The scientific rationale behind the experiment is rooted in basic ocean chemistry. Since the beginning of industrialisation, atmospheric carbon dioxide concentrations have risen sharply, and a substantial fraction of that gas has dissolved into the sea. This influx has shifted seawater chemistry toward progressively lower pH, a process known as ocean acidification, which places particular stress on organisms that build calcareous structures such as mussels, corals and many planktonic species. At the same time, the added carbon dioxide is a principal driver of global warming. Increasing the alkalinity of seawater raises its buffering capacity, which counteracts acidification and allows the ocean to take up additional CO2 and convert it into more stable dissolved forms. In principle, the chemistry is straightforward. What remains uncertain is how marine ecosystems respond to different alkalinity sources, concentrations and modes of delivery, and where the ecological tolerance limits lie.

That uncertainty is precisely what the Taliarte experiment is designed to address. According to Emeritus Professor Ulf Riebesell, a marine biologist at GEOMAR and co-lead of the study, this year’s trial is about comparing a liquid source of alkalinity with ground concrete rubble and assessing how well both substances are tolerated by the marine environment. The work forms part of the international research project OceanAlkAlign, which aims to standardise measurement and assessment methods for Ocean Alkalinity Enhancement and thereby create a robust scientific foundation for future decisions about whether, where and how such techniques might ever be deployed. The choice of location is no accident: the Canary Islands Marine Research Institute PLOCAN, a long-standing research partner of GEOMAR, sits directly beside the Taliarte harbour, from which the mesocosms are deployed, filled and monitored throughout the entire duration of the experiment.

The tools of the trade are the KOSMOS mesocosms, short for Kiel Off-Shore Mesocosms for Future Ocean Simulations, which GEOMAR has been developing and deploying since 2006. These enclosures function like enormous test tubes suspended in the sea, each enclosing a natural slice of the coastal ecosystem, including planktonic food webs, microbial communities and the biogeochemical processes that connect them. Because the water inside each mesocosm is isolated from the surrounding ocean, researchers can manipulate conditions in individual units and compare outcomes against untreated controls with a precision that would be impossible in the open sea. Across 23 experiments to date, the KOSMOS platform has been used to investigate ocean acidification, warming, nutrient dynamics and potential countermeasures including artificial upwelling and various alkalinity enhancement approaches, making it one of the most experienced experimental infrastructures of its kind in the world.

The experimental design in Gran Canaria follows a comparative logic. Natural plankton communities will be observed over several weeks inside the twelve mesocosms. Some of the systems receive increasing quantities of finely ground concrete rubble, while others are treated with liquid sodium hydroxide as a reference representing pure dissolved alkalinity. A further set of enclosures remains untouched as controls. Throughout the seven-week run, the team will track changes in the seawater carbon dioxide system, including pH and total alkalinity, alongside the degree of CO2 uptake achieved. In parallel, an extensive suite of biological measurements will capture the composition and productivity of the phytoplankton, the responses of zooplankton, microbial process rates, and any indications of shifts rippling through the food web. Together, these data streams allow the researchers to evaluate both the effectiveness of each alkalinity source and any potential side effects along the chain from microbes to grazers.

The choice of concrete as a candidate material is driven by a striking waste-management reality. Concrete rubble is one of the largest waste streams on the planet, with an estimated five billion tonnes generated every year, and only a fraction of that material has so far been reused. Because concrete contains cement, which is inherently alkaline, finely ground demolition waste could in principle serve as a cheap and abundant source of alkalinity for the ocean. Model estimates are already exploring whether large quantities of carbon dioxide could be sequestered in this way over the long term, potentially turning a costly disposal problem into a component of climate mitigation. The appeal is obvious: if a material that already exists in staggering quantities can safely enhance the ocean’s carbon uptake, the economics and logistics of carbon dioxide removal could look very different from approaches that require manufacturing alkalinity from scratch.

Yet the researchers are emphatic that apparent availability does not equal environmental safety. A material that seems unproblematic on land can behave very differently at sea. Ground concrete introduces not only alkalinity but also solid particles, which can increase turbidity in the water column and potentially harm microorganisms, with consequences that could propagate through the food web. Associate Professor Kai Schulz of Southern Cross University in Australia, co-leader of the experiment, stresses that a waste product does not automatically become a sustainable solution simply because it is available. In his view, the field needs data showing under what conditions Ocean Alkalinity Enhancement could be ecologically acceptable and where its limits lie. That caution reflects a broader tension in the emerging carbon removal landscape, where the urgency of climate targets creates pressure to scale technologies faster than the ecological evidence base can comfortably support.

The wider context makes the stakes clear. According to many current scenarios, emission reductions alone will not be sufficient to meet the goals of the Paris Agreement, which has pushed methods for active carbon dioxide removal to the forefront of climate policy discussions. Among the portfolio of candidate approaches, Ocean Alkalinity Enhancement is regarded as an option with high potential, provided it can be implemented effectively and in an environmentally responsible manner. The ocean already absorbs a substantial share of humanity’s carbon emissions each year, and even modest enhancements to that capacity, applied across vast areas, could translate into gigatonne-scale removal. But the same vastness that makes the ocean attractive as a carbon sink also makes mistakes difficult to reverse, which is why controlled, ecosystem-level experiments like the one in Taliarte are considered essential prerequisites before any larger deployment could be responsibly considered.

The ultimate objective of the Gran Canaria study is to help define what the researchers describe as a safe operating space for alkalinity enhancement. The results are intended to inform threshold values: which dosages alter the water chemistry in the desired way, and at what point ecological effects become apparent? What differences emerge between solid particulate sources and dissolved alkalinity? And how can findings from laboratory studies, mesocosm experiments and open-ocean field trials be combined into a body of evidence robust enough to guide real-world decisions? Schulz distils the philosophy of the project into a simple maxim: understanding before scaling up. If Ocean Alkalinity Enhancement is ever to be discussed at a meaningful scale, he argues, it must rest solely on transparent data regarding both benefits and risks.

For now, the twelve floating enclosures in Taliarte harbour will continue their seven-week vigil, quietly recording how plankton communities, microbes and water chemistry respond to each dose of ground concrete or sodium hydroxide. Whatever the outcome, the experiment marks a rare moment when two enormous problems, the mounting mountains of construction demolition waste and the urgent need to remove carbon dioxide from the atmosphere, are tested against each other in the same seawater. If the rubble proves both effective and benign, it could open a genuinely circular pathway from demolition site to ocean carbon sink. If it reveals unexpected harms, the results will have done exactly what careful science is meant to do: draw a clear line around what is safe before anyone is tempted to cross it.

Subject of Research: Ocean alkalinity enhancement using ground concrete demolition waste tested in mesocosm experiments

Article Title: Construction waste for climate protection in the ocean?

Article References: Construction waste for climate protection in the ocean?. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: ocean alkalinity enhancement, carbon dioxide removal, concrete rubble, mesocosm experiment, ocean acidification, GEOMAR, KOSMOS, Gran Canaria, PLOCAN, OceanAlkAlign, marine ecosystems, climate mitigation

Researchers led by Harm van Kuppevelt of the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin, working with colleagues at Brandenburg University of Technology, the University of Southern Denmark and Aarhus University, set out to test how long iron-bound phosphorus actually survives in sediment when sulfide is present. Their findings, published in the journal Biogeochemistry, reveal a slow chemical heist in which sulfide strips iron away from phosphorus, converting a durable mineral vault into a leaky one. The work matters because it identifies a measurable early warning signal, porewater sulfide concentration, that could tell lake managers their iron treatment is losing its grip before blooms return.

The mineral at the heart of the story is vivianite, an iron phosphate with the chemical formula Fe(II)3(PO4)2·8H2O. When iron is added to eutrophic lake sediment under anoxic conditions, dissolved phosphorus can precipitate with ferrous iron to form this pale blue-green mineral. Vivianite is prized by restoration ecologists because it is redox-stable: unlike the loosely adsorbed phosphorus that clings to iron oxides and dissolves the moment oxygen disappears, vivianite holds its phosphorus even in oxygen-free sediment. In theory, once phosphorus is locked into vivianite, it should stay buried for years, breaking the feedback loop that keeps eutrophic lakes green long after external nutrient inputs have been reduced.

Theory, however, meets a complication in sulfate-rich waters. Many lakes affected by salinization, seawater intrusion, agricultural runoff or drought-induced water level changes carry elevated sulfate concentrations. In anoxic sediment, microbes respire sulfate instead of oxygen, producing sulfide as a byproduct. Sulfide is a ferocious chemical competitor for iron, binding it into iron sulfide minerals such as amorphous FeS and the far more stable pyrite. If sulfide outcompetes phosphate for the iron in vivianite, the mineral should dissolve, releasing its phosphorus into the porewater and, potentially, back into the overlying lake. Whether and how fast this happens in realistic sediment conditions was, until now, poorly quantified.

To find out, the team built a controlled model system in the laboratory: a 96-day mesocosm experiment using lake sediment that had been amended with iron and phosphorus and deliberately enriched with vivianite during an anoxic pre-incubation period. This ensured that the sediment started with a substantial pool of iron-bound phosphorus in the very mineral form that iron treatments are meant to create. The sediment was then incubated under oxic overlying water at two sulfate levels, one low, below 100 micromoles per liter, and one high, around one millimole per liter, mimicking the range found in freshwater systems under different degrees of sulfate influence.

The experimental design was deliberately multi-pronged, because no single technique can capture the full picture of what happens to iron and phosphorus in sediment. The researchers measured porewater profiles with microsensors and with diffusive gradients in thin films, known as DGT, a technique that samples dissolved solutes at high spatial resolution in the sediment’s microscopic pore spaces. They complemented these measurements with sequential chemical extraction of the solid phase, which separates phosphorus into operationally defined pools of decreasing reactivity, and with scanning electron microscopy coupled to energy dispersive spectroscopy, which reveals the elemental composition of individual mineral grains. X-ray diffraction completed the toolkit by identifying crystalline mineral phases.

The results told a clear and sobering story. Under oxic overlying water, vivianite persisted in the sediment, confirming its reputation as a robust phosphorus sink when sulfide is scarce. But as sulfate-reducing microbes accumulated sulfide in the sediment, the mineral was progressively destabilized. The sulfide drove coupled dissolution-reprecipitation reactions: vivianite dissolved, its ferrous iron was captured by sulfide, and new amorphous iron sulfides formed, some of which matured into pyrite. The phosphorus released in the process did not simply vanish. A portion of it was re-adsorbed onto freshly precipitated iron(III) phases in the oxygenated surface layer, where oxic conditions allowed iron oxides to form and grab dissolved phosphate. Yet this rescue operation was only partial. The net effect in both treatments was a decline in the sediment’s total phosphorus-binding capacity and a measurable loss of total solid-phase phosphorus.

The chemistry behind this transformation is worth appreciating in detail, because it illustrates why sulfidic conditions are so corrosive to iron-based phosphorus retention. Vivianite owes its stability to the strong bonds between ferrous iron and phosphate within its crystal lattice. Sulfide attacks this stability on two fronts. First, dissolved sulfide is a stronger ligand for ferrous iron than phosphate under the relevant conditions, so it thermodynamically favors the formation of iron sulfides. Second, once iron sulfides such as pyrite form, they are kinetically inert, meaning the iron is effectively removed from the phosphorus cycle for good. The dissolution-reprecipitation sequence observed in the mesocosms, in which vivianite-bound iron was converted into amorphous FeS and ultimately pyrite, therefore represents a one-way ratchet: each sulfide molecule that captures an iron atom permanently reduces the sediment’s capacity to hold phosphorus in a redox-stable mineral form.

Importantly, the study also shows that the fate of released phosphorus depends on the redox structure of the sediment. The oxic surface layer acted as a partial safety net, because oxygen diffusing down from the overlying water allowed fresh iron(III) oxides to precipitate and re-bind some of the liberated phosphate. This finding suggests that lakes with a well-oxygenated sediment-water interface may temporarily buffer the phosphorus release triggered by sulfide-driven vivianite dissolution. But the buffer is finite, and the underlying loss of iron-binding capacity continues as long as sulfate reduction proceeds. In lakes that stratify in summer and develop anoxic bottom waters, that safety net disappears precisely when internal loading pressures are highest, raising the risk that sulfide-driven phosphorus release coincides with the season of maximum algal growth.

For lake managers, the practical message is that iron treatments should not be treated as permanent fixes, particularly in systems with rising sulfate loads. The authors highlight that monitoring porewater sulfide concentrations could serve as a practical early warning indicator of declining phosphorus retention capacity. Sulfide is relatively straightforward to measure with microsensors, colorimetric methods or peeper samplers, and rising sulfide levels in sediment porewater would signal that vivianite and other iron-bound phosphorus pools are under chemical attack. Such monitoring could inform decisions about whether repeated iron dosing is needed, whether sulfate inputs from salinization or pollution should be controlled, and whether the expected longevity of a restoration investment needs to be revised downward.

The research also carries a broader environmental warning. Sulfate concentrations in inland waters are increasing worldwide due to seawater intrusion into coastal aquifers, road salt application, mining discharge, acid sulfate soil runoff and reduced dilution during droughts. Each increment of sulfate is a potential increment of sulfide, and each increment of sulfide erodes the iron-phosphorus chemistry on which many restoration strategies depend. The study’s controlled mesocosm results now provide a mechanistic chain of evidence connecting sulfate availability, microbial sulfate reduction, sulfide accumulation, vivianite dissolution, iron sulfide formation and net phosphorus loss from sediment. As climate change and land use intensification push more sulfate into lakes, the fragile mineral vault that keeps phosphorus buried may be opening in far more places than managers currently realize, and the smell of rotten eggs rising from a lake bed may be the first clue.

Subject of Research: The persistence and sulfide-driven destabilization of iron-bound phosphorus, particularly vivianite, in lake sediment under oxic and sulfidic conditions.

Article Title: Investigating the persistence of iron-bound phosphorus in lake sediment under sulfidic conditions

Article References: van Kuppevelt, H., Hupfer, M., Reitzel, K., Sudo, M. L., & Marzocchi, U. (2026). Investigating the persistence of iron-bound phosphorus in lake sediment under sulfidic conditions. Biogeochemistry, 169(5), Article 55. https://doi.org/10.1007/s10533-026-01375-3

Image Credits: AI Generated

DOI: 10.1007/s10533-026-01375-3

Keywords: vivianite, iron-bound phosphorus, internal phosphorus loading, lake restoration, sulfate reduction, sulfide, phosphorus mobilization, eutrophic lakes, sediment biogeochemistry, pyrite, porewater, mesocosm experiment