Source
- Boilard, Dubé, Gruet, Mercière, Hernandez-Agreda & Derome (2020), LIT-712, read in NOTE-556: §1 (bleaching routes), §2.2 (host regulation of Symbiodiniaceae), §2.3.1 (bacterial regulation, microbial shifts), §3.2 (shuffling, switching, mutualism turning parasitic).
- Goulet, Erill, Ascunce, Finley & Javan (2020), LIT-709, read in NOTE-555: the Discussion sections on bleaching and on measuring several partners.
Both are reviews. Every result below is a cited study that the record has not read, reported as the reviews report it.
What the account says
Bleaching is an outcome, not a mechanism. It is defined by what is seen: fewer Symbiodiniaceae, or less pigment, so that the skeleton shows through the tissue. Goulet et al. (LIT-709) point out that this definition “does not distinguish what drives the Symbiodiniaceae loss”. They give the case that shows the routes coming apart. In two Caribbean octocorals at 3 °C above ambient, symbiont density fell 26% in Eunicea tourneforti and 35% in Pseudoplexaura crucis. Normalised to host lipid, the first had fewer symbionts per host cell. In the second, lipid fell 19% and symbionts per unit lipid did not change, so host cells were lost along with their symbionts. The same observation, a fall in symbiont density, came about by two routes. Only measuring host and symbiont together showed it.
The routes are failures of a regulated exchange. Boilard et al. (LIT-712) list them on the host side as apoptosis, autophagy, exocytosis, detachment and necrosis, by which the host rejects the algae. On the algal side, pigments are lost when thylakoids are exposed to free radicals, with ROS production in the algae “the most likely cause” of expulsion. Bleaching may also follow a shift of the symbiosis “from a mutualistic to a parasitic relationship” when nutrients fall under heat. What breaks down is a relation the review describes as regulated in normal conditions: “the coral host has the ability to regulate the density and diversity of their associated Symbiodiniaceae” through tissue compounds, digestion of symbionts and limits on nitrogen and phosphorus, and nitrifying and denitrifying bacteria regulate dinoflagellate density through the nitrogen to phosphorus balance (§§2.2, 2.3.1). Goulet et al. report that whether the host expels the algae or the algae “abandon a sinking ship” is “still debated”. In some species bleaching varies with the symbiont type hosted, and in others it does not.
So the claim. “Coral bleaching” names the common end state of several distinct breakdowns of a regulated host–symbiont relation. It is not one mechanism of symbiont loss. That makes it a regulatory-disruption case in a precise sense. In normal conditions the partners’ densities and exchanges are held in a range by control on more than one side. Bleaching is what is seen when that control fails, and it can fail at different points.
What this does not say
- It does not say the microbiome drives bleaching. Boilard et al.’s title defines bleaching as “microbial dysbiosis”, and candidate claim (a) as first put to this record framed it as “host–symbiont–microbiome” dysregulation. The source does not support the microbial part. Bleaching “is sometimes accompanied” by bacterial shifts, and “current knowledge makes it impossible to disentangle” whether Vibrio increases are cause or consequence (§2.3.1). Microbiome changes occur “without inferring a causative link” to bleaching states (§3.1). Bacteria are named here only as one of the regulators of the normal exchange, through nitrogen, as reported.
- It does not adopt the three-stage definition (adaptive dysbiosis without and with symbiont restructuring, then traumatic dysbiosis). The stages have no operational boundaries, and their “adaptive” label is untested.
- Evidence for “more than one route” is thin. It is one octocoral study, using lipid as a proxy for host biomass, plus the review-level list of cellular pathways. Scleractinian corals, the reef builders, are not directly represented in the route-separation evidence.
- “Regulated” does not mean one regulator. The account claims that the exchange is held in range by controls on more than one side. It does not claim the holobiont has a unified control system, or that it is one individual.
- It says nothing about bleaching as adaptive. Whether shuffling or switching symbionts after bleaching benefits the coral is a separate question (the Adaptive Bleaching Hypothesis), and Goulet et al. warn that its usual statement builds in host control.
Connections
- The individuality holdings. Whether a regulated host–symbiont exchange makes the holobiont one physiological individual is the question Wilson and Barker (LIT-168) leave open for holobionts and Baedke (LIT-167) answers in the negative. This account is compatible with either answer. It needs only that the exchange is regulated, not that the regulated whole is an individual.
- THEORY-072 draws the line between organisms and dissipative structures, and says that the holobiont boundary question is a different one. This account bears on neither. THEORY-071 uses “regulates” for an agent modulating its coupling with the environment against its own norms. The regulation here is physiological homeostasis between partners, and nothing about agency follows.
- THEORY-058 treats disorder as an alternative stable state kept up by loops. The bleaching literature reviewed here reports both reversible bleaching (recovery, symbiont reversion within 2–3 years) and irreversible bleaching (death). It shows no hysteresis and claims none. No relation is declared.