Recent discoveries have highlighted a fascinating paradox in coral immunology: the seawater microbiome surrounding corals provides clearer disease signals than coral tissue microbiomes themselves. While current models use this purely as an early warning system, we are missing the mechanistic link. Why does the adjacent water column shift before the host tissue?

I propose the Exudate-Boundary Hypothesis, which posits that the synergistic effects of thermal stress and ocean acidification (OA) force a metabolic reallocation in corals that fundamentally alters the biochemical composition of their surface mucus layer (exudate). This biochemical degradation transforms the adjacent seawater from a protective microbial halo into an opportunistic pathobiome before actual tissue invasion occurs.

The Mechanistic Breakdown

We know that Ocean acidification from atmospheric CO2 absorption impairs corals' ability to build calcium carbonate skeletons, exacerbating thermal stress effects. However, the historical focus on calcification has overshadowed the immense energetic cost of maintaining pH homeostasis in soft tissues.

Under combined OA and thermal stress, I hypothesize that corals must divert ATP away from the synthesis of complex mucopolysaccharides and antimicrobial peptides (AMPs) to maintain intracellular pH and basic metabolic function. This occurs precisely at the threshold where coral microbiomes—comprising bacteria, fungi, and viruses—play a critical protective role during heat stress.

This metabolic diversion results in a "thinning" of the chemical boundary layer:

1. Reduced molecular complexity in the expelled mucus starves the symbiotic, beneficial microbes residing in the immediate seawater envelope.
2. Decreased AMP secretion removes the chemical suppression of opportunistic, free-floating copiotrophs, allowing them to hyper-proliferate at the coral-water interface.
3. This boundary layer disruption offers a mechanistic explanation for why While several bacterial groups associate with stony coral tissue loss disease (SCTLD), the specific causative pathogen remains unknown. SCTLD and similar afflictions may not be driven by a single primary pathogen, but rather by the metabolic collapse of the exudate-boundary layer, which permits secondary polymicrobial swarms to colonize previously healthy tissue.

Testable Predictions

If the Exudate-Boundary Hypothesis holds true, we can falsify it through targeted mesocosm experiments:

• Prediction 1: Under combined OA (low pH) and thermal stress, the concentration of AMPs and complex glycoproteins in coral exudate will decrease significantly prior to any detectable zooxanthellae expulsion or tissue necrosis.
• Prediction 2: The shift in the adjacent seawater microbiome is directly driven by the altered metabolomic profile of the coral mucus. We can test this by extracting stress-profiled mucus, applying it to inert artificial substrates in sterile seawater, and observing if it recruits a "diseased" microbial signature from raw seawater faster than healthy mucus.
• Prediction 3: Artificially supplementing the boundary layer of stressed corals with exogenous, bio-compatible muco-adhesives loaded with specific AMPs will delay the onset of symbiosis breakdown.

By shifting our focus from isolated tissue biopsies to the metabolic and physical dynamics of the coral-seawater interface, we can move beyond correlation. Understanding the precise biochemical triggers of boundary layer collapse is essential for designing next-generation interventions that stabilize the reef microbiome when endogenous host defenses fail.