Adding Aluminum to Seawater Increases Net Carbon Fixation of Marine Diatoms

The key to tackle climate change and global warming is decreasing net greenhouse gas emissions to zero as quickly as possible and achieving 'carbon neutrality.'

On Earth, oceans are considered the biggest active carbon pool with great ability to help reach negative emissions by acting as a carbon sink.

In recent times, scientists have discovered that when a negligible quantity of aluminum is added to achieve concentrations in the 10x nanomolar (nM) range, the net fixation of CO₂ by marine diatoms can be increased and their decomposition can be reduced.

This could help enhance the capability of the ocean to absorb CO₂ and sequester carbon at deep ocean depths.

The study reported on May 3^rd, 2021, in the journal Limnology and Oceanography was performed by a collaborative team under the guidance of Professor Yehui Tan from the South China Sea Institute of Oceanology (SCSIO) of the Chinese Academy of Sciences and Professor Peter G.C. Campbell from the Eau Terre Environnement Research Center of the National Institute of Scientific Research in Canada.

As per the previous 'iron hypothesis,' when a small quantity of iron is added to the iron-limited but nutrient-rich oceans, it could considerably boost the marine phytoplankton (microalgae) growth and their absorption of CO₂, and the resultant burial of organic matter present in the ocean.

But the findings of artificial iron fertilization experiments did not completely support the 'iron hypothesis' and further studies indicated that overlooking the impacts of aluminum and other elements might be the cause.

In fact, natural iron fertilization, as caused by dust deposition, upwelling and hydrothermal venting, provides the ocean not only iron, but also aluminum and other elements. Aluminum concentrations in the upper ocean are usually one order of magnitude higher than those of iron.

Yehui Tan, Professor, South China Sea Institute of Oceanology, Chinese Academy of Sciences

The research group under Professor Tan together with their collaborators have discovered that aluminum might not only enhance the usage efficiency of dissolved organic phosphorus and iron by marine phytoplankton, thereby improving carbon fixation in the upper ocean, but may also decrease the biogenic organic carbon’s decomposition rate and enhance the sequestration and export of carbon in deep ocean depths.

The researchers also discovered a considerable negative correlation present between aluminum input to the Southern Ocean and atmospheric CO₂ concentration in the past 160,000 years.

Depending on their results regarding aluminum, the original 'iron hypothesis' was enhanced by suggesting the 'iron-aluminum hypothesis' to better describe the parts of the two elements in climate change.

In this research, the scientists made use of radiocarbon (¹⁴C) as a tracer to demonstrate that adding aluminum to seawater to get trace concentrations (for example., 40 nM) raised net carbon fixation of marine diatoms from 10% to 30%.

Above all, this study demonstrated that environmentally relevant, low concentrations of aluminum can decrease the day-to-day decomposition rate of marine diatom-produced particulate organic carbon by 50% or more.

Calculations performed on the basis of the new data indicate that adding aluminum at a concentration of 40 nM or lower to the ocean might raise the quantity of particulate organic carbon exported to depths of 1,000 m and deeper by 1 to 3 orders of magnitude.

This will considerably raise the carbon sink capacity of the ocean and sequester carbon in the ocean for a longer period, thereby bettering climate change.

Journal Reference:

Zhou, L., et al. (2021) Aluminum increases net carbon fixation by marine diatoms and decreases their decomposition: Evidence for the iron–aluminum hypothesis. Limnology and Oceanography. doi.org/10.1002/lno.11784.