Aluminum plays a thus-far unsuspected role in carbon dioxide (CO2) capture on the ocean floor. This is what the results of a study published in May in the prestigious journal Limnology and Oceanography suggest. The research, carried out at the Institut national de la recherche scientifique (INRS), positions aluminum as a new variable in the fight against climate change that must be thoroughly examined.
“We have always seen aluminum as a non-essential and sometimes harmful element, but our results challenge this perception. We were able to show for the first time that aluminum would have a positive effect on CO2 storage!” says Dr. Claude Fortin, professor at the INRS and one of the study’s co-authors.
The research team showed that the presence of moderate concentrations of aluminum increased phytoplankton’s CO2 fixation. These microalgae absorb CO2 via photosynthesis. Often referred to as the “biological pump” of the oceans, phytoplankton are composed of microalgae that assimilate CO2 through the process of photosynthesis and thus send some of the carbon to the depths. This naturally reduces global warming.
However, during the sedimentation of phytoplankton towards the bottom of the oceans, a part of this “fixed” carbon is degraded by bacteria, which regenerates the CO2. The study shows that aluminum present would slow down this degradation process.
The scientists used carbon-14 (14CO2) as a tracer to arrive at these results. The team tracked CO2 fixation by three marine diatom species. Diatoms are algae and a key component of phytoplankton. The team exposed the diatoms to different concentrations of dissolved aluminum and observed their decomposition. They observed that adding relatively low doses of aluminum to the water increased CO2 fixation by 10% to 30% in these three species of algae, and that diatom decomposition rates could be reduced by more than half.
The role of aluminum in this phenomenon is not intuitive and surprised the scientists, who have hinted at a possible explanation. A lack of iron in certain parts of the ocean limits photosynthesis by phytoplankton. Earlier studies led by Dr. Linbin Zhou, first author and oceanography researcher at Chinese Academy of Science’s South China Sea Institute of Oceanology (SCSIO), showed that the presence of aluminum stimulates the growth of phytoplankton, possibly by favoring the uptake of iron, which would then increase CO2 capture.
“The idea of fertilizing the surface of the ocean with iron (known as the “iron hypothesis”) so that phytoplankton absorb greater amounts of CO2 caught many researchers’ attention in the 1990s,” said Peter Campbell, professor emeritus at the INRS and co-author of the study. “However, tests were inconclusive. Phytoplankton growth was stimulated, but carbon fixation was not permanent. Much of the CO2 was released again as the algal cells degraded.”
According to Dr. Campbell, a global expert in ecotoxicology and biogeochemistry, the link between aluminum and iron cycles is a topic that merits further attention. “How can aluminum increase the availability of iron? Could such a reaction occur naturally? These are the questions we will have to answer,” he added.
According to the researchers, these results could have an impact on climate models by incorporating the “positive” effect of aluminum on carbon sequestration in the deep ocean.
“Aluminum could become an additional variable in future models based on oceanographic data. It is possible to imagine a systematic measurement of aluminum deposition to the ocean surface,” said Dr. Fortin. “This would allow models to be refined and better predictions to be made about ocean feedback in relation to rising CO2 levels in the atmosphere.”
This study is the result of work carried about by professor Yehui Tan and Dr. Linbin Zhou of the Chinese Academy of Science’s South China Sea Institute of Oceanology (SCSIO), in collaboration with INRS Professors, Peter Campbell and Claude Fortin. Linbin Zhou was an invited researcher from May 2016 to June 2017 at the Eau Terre Environnement Research Centre at the INRS in Quebec City.
The study Aluminum increases net carbon fixation by marine diatoms and decreases their decomposition: Evidence for the iron–aluminum hypothesis, published in Limnology and Oceanography received financial support from the Key Special Project for Introduced Talents Team of the Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), the National Key Basic Research Program of China, the National Natural Science Foundation of China, the National Sciences and Engineering Research Council of Canada (NSERC), the Guangdong Basic and Applied Basic Research Foundation, and the Science and Technology Planning Project of Guangdong Province, China.