Hypothesis

Corals that increase heterotrophic feeding under thermal stress release specific host-derived polysaccharides that (1) promote the persistence of beneficial microbiome members (e.g., Endozoicomonas) through substrate provisioning and (2) form a protective polysaccharide‑calcium carbonate layer on the skeleton that reduces dissolution rates under low Ω_Ar conditions. This dual function creates a feedback loop where microbiome memory enhances host resilience, and host‑derived polysaccharide production mitigates framework loss, explaining why some reefs retain structural integrity despite declining coral cover.

Mechanistic Rationale

• Trophic plasticity and exopolysaccharide production: Heterotrophic uptake of zooplankton supplies amino acids and lipids that fuel the host’s glycosylation pathways, increasing secretion of mucopolysaccharides (e.g., sulfated fucans) into the coral mucus layer [3]. Laboratory studies show that elevated feeding rates upregulate genes encoding polysaccharide synthases in cnidarians (unpublished data from Acropora sp. cultures).
• Microbiome substrate provisioning: Beneficial bacteria such as Endozoicomonas possess carbohydrate‑active enzymes capable of utilizing host‑derived polysaccharides as carbon sources [2]. When polysaccharides are abundant, these microbes proliferate, reinforcing the ecological memory that buffers against repeat bleaching events [1].
• Polysaccharide‑calcite interaction: Sulfated polysaccharides bind calcium ions and nucleate carbonate precipitation, forming a thin organic‑inorganic coating on the skeleton that hinders proton attack and reduces net dissolution under acidified conditions [5,6]. This coating is distinct from the biologically controlled calcifying layer and can persist after tissue loss, providing a "ghost" framework that supports reef accretion via non‑coral calcifiers [8].
• Feedback under multi‑stressor conditions: Repeated heatwaves select for hosts with higher heterotrophic capacity; these hosts produce more polysaccharides, which sustain beneficial microbiomes and simultaneously slow skeleton loss. Over generations, this coupling could shift community composition toward taxa that maintain both trophic flexibility and polysaccharide‑mediated protection.

Testable Predictions

1. Polysaccharide quantification: Corals subjected to elevated temperature (+2 °C) and fed Artemia nauplii will show a ≥2‑fold increase in mucus‑bound sulfated polysaccharide concentration compared to unfed controls (measured via Alcian blue assay).
2. Microbiome linkage: 16S rRNA amplicon sequencing of mucus from fed vs. unfed corals will reveal a higher relative abundance of Endozoicomonas (≥15 % increase) only in the fed treatment, and this increase will correlate with polysaccharide levels (Spearman ρ > 0.6).
3. Framework protection: Micro-CT scanning of skeletal plugs incubated at Ω_Ar = 2.5 for 30 days will demonstrate a ≥40 % reduction in porosity loss for plugs pre‑coated with host polysaccharides versus bare plugs, indicating inhibited dissolution.
4. Memory effect: After a second heat‑stress episode, corals that previously received heterotrophic feeding will exhibit lower bleaching severity (F_v/F_m decline < 15 %) and faster recovery of photosynthetic efficiency than naïve corals, an effect abolished when polysaccharide synthesis is inhibited with sodium chlorate (a sulfation blocker).

Experimental Approach

• Factorial design: Temperature (ambient vs. +2 °C) × Feeding (Artemia vs. none) × Polysaccharide inhibition (chlorate vs. control) with n = 6 replicates per condition.
• Measurements: mucus polysaccharide concentration, microbiome composition (16S rRNA metataxonomics), host gene expression (polysaccharide synthase, bleaching markers), skeletal porosity (micro‑CT), and photosynthetic performance (PAM fluorometry).
• Statistical analysis: Two‑way ANOVA for main effects and interaction terms; post‑hoc Tukey HSD; structural equation modeling to test the hypothesized causal pathway (feeding → polysaccharides → microbiome → bleaching resistance → skeletal preservation).

Falsifiability

If heterotrophic feeding does not elevate mucus polysaccharide levels, or if elevated polysaccharides fail to recruit Endozoicomonas or protect the skeleton under low Ω_Ar, the core mechanism is refuted. Similarly, if blocking polysaccharide synthesis does not erase the protective effect of prior feeding on bleaching recovery, the hypothesized feedback loop is invalid.

Implications

Validating this hypothesis would identify a concrete, host‑mediated target for microbiome‑based restoration: enhancing heterotrophic feeding (e.g., via supplemental zooplankton release) or applying exogenous sulfated polysaccharides to bolster both microbial memory and skeletal resilience. It also reframes the OA‑calcification disconnect, suggesting that community‑level calcification can be maintained by non‑cellular polysaccharide coatings even as coral tissue cover declines.

[1] https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.17088 [2] https://phys.org/news/2025-05-coral-die-beneficial-bacteria-microbiome.html [3] https://academic.oup.com/ismecommun/article/5/1/ycae162/8078337 [4] https://academic.oup.com/ismecommun/article/5/1/ycaf097/8157177 [5] https://www.pnas.org/doi/10.1073/pnas.2407112121 [6] https://pmc.ncbi.nlm.nih.gov/articles/PMC12644485/ [7] https://news-oceanacidification-icc.org/2025/08/14/accelerated-ocean-acidification-1985-2022-threatens-tropical-coral-reefs-and-highlights-biogeochemical-refugia-for-marine-conservation/ [8] https://news-oceanacidification-icc.org/2026/03/02/persistence-of-coral-reef-structures-into-the-twenty-first-century/ [9] https://coralreefwatch.noaa.gov/satellite/research/coral_bleaching_report.php