Can a thin high-pH layer speed coral growth?

Can a coral grow faster when alkalinity is raised only within millimetres of its base? Kiel and colleagues tested an electrochemical system that changed the chemistry beside a steel cathode without measurably changing the aquarium’s bulk water. The answer depended on coral height.

Study at a glance: two 60-day flow-through experiments used Atlantic nursery corals. The clearest result came from 48 Pseudodiploria clivosa microfragments from six genets, nested within four 50-litre aquaria: two electrochemical tanks and two control tanks. A separate experiment used 56 Acropora cervicornis fragments across four 150-litre aquaria.

The evidence

The electrochemical treatment was assigned at aquarium level in the brain-coral experiment: two tanks per system condition.

The central caveat: 48 brain-coral fragments do not equal 48 independent treatment replicates. The electrochemical system was represented by two tanks and the control by two tanks. Fragment- and genet-level replication improves precision inside those tanks, but it cannot replace independent aquaria.

What remains unresolved?

The design has several useful strengths. The researchers included inert substrate controls, blank powered cathodes to estimate non-biological mineral gain, two fragment heights chosen around the measured pH layer, repeated growth measurements, multiple genets and a second coral morphology. All 48 brain-coral fragments survived, and final Fv/Fm did not differ by system or height.

The largest uncertainty is scale. Two tanks per electrochemical condition make the result promising rather than decisive. Steel cathodes and acrylic controls also differ in material as well as electrical state. The blank-cathode correction addresses mineral mass deposited on the hardware, but it does not turn the comparison into a material-matched sham circuit.

Flow changed the treatment itself. At 3 cm s⁻¹, the pH measured 5 mm above the cathode was 8.03—nearly the 8.02 bulk value—and the estimated boundary layer was only 5.92 ± 1.44 mm thick. That makes pump placement, local velocity and coral geometry part of the intervention, not background details.

What this means

What the study found: in this flow-through system, five-millimetre brain-coral microfragments positioned within a locally elevated-pH layer calcified and spread tissue faster than controls for 60 days. Taller brain-coral fragments and five-centimetre staghorn fragments did not receive a measurable biological growth benefit.

Reef Cloverson interpretation: this is not evidence that raising whole-tank alkalinity by electrolysis will speed every coral. It is evidence for a geometry-dependent treatment zone. The most useful next question is whether the same advantage survives when tank replication is increased and local flow is deliberately varied while electrical current and fragment height are held constant.

Could we replicate this?

Published protocol: the brain-coral experiment used four independent 50-litre flow-through aquaria, two per system condition. Each tank held one 5 mm and one 15 mm fragment from each of six genets. Treatment fragments sat on steel cathodes operated at 1 A m⁻²; controls sat on similarly sized inert acrylic pucks. The run lasted 60 days after a one-week healing period. Corals received filtered Biscayne Bay seawater, 27°C temperature control, programmed LED light and twice-weekly target feeding.

The powered system used a platinized titanium-mesh anode inside an actively evacuated chamber. That safeguard removed acidic and oxidizing products generated at the anode. Growth endpoints combined buoyant weighing, photographs analyzed in Fiji and a final imaging-PAM measurement. The team also used microsensors and laboratory carbonate-chemistry instruments to map what the system actually changed.

Proposed adaptation—not performed by the authors: a nursery pilot could focus on planar tissue spread, use independently controlled treatment and sham-control tanks, block fragments by genet, standardize fragment height, document local velocity at every plug and photograph on a fixed schedule. Bare powered substrates would still be needed to track mineral deposition. A pilot without buoyant weighing could test tissue expansion, but it could not reproduce the calcification result; one without a micro-pH method could not confirm that fragments remained inside the intended chemical layer.

This is not a casual plug-and-play aquarium intervention. Electrolysis can generate hazardous anodic products, and the published apparatus used engineered evacuation, regulated power, corrosion-resistant components and chemical monitoring. A smaller setup could test measurement repeatability, but it could not establish nursery-scale effectiveness or field safety.

U.S. research resources: the work connects the University of Miami’s Rosenstiel School and Cooperative Institute for Marine and Atmospheric Studies with NOAA’s Atlantic Oceanographic and Meteorological Laboratory in Miami. The authors released supporting data and scripts publicly, and PANGAEA archived the chemistry and growth dataset. These are methods resources, not offers of specimens, facility access or collaboration.

Methods: How they did it

The team first characterized the apparatus with three-hour seawater incubations at 0.5, 1 and 3 A m⁻², then collected replicate pH microprofiles while changing flow speed and current density. Hyperbolic-tangent models estimated the thickness of the altered layer above the cathode.

For P. clivosa, 48 square microfragments from six genets were randomized among four tanks and evenly divided between 5 and 15 mm heights. The two powered tanks received cathodes plus three partially masked blank cathodes; the two controls received acrylic pucks. Mass was measured repeatedly by buoyant weighing. Photographs tracked live planar area. Mixed-effects models accounted for aquarium nesting, genet and repeated measurements, with Tukey adjustment for pairwise comparisons.

The separate A. cervicornis test used 56 five-centimetre fragments from seven genets in four 150-litre aquaria, with paired powered and inert substrates inside each aquarium. Apparent extra mass on powered plugs disappeared after the researchers subtracted abiotic mineral deposition measured on bare cathodes.

Funding source

The paper reports support from the U.S. National Science Foundation Graduate Research Fellowship Program, grant 1938060, awarded to Patrick M. Kiel; the University of Miami Laboratory for Integrative Knowledge project “Engineering Corals for Climate Change Resilience”; University of Miami start-up funding to Vivek N. Prakash; and NOAA’s Coral Reef Conservation Program. The acknowledgements also identify coral, laboratory and discussion support from University of Miami and NOAA colleagues. The authors declared no competing interests.

Sources

Kiel et al. (2026), Coral Reefs, 45:737–752 · primary paper

Supporting data and analysis scripts · GitHub

Archived chemistry and coral-growth dataset · PANGAEA

Institutional research summary · NOAA AOML