Bleached coral reef with a giant, wavering pH meter in the background.

Coral Reefs Under Stress: Are They Sending False Alarms?

Reese Marlowe in Climate & Environment December 2025 • 5 min read.

"New research reveals that coral's pH levels, often seen as reliable indicators, might not always tell the full story of bleaching events."

Coral reefs are among the most vibrant and biodiverse ecosystems on our planet, but they are also incredibly sensitive to changes in their environment. Rising ocean temperatures and increasing acidity, driven by climate change, pose significant threats to these underwater habitats. Scientists are racing against time to understand and monitor the health of coral reefs to protect them from further decline.

One of the key methods used to assess the health of coral reefs is by examining the chemical composition of coral skeletons. In particular, the levels of boron isotopes within coral skeletons have been used as a proxy to determine seawater pH, providing valuable insights into past and present ocean conditions. However, a recent study is challenging some of these assumptions, suggesting that these indicators might not always be as reliable as previously thought, especially during short-term stress events like coral bleaching.

Coral bleaching occurs when corals expel the symbiotic algae living in their tissues due to stress from high water temperatures. This process causes corals to turn white and can lead to coral death if the stress is prolonged. While scientists have traditionally looked at boron isotopes to understand the impact of bleaching events, new research indicates that these isotopes may not accurately reflect the immediate effects of bleaching on coral health. This discovery could change how we monitor and interpret the health of coral reefs worldwide.

The Boron Isotope Puzzle: What Does It Really Tell Us?

Bleached coral reef with a giant, wavering pH meter in the background.

The study, published in PLOS ONE, investigated how coral bleaching affects boron isotopes in the skeletons of Caribbean corals. Researchers conducted controlled bleaching experiments on three common coral species: Porites divaricata, Porites astreoides, and Orbicella faveolata. These corals were subjected to elevated temperatures to simulate bleaching conditions, and their skeletal composition was analyzed for various isotopes and trace elements.

Contrary to expectations, the study found that coral bleaching did not significantly affect the boron isotopic signature (δ¹¹B) in any of the coral species tested. This was a surprising result, as previous studies had interpreted decreases in boron isotopes as indicators of bleaching events. The researchers also measured other indicators, such as carbon and oxygen isotopes, and found that these were significantly altered during bleaching, suggesting that they might be more reliable markers for short-term stress.

Boron Isotopes (δ¹¹B): Remained stable, not reflecting bleaching stress.
Carbon (δ¹³C) and Oxygen (δ¹⁸O) Isotopes: Showed major changes, indicating stress.
Strontium/Calcium (Sr/Ca) Ratios: Unreliable in recording temperature changes during bleaching.
Magnesium/Calcium (Mg/Ca), Uranium/Calcium (U/Ca), and Barium/Calcium (Ba/Ca) Ratios: Affected in some species, but patterns were inconsistent.

These findings suggest that while boron isotopes are useful for understanding long-term pH changes in the ocean, they may not be sensitive enough to detect the immediate impacts of bleaching events. The stability of boron isotopes during bleaching indicates that the process of pH regulation within coral tissues remains robust, even under stress. This insight is crucial for refining how we use geochemical proxies to assess coral reef health and reconstruct past environmental conditions.

What Does This Mean for Coral Reef Monitoring?

The study's conclusion calls for a reassessment of how scientists monitor coral reefs and interpret past bleaching events. The reliability of boron isotopes in reflecting seawater pH remains intact, but their insensitivity to short-term bleaching suggests that other indicators, such as carbon and oxygen isotopes, may provide a more accurate picture of immediate coral stress. Future research should focus on identifying and validating these alternative proxies to improve our ability to detect and respond to coral bleaching events, ensuring the preservation of these invaluable ecosystems. This research is important because it helps scientists learn more about how future climate change and increasing ocean acidification will affect ocean chemistry and marine ecosystems.

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This article is based on research published under:

DOI-LINK: 10.1371/journal.pone.0112011, Alternate LINK

Title: Short-Term Coral Bleaching Is Not Recorded By Skeletal Boron Isotopes

Subject: Multidisciplinary

Journal: PLoS ONE

Publisher: Public Library of Science (PLoS)

Authors: Verena Schoepf, Malcolm T. Mcculloch, Mark E. Warner, Stephen J. Levas, Yohei Matsui, Matthew D. Aschaffenburg, Andréa G. Grottoli

Published: 2014-11-14

Everything You Need To Know

What is coral bleaching, and why is it significant in the context of this research?

Coral bleaching occurs when corals expel the symbiotic algae living in their tissues due to stress from high water temperatures. This causes the corals to turn white and can lead to death. The research examined the effects of this process on the chemical composition of coral skeletons, specifically, the levels of boron isotopes. Understanding this process is vital as it is a primary indicator of coral health and a direct consequence of rising ocean temperatures and climate change.

What are Boron Isotopes, and what role do they play in assessing coral health?

Boron isotopes (δ¹¹B) are chemical markers used to assess seawater pH levels, which can indicate the health of coral reefs. The recent study found that during coral bleaching, the boron isotopic signature (δ¹¹B) remained stable in three coral species (Porites divaricata, Porites astreoides, and Orbicella faveolata), even under stress. This result challenges the previous assumptions, suggesting that Boron Isotopes may not be a reliable indicator of short-term coral bleaching stress. While still useful for understanding long-term pH changes, their insensitivity to short-term bleaching events reduces their effectiveness as an immediate coral health monitoring tool.

Besides Boron Isotopes, what other indicators are used to assess coral health?

Scientists use several indicators to assess coral health. Besides Boron Isotopes, carbon (δ¹³C) and oxygen (δ¹⁸O) isotopes are also examined. The research revealed that these isotopes showed significant changes during coral bleaching, unlike the stable behavior observed in Boron Isotopes. Furthermore, Strontium/Calcium (Sr/Ca) ratios were found unreliable in recording temperature changes during bleaching. Magnesium/Calcium (Mg/Ca), Uranium/Calcium (U/Ca), and Barium/Calcium (Ba/Ca) Ratios were affected in some species, but patterns were inconsistent. These results highlight that different indicators offer varied insights into coral stress, with some being more responsive to short-term events like bleaching.

What are the implications of this research for coral reef monitoring?

The implications of this research are significant for coral reef monitoring because it challenges the reliance on boron isotopes as the sole indicator of coral bleaching. The findings call for a reassessment of how scientists interpret past bleaching events. The research highlights the need to identify and validate alternative proxies, such as carbon and oxygen isotopes, to gain a more accurate picture of immediate coral stress. Improving our ability to detect and respond to coral bleaching events is essential for preserving these vital ecosystems, which are increasingly threatened by climate change and rising ocean temperatures.

How does this research influence the direction of future studies on coral reefs?

Future research should focus on identifying and validating alternative proxies, such as carbon and oxygen isotopes, to improve our ability to detect and respond to coral bleaching events. The stability of boron isotopes during bleaching suggests that the process of pH regulation within coral tissues remains robust, even under stress. This insight is crucial for refining how we use geochemical proxies to assess coral reef health and reconstruct past environmental conditions. This research is important because it helps scientists learn more about how future climate change and increasing ocean acidification will affect ocean chemistry and marine ecosystems.