Fertile seas

Feeding diatoms

Diatoms have a remarkable ability to multiply quickly when nutrients are available, producing massive blooms. When the nutrient supply is exhausted, they can no longer multiply and they die off very quickly. The whole process has been likened to a financial ‘boom-and-bust cycle’.

In nature, such sudden blooms are observed, for instance, when an iceberg splits from an Antarctic glacier and delivers minerals to the relatively nutrient-poor waters of the Southern Ocean. Scientists have discovered that iron is often the limiting factor in diatom growth in the oceans. Phytoplankton organisms need iron as a cofactor in the enzyme nitrogenase, which turns molecular nitrogen into biologically usable compounds.

This finding has led to two questions that can be addressed experimentally: whether fertilising the oceans with iron could produce artificial and controlled bloom-and-bust cycles; and whether carbon pulled down by blooming algae could be removed from circulation for long enough to help in the fight against climate change.

There have been around a dozen such experiments so far, and the answer to the first question seems to be a resounding ‘yes’. Whenever researchers released soluble iron compounds into the Southern Ocean, diatoms reliably responded with a rapid bloom. Recent research into the physiology of diatoms is beginning to elucidate how these organisms respond to the sudden availability of iron, and why, of all plankton species, they are the quickest to benefit from it. Results show that the activity of hundreds of diatom genes changes very rapidly when iron becomes available, linking iron uptake to metabolic reactions including nitrate assimilation, photosynthesis, the urea cycle, and carbohydrate synthesis (Proc. Natl. Acad. Sci. U.S.A, 2012, 109, E317).

The second question, whether the carbon they fix in their bloom phase will actually be removed from circulation by their bust phase, is much harder to answer and is more controversial.

In July 2012, a team led by Victor Smetacek and Christine Klaas, from the Alfred Wegener Institute for Polar and Marine Research at Bremerhaven in Germany, published the results of a European iron fertilisation experiment (EIFEX) done in 2004 on board the German research vessel Polarstern in the Southern Ocean (Nature, 2012, 487, 313).

The researchers used the core of a stable eddy in the ocean as their test tube – this core had very little exchange with horizontally neighbouring areas of the ocean, and represented a well-defined and stable vertical column down to the sea floor. In a circular study area of 167km², the researchers released iron(II) sulphate to increase the iron concentration in the upper 100m of the sea water by a factor of five, to 1.5nM. Two weeks later, they released the same amount of iron to the area of the spreading bloom, enhancing the iron concentration by a further 0.34nM.

Over a period of 40 days after the first release, the team monitored both the water circulation and mixing of the iron, and the biological response to the fertilisation. They detected the bloom-and-bust cycle through a variety of substance concentrations in water samples collected from various depths, and in chlorophyll profiles and particle concentrations in the water column down to the sea floor. The chlorophyll concentration showed dramatic increases in the storm-mixed surface layer down to 100m, beginning four days after fertilisation, and reaching a peak after 24 days. After that point, the sinking of dead cells, often within larger aggregates, decreased the concentration of all biological substances in the surface layer.

The bloom in this experiment was much stronger than in the similar LOHAFEX experiment, which Smetacek’s team conducted later in 2009. LOHAFEX was carried out in a part of the Antarctic Circumpolar Current which has very low concentrations of silicate because it is all taken up by diatoms there. So diatoms were also stimulated by the iron and grew, but nothing like the diatoms in the silicate-rich water of the EIFEX eddy core,’ Smetacek explains. ‘Carbon sequestration will only function if mediated by diatoms because they are the best protected [from grazing].’

The measurements suggest that at least half the biomass produced in the bloom sank below a depth of 1000m. From this, the researchers concluded that much of this material would have reached the sea floor, where it would be safely sequestered for decades, centuries, or even longer.

Expressed as an efficiency ratio, the results suggest that each mole of iron released in the experiment will have led to the sequestration of 13,000 moles of carbon. This ratio is higher than in other fertilisation experiments reported so far, but laboratory studies suggest that it could be improved even further.

Sallie Chisholm, oceanographer at Massachusetts Institute of Technology (MIT), US, comments that ‘small scale ocean experiments are powerful tools for understanding how ecosystems work and we have learned a lot from them. But the preoccupation with geoengineering applications has led oceanographers to focus their measurements on one single thing – the export of carbon to the deep sea – at the expense of measuring all of the different responses of the ecosystem that can give us critical insights into how these systems work.’

Chisholm adds: ‘The focus on carbon export gives the impression that these experiments are designed to study the efficacy of ocean fertilisation for carbon sequestration. But, in fact, results from these types of experiments cannot be scaled up to “geoengineering proportions” – that is, magnitudes that could make ocean fertilisation a significant component of the portfolio of climate mitigation options.’

Commercial interest

Since the first fertilisation experiments in the late 1990s, several companies have sprung up aiming to commercialise this approach. One of them, California-based Planktos, created controversy when its plans to release iron filings near the Galapagos Islands and near the Canary Islands led to protests, finally blocked by local authorities.

The company then ran into financial difficulties until it officially abandoned its quest to commercialise iron fertilisation in early 2008. Rival companies, Climos in the US and Ocean Nourishment Corporation (ONC) in Australia, have also gone quiet in recent years.

However, the former chief executive of Planktos, Russ George, returned to the headlines in October 2012, after he released around 100t of iron sulphate into the Pacific off the Canadian coast. Early press reports suggested he had won the support of the population of the nearby Haida Gwaii Islands by presenting the endeavour as a ‘salmon restoration project’.

George said that scientists would be monitoring the biological impact of the fertilisation, but did not reveal names of researchers involved.

Many experts in the field are critical of commercial iron fertilisation projects. Smetacek wants to keep commercial interest out of the endeavour: ‘Under all circumstances, the experiments and later large-scale applications must remain in the hands of scientists employed by international, non-profit agencies analogous to the IAEA (International Atomic Energy Agency) or WHO (World Health Organisation),’ he says.

Others have argued that we still know far too little about the effects of large- scale release of iron on the ecosystems. For instance, blooms would deplete the water of oxygen, which could endanger animals in the fertilised surface waters. Under certain conditions, the fertilisation could encourage unwanted types of algae, including those that release toxins. Moreover, the decomposing phytoplankton could boost the growth of bacteria that produce nitrous oxide or methane, which are both more potent greenhouse gases than CO₂ and could offset any positive contribution to carbon sequestration the fertilisation might make.

Chisholm warns that ocean fertilisation scaled up might carry large risks and bring little benefit: ‘Many people have calculated that if you fertilised all of the low iron regions of the oceans for 100 years, and if all the resulting carbon settled to the deep, it would have a minimal effect on the trajectory of CO₂ in the atmosphere. Not to mention that in fertilising the oceans you – by design – change the structure of the food web.

‘We have damaged many coastal ecosystems by unintentional nutrient enrichment from land run-off. Let’s not make the same mistake intentionally in the open oceans. Small-scale experiments do not cause problems because they are ephemeral, but large- scale implementation is a different matter.’

Marine biologist Chris Bowler from the Ecole Nationale Supérieure at Paris comments: ‘At this time, the handful of large-scale iron fertilisation experiments that have been done are simply not enough for us to allow an accurate cost-benefit analysis to weigh the benefits of iron fertilisation against the collateral ecological costs. We need more studies like EIFEX that assess carbon export, and we need even longer term experiments that assess plankton ecosystem community changes.’

Other CO₂ sequestering schemes

Various options to counteract CO₂ emissions on a global scale have been discussed in recent years, though all have their fair share of uncertainties. In September 2009, the Royal Society published a review of the possible approaches, dividing them into two main groups: those that remove CO₂ from the atmosphere; and those that reflect the Sun’s energy back into space.

The latter schemes include injecting aerosols into the stratosphere and the positioning of mirrors in space. Apart from the technical difficulties involved and the requirement for international agreement and collaboration, these suffer from the fundamental flaw that they would allow the CO₂ to build up further. Thus, ocean acidification, which already poses a significant threat to coral reefs, would continue. The Royal Society report concluded that sun-blocking schemes should only be considered as short-term emergency measures, if at all.

Carbon dioxide removal schemes range from the obvious to the fanciful. Planting trees and improving land management to encourage natural carbon sinks, such as mangrove wetlands, are among the measures that are easy to implement, and won’t upset ecologists. ‘Another option,’ says Smetacek, ‘is converting plant biomass into elemental carbon (charcoal), which is resistant to breakdown and, if added to the soil, significantly improves its quality for plant growth. This technique is known as biochar and could be carried out in decentralised way.’ However, if carbon emissions continue to grow as they have until now, these measures will not be able to stop the rise in atmospheric CO₂ concentrations.

Carbon dioxide removal could be achieved with technical devices, sometimes called ‘artificial trees’. Klaus Lackner from the Lenfest Center for Sustainable Energy (LCSE) at Columbia University, US, has argued for this approach since 1999. Recently, his group published a new carbon withdrawal method based on a ‘moisture swing’ (Environ. Sci. Technol., 2011, 45, 6670). The team has developed a material that absorbs CO₂ when it is dry and releases it when it is wet, so the gas can be washed off the device in repeated cycles. Lackner has estimated that the cost of CO₂ removal could be brought down to $30/tonne, which is competitive with devices capturing the gas at source in large emitters.