Kathryn Lee Morrissey, Ljiljana Iveša, Soria Delva, Sofie D'Hondt, Anne Willems, Olivier De Clerck
4 min
Marine macroalgae host complex bacterial communities that are vital for their ecological success. As climate change increases the frequency of extreme environmental events, it is critical to understand how these algal-associated microbiomes respond to stress. This study investigates how nutrient enrichment and temperature stress affect the taxonomic and functional stability of bacterial communities associated with different parts of the green alga Caulerpa cylindracea.
The researchers conducted in situ factorial experiments using semi-closed mesocosms to expose C. cylindracea to pulse heatwaves and nutrient enrichment. They sampled three distinct morphological niches—the endomicrobiome (internal), epimicrobiome (surface), and rhizomicrobiome (root-like structures)—before stress, immediately after a 3-day stress period, and after a 9-day recovery period. Bacterial communities were characterized using 16S rRNA gene sequencing, and stability was quantified using indices for resistance (the ability to remain stable during stress) and resilience (the ability to return to the original state after recovery).
The study reveals that bacterial community responses are highly dependent on their specific morphological niche. The endomicrobiome demonstrated the highest taxonomic and functional resistance to environmental perturbations, yet it exhibited the lowest resilience, suggesting that once these internal communities are disturbed, they struggle to return to their original state. Conversely, the rhizomicrobiome showed the lowest resistance to stress, with evidence that combined temperature and nutrient stressors have additive, compounding effects on these communities. The epimicrobiome was particularly sensitive to nutrient enrichment, showing significant taxonomic and functional shifts. Overall, the results indicate that while taxonomic composition is often sensitive to stress, functional profiles can sometimes show higher resilience, potentially due to functional redundancy within the microbial community.
Understanding the stability of algal-associated microbiomes is essential for predicting the health of marine ecosystems under climate change. Because these bacteria can influence host adaptation and defense, shifts in their composition or function may signal a decline in algal health or the proliferation of opportunistic pathogens. This research highlights that the "holobiont" is not a uniform entity; rather, different niches within the same organism respond uniquely to environmental change, which must be considered when assessing the vulnerability of marine macroalgae.
Abstract Algal‐associated bacteria are fundamental to the ecological success of marine green macroalgae such as Caulerpa. The resistance and resilience of algal‐associated microbiota to environmental stress can promote algal health and genetic adaptation to changing environments. The composition of bacterial communities has been shown to be unique to algal morphological niches. Therefore, the level of response to various environmental perturbations may in fact be different for each niche‐specific community. Factorial in situ experiments were set up to investigate the effect of nutrient enrichment and temperature stress on the bacterial communities associated with Caulerpa cylindracea. Bacteria were characterized using the 16S rRNA gene, and the community compositions were compared between different parts of the algal thallus (endo‐, epi‐, and rhizomicrobiome). Resistance and resilience were calculated to further understand the changes of microbial composition in response to perturbations. The results of this study provide evidence that nutrient enrichment has a significant influence on the taxonomic and functional structure of the epimicrobiota, with a low community resistance index observed for both. Temperature and nutrient stress had a significant effect on the rhizomicrobiota taxonomic composition, exhibiting the lowest overall resistance to change. The functional performance of the rhizomicrobiota had low resilience to the combination of stressors, indicating potential additive effects. Interestingly, the endomicrobiota had the highest overall resistance, yet the lowest overall resilience to environmental stress. This further contributes to our understanding of algal microbiome dynamics in response to environmental changes.
Alex: Does the functional data back that up, or is this purely a taxonomic pattern—species turning over without changing what the community actually does? [[RP_SECTION:functional-redundancy-and-metabolic-prof|Functional redundancy and metabolic profiling]]
Sam: That's exactly the distinction they went after with predicted metabolic profiling. Taxonomic composition shifted across all three niches under stress. But the predicted functional capacity—the KEGG orthology profiles—stayed largely stable in the endomicrobiome even as which species were present changed. That's functional redundancy: different taxa, same job getting done.
Alex: And the epimicrobiome didn't show that redundancy?
Sam: Not to the same degree—its functional profile shifted meaningfully following nutrient enrichment. So you get a cleaner version of the same trade-off at the functional level. The endomicrobiome prioritizes metabolic consistency, locked into a narrow set of functions that's efficient until the environment crosses a threshold it can't absorb. The epimicrobiome trades some of that consistency for flexibility—composition and function both move, but that movement is what lets the holobiont keep functioning as conditions change.
Alex: And the rhizomicrobiome loses on both fronts again—no structural insulation like the endosphere, and it can't recruit replacement taxa as readily as the episphere. [[RP_SECTION:holobiont-stability-as-a-mosaic|Holobiont stability as a mosaic]]
Sam: Correct. Its functional profile stayed significantly altered even after the stress was lifted—the only niche where that was true. It has neither the fortress protection of the endosphere nor the rapid-recruitment flexibility of the episphere. Which is why the authors frame holobiont stability as a mosaic rather than a single property of the organism. Each niche is doing something different, and the weakest link—here, the holdfast—may set the practical limit on how the whole system responds to warming, however stable the core stays.
Alex: If you want the Orwin-Wardle breakdowns and the full metabolic shift data behind that mosaic picture, you can generate a deep dive of this paper. The paper has the rest either way.
Sam: Thanks for listening.