The study's co-authors, including MIT CGCS research scientist Stephanie Dutkiewicz, argue that ecosystem models should account for the diversity of diatoms to better predict the fate of these micro-algae and their role in the biological carbon pump of tomorrow's ocean.
The oceans, by absorbing some of the CO2 present in the atmosphere, contribute to regulation of the climate on a global scale. Through photosynthesis, surface micro-algae transform atmospheric CO2 into organic carbon. Because organic matter is denser than seawater, this process transfers organic carbon to the deep ocean where it remains on time scales of more than a century. The transfer of CO2 from the surface to the deep ocean is called the “biological pump."
Now, a group of scientists from European and North American research institutions and universities, among them Stephanie Dutkiewicz and Oliver Jahn from the Center and Global Change Science and the Department of Earth, Atmospheric and Planetary Sciences at MIT, reviews the science and suggest that diatoms (micro-algae with siliceous shell) play an essential, and larger than previously thought, role in the biological carbon pump.
For organic matter sedimenting in the oceans, silica acts as a ballast that enables massive and episodic transfer of marine snow to the deep ocean. In their recent review article published in Nature Geoscience December 18, 2017, the authors review the potential of diatoms to transfer organic carbon to the deep ocean, and suggest it has been underestimated. They also find that all diatoms are not equal in their capacity to export carbon out of the surface ocean. Their work, which involved a combination of physical, chemical, and biological modeling, also establishes that current ecosystem models must take the diversity of diatoms into account to better predict the fate of these microalgae and their role in the biological carbon pump of tomorrow's warmer and acidified ocean.
For millions of years, diatoms have been the mainstay of food chains that underpin ocean biological productivity and actively participate in the transfer of carbon from the surface to the deep layers of the ocean. When diatoms die and fall out of the surface ocean, the organic carbon they have synthesized is easily degraded during descent to be almost entirely reconstituted into carbon dioxide gas before reaching a depth of 1000 meters.
However, from data collected in the current ocean as well as from paleo-ocean data, the researchers show that diatoms can episodically carry massive amounts of organic carbon to the deepest layers of the ocean. They also find that not all diatoms exhibit the same potential to transfer carbon, with the potential of different diatom species varying according to the size of the diatoms, the shape of their cells, and their degree of silicification (the silicon / carbon ratio of their shells). The researchers also saw a dependence on the biogeochemical environment in which they evolve.
Diatom abundance in the world oceans (moles of carbon per cubic meter) during boreal spring (left) and autumn (right) as simulated by the DARWIN MIT model (Tréguer et al. 2017), 18 km resolution (ECCO2 physical model ) © MIT
Currently, diatoms are present on a planetary scale and dominate the other marine phytoplankton species in cold and turbulent waters. A hotter and more stratified future, models predict an overall decline of these siliceous micro-algae (with the exception of the Southern Ocean), although the authors acknowledge adaptations of these species to climate change and ocean acidification may contradict these predictions.
This international study highlights the value of developing strategies that take a multidisciplinary approach combining physical, biogeochemical and biological approaches, on global to local scales, to understand how the ocean carbon biological pump works now, and to make predictions of future ocean scenarios.
“A clear picture of large scale patterns of diatom biogeography is difficult to obtain given the paucity of observations of diatom abundances, “ says Dutkiewicz. “Oliver Jahn and I provided output from our coupled physical, biogeochemical and ecosystem model to provide an illustration of the diatom biogeography. By including a diversity of diatoms in the model we provided examples of how estimates of carbon export are dependent on this diversity. Additional work in collaboration with Marina Levy (another co-author) highlighted that diatoms are also found in higher abundances in fronts in the ocean.”
Story Image Credit: NASA's Goddard Space Flight Center