Geoengineering the ocean – a viable solution to our climate crisis?

© August 2012, NASA archives

Satellite image of an algal bloom west of Haida Gwaii, an archipelago off British Columbia, caused by fertilizing the ocean using 120 tonnes of iron sulphate. 

The Microbiology Society is undertaking a project entitled A Sustainable Future as part of our 75th Anniversary, which aims to highlight the Sustainable Development Goals (SDGs) to our members and empower them to use their research to evidence and impact the goals. Earlier this year, we put a call out to our members to submit case studies in the following three areas: antimicrobial resistance, soil health and the circular economy.

This case study is written by Dr Elena Kazamia, who is a Research Scientist at the Institut de Biologie de l’Ecole Normale Supérieure (IBENS) and a member of the Microbiology Society. It focuses on the circular economy; an alternative to a traditional linear economy (make, use, dispose), in which we keep resources in use for as long as possible, extract the maximum value from them while in use, then recover and regenerate products and materials at the end of each service life.

Accounting for less than 1% of the photosynthetic biomass, ocean-dwelling microalgae are responsible for half of the global carbon fixation. Invisible to the human eye, microalgae are major players in the planet’s carbon cycle. When microalgae die, a minute portion of their biomass is transported to the ocean deep undegraded (the rest is decomposed in the water column by bacteria).

This process, described as the “biological carbon pump”, collectively generates a considerable carbon sink1. Eventually, the debris is brought back to the surface ocean where it fertilises the growth of ocean microbes. But the process is slow, taking place on the scale of hundreds to thousands of years. The idea of strengthening the biological carbon pump by fertilising the ocean, and thus increasing its algal density, has been proposed as a geoengineering solution to the ongoing climate crisis.

The premise is simple – perhaps by speeding algal growth and their subsequent death we can lock away in the ocean deep the excess carbon dioxide that we have been emitting by burning fossil fuels. One way to think of this is as controlled eutrophication. A cost-effective and plausible strategy for large-scale ocean fertilisation emerged in the 1990’s, with the discovery that the growth of algae in vast regions of the ocean, was limited by iron2. Infamously, the oceanographer Professor John Martin quipped, only half-joking, “Give me half a tanker of iron, and I will give you the next ice age”. Experiments showed that it was possible to stimulate algal blooms visible from space by simply adding iron filings to ocean waters3. Commercial ventures saw the financial benefit of iron fertilisation as an inexpensive strategy for generating carbon credits in a growing market.

What followed was a thoroughly reasoned backlash from expert oceanographers4. They argued that we have little knowledge of how to control the strength of the biological carbon pump, even if we got a handle on stimulating algal blooms. In their opinion, such geoengineering measures ignored the knock-on effects on community dynamics, by way of altering food webs and upsetting ecosystem balances. There were also risks to changing other nutrient cycles besides that of carbon, and creating artificial anoxic zones in the ocean, similar to what we have seen in eutrophicated lakes. Geoengineering proposals were taking a gamble with the only ocean we have. Thirty years on, the idea of ocean fertilisation has not gone away. Despite a global ban on commercial large-scale ocean fertilisation, research proposals continue to pitch this as a strategy for CO2 sequenstration5.

In my research, I study the response of diatoms – the most prolific group of ocean microalgae – to different iron sources6. My research starts in the laboratory where I investigate the iron physiology of model species. I extend my analyses to ocean models, which are based on community samples of algae collected during global circumnavigation projects, such as Tara Oceans7. Invariably, together with colleagues, I find that iron physiologies are species-specific, and iron stimulation would have complex knock-on effects on the microbial community structure, which would vary with location and season.

In the past 30 years, we have made progress in understanding the structure of algal communities largely attributed to the ‘omics revolution’. We have a clearer perspective on the diversity, distribution and evolutionary history of microalgae. However, we remain far from understanding the function and dynamics of ocean systems. This is a challenge that requires interdisciplinary action and a stronger emphasis on community ecology. Continued research into the growth response of various algal groups to iron stimulation is important, so that we can gain an insight into the balances that we seek to disrupt through geoengineering.

It is my opinion that to bargain on cascading effects through an ecosystem by fertilising the ocean remains a risky strategy, which should not be admissible at this time of environmental crisis. A more direct approach to decreasing atmospheric carbon dioxide emissions is our only option for a sustainable future. For this, drastic cuts in emissions are a necessary first step.  


Henson, S.A., Sanders, R., Madsen, E., Morris, P.J., Le Moigne, F., Quartly, G.D., 2011. A reduced estimate of the strength of the ocean's biological carbon pump. Geophysical Research Letters. 38(4).
Geider, R.J. and La Roche, J., 1994. The role of iron in phytoplankton photosynthesis, and the potential for iron-limitation of primary productivity in the sea. Photosynthesis research39(3), pp.275-301.
Coale, K.H., Johnson, K.S., Fitzwater, S.E., Gordon, R.M., Tanner, S., Chavez, F.P., Ferioli, L., Sakamoto, C., Rogers, P., Millero, F. and Steinberg, P., 1996. A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean. Nature383(6600), pp.495-501.
Chisholm, S.W., Falkowski, P.G. and Cullen, J.J., 2001. Dis-Crediting Ocean Fertilization. Science294, p.309.
Emerson, D., 2019. Biogenic iron dust: A novel approach to ocean iron fertilization as a means of large scale removal of carbon dioxide from the atmosphere. Frontiers in Marine Science6, p.22.
Bork, P., Bowler, C., De Vargas, C., Gorsky, G., Karsenti, E. and Wincker, P., 2015. Tara Oceans studies plankton at planetary scale.

 About the author

Dr Elena Kazamia is a Research Scientist at the Institut de Biologie de l’Ecole Normale Supérieure (IBENS) and a member of the Microbiology Society. More information about her work is available here.