A few weeks ago I wrote about marine carbon drawdown and restoring ocean fertility.[1] I want to return to that subject because the more I look at it, the more I think the early verdict on ocean iron fertilization - and perhaps on marine carbon dioxide removal more broadly - was too quick.
The standard story goes something like this: scientists tried adding iron to parts of the ocean, they got phytoplankton blooms, but they did not get convincing proof of large-scale carbon sequestration, so the field cooled off. That story is true, but it leaves out the most important part: the trials did not disprove the idea; they exposed the limits of the experiments.
What the Early OIF Trials Actually Proved
The early field trials of ocean iron fertilization (OIF) established one thing clearly: if you add iron to high-nutrient, low-chlorophyll waters, you can stimulate phytoplankton growth.[2][3][4] That matters. The biological mechanism was not imaginary. It worked.
What those trials mostly did not establish was the thing people ultimately cared about: durable carbon sequestration. In other words, yes, blooms formed. But did a meaningful fraction of the carbon fixed by those blooms sink deep enough, and stay isolated long enough, to count as genuine atmospheric drawdown? In most cases, the answer was not clearly yes.[2][3][4]
That distinction turned out to be decisive. The issue was never just whether iron could make something grow. The issue was whether it could strengthen the biological carbon pump in a way that mattered climatically.
Why the Retreat from OIF Was Premature
This is where I think the wrong lesson was drawn.
The early trials showed surface bloom yes, verified sequestration mostly no. They also raised the concern of nutrient robbing: if a bloom uses nitrate, phosphate, and silicate in one place, those nutrients may no longer be available downstream where currents would otherwise have carried them.[4] So even an impressive local bloom may overstate the global benefit.
The trials also flagged another issue that future work needs to confront directly: what kind of phytoplankton are being favored. It is not enough to say that biomass increased. One has to ask whether enrichment might under some conditions favor harmful or toxin-producing species. That is why future trials need to monitor species composition, food-web response, and the risk of harmful algal blooms, not just chlorophyll.[2][5]
These are serious concerns. But they are not reasons to walk away. They are reasons to do the accounting properly and to design better experiments.
Why Marine CDR Deserves Serious Attention
The reason this matters is scale.
Contemporary discussions continue to cite the potential for drawing down gigatons of CO2 per year.[4][6][7] Peter Fiekowsky and others in the climate restoration world have made even larger claims, in the tens of gigatons of CO2 range. Whether those higher-end estimates are right is exactly what needs to be tested.
And scale is where marine CDR becomes especially interesting. Many carbon removal approaches may well have a role to play, but scaling them is difficult. Land-based options run into limits of land, water, permanence, and social acceptance. Industrial approaches such as direct air capture and carbon capture and storage face major infrastructure, energy, and cost hurdles. A recent technical review of scaling carbon capture and storage to gigaton capacity underscores how formidable those system-level challenges really are.[8]
That is part of why the National Academies put such weight on the ocean. In its report A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration, NASEM concluded that ocean-based approaches could offer very large removal potential and that this promise justifies a serious research program.[9] I take that as an important point. The report is not saying marine CDR is already proven. It is saying that, if one is thinking seriously about scale, the ocean is too important to ignore.
That seems exactly right to me.
Why This Is Really About Diatoms
One thing that often gets lost in these discussions is that the story is not really about phytoplankton in general. It is especially about diatoms. Diatoms are single-celled algae with glass-like shells made of silica; they sit near the base of marine food webs and are especially important because they can both feed zooplankton and sink efficiently when they die.
That combination is what makes them so important. Diatoms can support ocean abundance because they help feed the food web, and they can support climate stabilization because they are unusually effective at moving carbon downward. In that sense, they sit at the intersection of the two goals that make marine fertilization so compelling.[2][7]
So the real question is not whether we can make the ocean greener for a short time. The real question is whether we can favor the right organisms, in the right places, in a way that both strengthens marine ecosystems and enhances long-term carbon drawdown.
The Diatom Decline Makes This More Urgent
There is also a broader ecological backdrop that, to my mind, makes this more urgent.
There is evidence that diatoms have declined in some marine systems over recent decades, with warming, changing nutrient regimes, and ocean acidification all implicated.[10] I have not verified the stronger claim that global diatom abundance is now only about two-thirds of its 1950s level, so I am not going to repeat it here without a solid source. But the broader point is enough: the very organisms most useful for both marine food webs and carbon export are under stress in at least some important parts of the ocean.
That should concentrate the mind.
The Whale Connection
This brings me to what I regard as one of the most important and still underappreciated parts of the story: whales.
Great whales historically acted as a kind of biological nutrient pump. They fed at depth and released nutrient-rich fecal plumes near the surface, fertilizing the photic zone from above. Recent work shows that whale feces contain iron and other nutrients relevant to phytoplankton growth.[11] More importantly, recent research shows that whale excrement contains organic ligands that help keep iron bioavailable and can reduce copper toxicity in surface waters.[12]
That is a striking finding. It suggests that whale feces are not just raw waste. They are part of a highly functional nutrient-delivery system.
Put differently, whales did not merely live in ocean ecosystems. They helped maintain them.
The industrial destruction of whale populations likely weakened that nutrient-recycling loop dramatically. Positive Polar frames this as part of a roughly 90% loss in the great whale-mediated fertilization effect.[13] I would treat that specific number as the company’s estimate rather than settled consensus. But the basic point seems hard to dismiss: if whales once helped fertilize surface waters at scale, then modern oceans are likely operating with a diminished version of their former fertility.
A Biomimicry Idea That Deserves Testing
This is where the Positive Polar concept becomes genuinely interesting.
The company says it is developing methods to transform food and human waste generated on ships into materials that biomimic whale feces and urine, and it suggests using organic nanoparticle encapsulation to keep micronutrients in the photic zone longer rather than allowing them to sink quickly, as can happen with more conventional OIF approaches.[13][14]
If something like that worked, it would be more than a clever engineering trick. It would point toward a way for the shipping industry to help restore, at least in part, a nutrient-cycling service that whales once provided naturally.
At this stage that remains a hypothesis, not a demonstrated solution. Still, it is exactly the kind of hypothesis that ought to be tested rather than waved away.
Where I Come Out
To me, the basic point is this: the early trials of ocean iron fertilization were not a reason to stop. They were a reason to get serious.
They showed that iron can stimulate blooms. They did not reliably show durable sequestration. They highlighted nutrient robbing. They underscored the need to watch for harmful ecological shifts. In other words, they identified the hard questions. And then, somehow, many people treated those unanswered questions as if they were final answers.
I think that was a mistake.
If the upside is potentially enormous, if diatoms are central to both marine abundance and climate stabilization, and if the historical loss of whales has weakened a natural surface-fertilization pathway, then this is exactly the kind of subject that deserves disciplined, transparent, well-designed field research.
Not hype. Not dismissal. Research.
References
[1] Marine Carbon Drawdown: Restoring Ocean Fertility.
[2] Woods Hole Oceanographic Institution, Iron Fertilization.
[3] Yoon, J.-E. et al., Ocean iron fertilization experiments - past, present, and future, Biogeosciences (2018).
[4] Buesseler et al., The case for ocean iron fertilization field trials.
[5] Science News, Iron fertilization in ocean nourishes toxic algae.
[6] ExOIS, Assessing its potential as a climate solution.
[7] Woods Hole Oceanographic Institution, Scientists outline case for next-generation ocean iron fertilization field trials.
[9] National Academies, A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration.
[10] Ocean Acidification News, Decline of diatoms due to ocean acidification.
[11] University of Washington, Whale poop contains iron that may have helped fertilize past oceans.
[13] Positive Polar, Whale Biomimicry Research Program.
[14] Positive Polar, Sword.
