Core Chemistry of Calcification
Write the balanced chemical reaction for coral skeleton formation.
Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s).
Define Ωₐᵣₐ qualitatively.
A measure of how supersaturated seawater is with respect to aragonite (depends on [Ca²⁺][CO₃²⁻]/Ksp).
Two environmental factors (other than ions) that influence calcification rate.
Temperature and light (also pH).
State the simple rule for long-term reef growth vs. erosion.
Accretion > destructive processes.
A student says “Ca²⁺ is the limiting ion.” Challenge that claim using a single sentence.
In seawater, Ca²⁺ is ample; CO₃²⁻ availability is typically limiting for Ωₐᵣₐ and calcification.
Name the crystal form of CaCO₃ produced by reef-building corals.
Aragonite.
State the stress threshold for reefs commonly cited for today’s oceans.
Around Ωₐᵣₐ ≈ 3.3 (reef accretion ~zero/begins to go negative).
Why does light matter for many reef-building corals?
Photosynthesis by zooxanthellae energises calcification; more light → more energy (to a point).
If maps show Ωₐᵣₐ < 3.3 expanding across the tropics, what happens to global reef-building capacity?
It shrinks (reduced or negative net accretion).
Name the approximate % of nutrition corals gain from zooxanthellae in many reefs.
~95%.
In one sentence, state what determines the arrow’s direction in Ca²⁺ + CO₃²⁻ ⇌ CaCO₃.
The saturation/availability (concentration) of dissolved ions in seawater (relative to Ksp).
Give the carbonate-ion concentration often linked with that threshold.
~200 μmol kg⁻¹.
Briefly describe how lower pH affects carbonate availability.
More H⁺ shifts carbonate speciation toward HCO₃⁻/CO₂, reducing CO₃²⁻; Ωₐᵣₐ falls.
Connect rising atmospheric CO₂ to reef distribution in one causal chain.
↑CO₂ → ↓pH → ↓CO₃²⁻ → ↓Ωₐᵣₐ → weaker/slower calcification → range/construction limits.
Give one simple field observation that might hint Ωₐᵣₐ is chronically low at a site.
Poor skeletal density/fragile growth; low net accretion/erosion dominance.
Where—physically—does CaCO₃ precipitation happen in a coral?
In the calcifying fluid just above the skeleton.
Approximate atmospheric CO₂ level associated with Ωₐᵣₐ ≈ 3.3 in many oceans.
~480 ppm CO₂.
A warm-water spike adds thermal stress but also increases metabolic demand. Predict net effect on calcification during/after a heatwave.
Often reduced calcification (and/or bleaching) despite higher kinetics, due to symbiosis breakdown and acid–base stress.
Inshore turbid lagoon vs. clear outer shelf of equal temperature—where is Ωₐᵣₐ usually more favourable and why?
Clear outer shelf; less freshwater/acidification/dilution, typically higher CO₃²⁻/Ωₐᵣₐ.
Propose one classroom demonstration to model nematocyst discharge or the “arrow direction” idea.
Glove/balloon “nematocyst” pop lab; or dissolving/precipitation demo showing saturation controls.
If carbonate ion concentration drops while Ca²⁺ and Ksp stay constant, predict the shift.
Reaction shifts left (less precipitation/more dissolution).
Explain why Ωₐᵣₐ < 1 is catastrophic for corals.
Seawater is undersaturated—thermodynamics favour CaCO₃ dissolution (arrow left); skeletons dissolve.
A site has normal Ca²⁺, high light, but chronically low CO₃²⁻. Which single intervention would most directly improve calcification potential?
Raising Ωₐᵣₐ (i.e., increasing CO₃²⁻/alkalinity locally) rather than more light/heat.
Under a high-emissions pathway (e.g., RCP8.5), describe one likely 2050s outcome for reef frameworks.
Many reefs transition toward rubble/low-coral frameworks as accretion approaches zero/negative.
A dataset shows [CO₃²⁻] = 180 μmol kg⁻¹, Ωₐᵣₐ = 3.0 during summer. Suggest a mechanistic reason calcification slowed despite high light and warm water.
Carbonate ion scarcity (low Ωₐᵣₐ) constrained CaCO₃ precipitation despite energetic support from light/temperature.