The research, led by Anthony M. Bonacolta and Javier del Campo of the University of Miami’s Rosenstiel School of Marine, Atmospheric, and Earth Science, together with colleagues in Miami and Barcelona, focused on the mountainous star coral, Orbicella faveolata, a major reef-building species in the Caribbean and Gulf of Mexico. Under controlled heat-stress conditions in the laboratory, the team tracked the coral as it progressed from a healthy state through mid-bleaching stages to full bleaching. What makes the study unusual is its resolution: rather than averaging gene activity across an entire coral fragment, the researchers sequenced RNA from individual cells, allowing them to see which cell types were responding, when, and in what direction.

The technical foundation of the work is single-cell RNA sequencing, or scRNA-seq, a method that captures the transcriptome, the complete set of active genes, of thousands of individual cells simultaneously. The team combined this with marker gene metabarcoding targeting three genetic regions at once: the 16S rRNA gene for bacteria and archaea, the 18S rRNA gene for microbial eukaryotes, and the ITS2 region to distinguish types of algal symbionts. Photochemical measurements of the algae’s photosynthetic performance rounded out the picture. Together, these approaches produced what the authors describe as a granular view of the microbial ecology of symbiotic breakdown, treating the coral not as a single organism but as a holobiont, an integrated ecosystem of host cells, algae, bacteria, and protists.

One of the most striking findings concerns the two co-dominant algal symbionts living within the same coral colonies. Orbicella faveolata hosts both Durusdinium and Breviolum, two genera within the family Symbiodiniaceae that provide the coral with most of its energy through photosynthesis. When heat stress arrived, the two symbionts did not respond identically. Their transcriptomic diverged particularly sharply in transcripts involved in nitrogen cycling, a metabolic arena that sits at the heart of the symbiotic bargain. The coral supplies nitrogen-poor compounds to its algae in exchange for carbon-rich photosynthate, and the new data suggest that the way each symbiont genus handles nitrogen under stress may shape how the partnership unravels.

The host side of the story proved equally revealing. The researchers identified distinct coral cell types, including gastrodermal cells, the digestive cells that physically house the algal symbionts, and gland cells known as mucocytes, which produce mucus. In the gastrodermal cells, the team observed something unexpected: the suppression of host heat-stress genes differed depending on which symbiont a given cell was hosting. In other words, the identity of the algal tenant appeared to influence how the coral cell itself expressed its stress response, a symbiont-specific effect visible at the level of individual host cells. This finding hints that the two partners in each hosting cell are chemically conversing in ways that standard bulk measurements would completely miss.

The mucocytes delivered perhaps the most dramatic cellular storyline. A subpopulation of these mucus-producing gland cells showed a distinct increase during heat stress, suggesting that they are not passive bystanders in bleaching but active participants in the response. Mucus production is a well-known feature of stressed corals, often interpreted as a protective or sloughing mechanism, but the single-cell data now place mucocytes squarely within the cellular choreography of the bleaching process. The finding gives researchers a specific cell type to watch, and potentially to target, when trying to understand why some colonies survive thermal anomalies that kill others.

Beyond the coral and its algae, the study broke new ground by documenting, for the first time, the concurrent shifts in both the prokaryotic and microeukaryotic microbiomes during experimental heat stress. The prokaryotic community told a coherent metabolic story. Early in the heat-stress response, suspected nitrifying bacteria, microbes that convert ammonia into nitrate, decreased, while suspected denitrifiers and nitrate reducers increased. The net effect is a rise in nitrogen availability within the coral’s internal environment. This matters because nitrogen availability is theorized to regulate the size of the Symbiodiniaceae population: when nitrogen is scarce, the coral can keep its algal partners in check. More nitrogen may decouple that regulation, allowing algal populations to expand or behave in ways the host can no longer control.

The consequences rippled outward through the rest of the holobiont. The higher nitrogen availability coincided with blooms of other microbes, including chlorophytes, a group of green algae, diatoms, and labyrinthulids, slime-net-producing protists that are often overlooked in coral studies. These opportunistic eukaryotic microbes may contribute to the negative feedback loops that characterize coral bleaching, in which the initial stress response creates conditions that accelerate further dysfunction. The picture that emerges is not a simple two-partner divorce between coral and alga but a cascading ecological rearrangement of an entire microscopic community, with nutrient cycling at its center.

The study’s conclusions position it as a starting point rather than an endpoint. By pairing scRNA-seq with the latest metabarcoding methodologies, the authors provide what they call a comprehensive view of the coral holobiont, one in which all members, from gastrodermal cells to denitrifying bacteria to labyrinthulid protists, are considered interconnected and important to the health of the whole. The cell-type-specific responses they documented, particularly within mucocytes and gastrodermal cells, along with the symbiont-specific suppression of heat-stress transcripts, offer concrete mechanistic threads for future work on symbiosis breakdown, coral mortality, and ultimately reef decline.

For a field racing against warming oceans, the significance of this cellular atlas is hard to overstate. Bleaching events are becoming more frequent and severe, and predictions of reef futures depend on understanding why some corals bleach and die while others endure. By revealing that the answer may lie partly in which symbiont genus a cell hosts, how its mucocytes respond, and whether its bacterial community tips from nitrification toward denitrification, the study converts a planetary catastrophe into a set of testable, cell-level hypotheses. The mountainous star coral that yielded these insights now serves as a reference point for the single-cell ecology of the holobiont, and the methods deployed here, from STARsolo-based RNA mapping to ANCOM-BC differential abundance analysis and cross-species cell-type matching against existing coral atlases, give other laboratories a template for extending this work to more species, more symbiont combinations, and the real thermal landscapes of dying and surviving reefs.

Subject of Research: Single-cell analysis of coral-algal symbiosis breakdown and holobiont microbiome shifts during heat stress

Article Title: Single-cell ecology of coral-algal symbiosis breakdown

Article References: Single-cell ecology of coral-algal symbiosis breakdown. (n.d.). https://doi.org/10.1186/s40168-026-02506-6

Image Credits: AI Generated

DOI: 10.1186/s40168-026-02506-6

Keywords: coral bleaching, single-cell transcriptomics, Symbiodiniaceae, holobiont, microbiome, Orbicella faveolata, Durusdinium, Breviolum, nitrogen cycling, metabarcoding, mucocytes, gastrodermal cells