for example, can only be dispersed short distances from parent trees, hindering their ability to track climate. Habitat disruptions from humans could make it even more difficult for ash trees to be able to take hold in new regions. If there are no adjacent habitats for ash trees, their seeds are under pressure to move even farther – a particular challenge for ash, which slows their migration movements even more.

Protecting the Fabric of Life

On the bright side, by identifying which plants have historically been most sensitive to changing climates, McGuire and Wang’s research can help conservation organizations like The Nature Conservancy prioritize land where biodiversity is most vulnerable to climate change.

As a final step, McGuire and Wang identified “climate fidelity hotspots,” regions that have historically exhibited strong climate fidelity whose plants will most urgently need to move as their climates change. They compared these hotspots to climate-resilient regions identified by The Nature Conservancy that could serve as refuge areas for those plants. While plants in these resilient regions can initially adapt to impending climate change by shifting their distributions locally, the plants will likely face major challenges when a region’s climate change capacity is exceeded due to lack of connectivity and habitat disruptions from humans. Refining these priorities helps stakeholders identify efficient strategies for allowing the fabric of life to thrive.

“I think that understanding climate fidelity, while a new and different idea, will be very important going forward, especially when thinking about how to prioritize protecting different plants in the face of climate change,” McGuire said. “It is important to be able to see that some plants and animals are more vulnerable to climate change, and this information can help build stronger strategies for protecting the biodiversity on the planet.”

Citation: Yue Wang, Silvia Pineda-Munoz, and Jenny L. McGuire, "Plants maintain climate fidelity in the face of dynamic climate change." PNAS (2023).

DOI: doi.org/10.1073/pnas.2201946119