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A research team from the Institut de Ciències del Mar (ICM-CSIC) has published a study in showing how ocean acidification and warming—two of the main consequences of global climate change—can simultaneously affect the structure, mineral composition, and microbiome of bryozoans, colonial invertebrates crucial for forming marine habitats. The findings point to potentially serious ecological consequences under a scenario of accelerated climate change.

The study characterizes for the first time the microbiome of Myriapora truncata, a habitat-forming species known as "false coral" and widely distributed throughout the Mediterranean. It also analyzes the response of this and another encrusting bryozoan species under future environmental conditions. False corals form three-dimensional structures that offer shelter to many species, as do other bryozoans that can even form reef-like systems—although corals usually receive more attention as primary marine habitat builders.

"Despite being a different phylum, very diverse and abundant globally, these small architects of the sea are often overlooked in studies on responses to environmental changes," explains Blanca Figuerola, ICM-CSIC researcher and lead author of the study. She emphasizes that this work opens a new window to better understand how bryozoans may respond to the ocean's rapid changes.

The researcher notes that "bryozoans play a very important ecological role," although little was previously known about their response to the combined effects of ocean acidification and warming. She adds that "their microbiome had been virtually unexplored."

A natural laboratory to predict future scenarios

To conduct the study, the team used a "natural laboratory": on the island of Ischia (Italy), volcanic COâ‚‚ bubbles from the seabed, which allow simulation of the ocean acidification conditions projected for the end of the century.

"This area offers a unique opportunity to study how marine species respond to acidification under natural conditions," explains Núria Teixidó, researcher at the Stazione Zoologica Anton Dohrn and last author of the article.

Using this approach, the researchers compared the morphology, skeleton mineralogy, and microbiome of colonies of two bryozoan species exposed and unexposed to these conditions. Results show that the species exhibit some acclimation capacity, modifying their skeletal mineralogy to become more resistant and maintaining a relatively stable microbiome composition.

"However, we observed a loss in functional microbial diversity, with a decline in genera potentially involved in key processes such as nutrition, defense, or resistance to environmental stress," Figuerola states.

These microbial shifts may have important long-term consequences, since the microbiome plays a fundamental role in bryozoan health and resilience. "Even if colonies look externally healthy, changes in the microbiome could serve as early bioindicators of environmental stress," adds Javier del Campo, researcher at the Institute of Evolutionary Biology (IBE, CSIC-UPF).

Warming amplifies the impacts

Over a five-year monitoring period, the study also considered the effects of rising temperatures—another key factor in climate change.

"The models used indicate that the combination of these two stressors intensifies the effects observed, significantly reducing the coverage of the encrusting bryozoan and increasing mortality. Although the species show some morphological plasticity, it is not enough to offset the combined impact of acidification and warming," says Pol Capdevila, researcher at the University of Barcelona.

To reach these conclusions, the team used advanced techniques such as modeling and computed microtomography to obtain, for the first time, 3D images of the internal skeleton structure of these species. These images are valuable both for research and for science communication and environmental education. The team is currently preparing a science animation for the general and educational public, in collaboration with the team at Cooked Illustrations, a visual storytelling studio.

Implications for marine conservation

The findings have important implications for the management and conservation of Mediterranean marine ecosystems, particularly in the context of climate change. Habitat-forming species like bryozoans are not only vulnerable but their disappearance could trigger cascading effects on many other species that rely on them for shelter or food.

The characterization of the microbiome and preliminary identification of potentially beneficial microorganisms open new research avenues to enhance the resilience of holobionts (host and its associated microbiome) through nature-based approaches.

This research line, initiated under the MedCalRes National Plan project, is now continuing with the HOLOCHANGE consolidation project and the National Plan MedAcidWarm, which aim to deepen the understanding of bryozoan–microbiome interactions to anticipate and mitigate climate change impacts.

"The complexity of the issue demands integrated analyses," concludes Figuerola. "This study shows how interdisciplinary approaches can help us anticipate future scenarios and more effectively protect marine ecosystems."