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Increases in certain algae could impact carbon cycle

American Association for the Advancement of Science

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IMAGE: This is the Coccolithophore aggregate from the Atlantic Meridional Transect cruise (AMT 24, November 2015): Umbellosphaera tenuis; Emiliania huxleyi; Syracosphaera protrudens; Discosphaera tubifera. view more

Credit: Amy Wyeth- Bigelow Laboratory for Ocean Sciences

Two new studies report dramatic changes in phytoplankton abundance and nature, changes that have important implications for storing excess carbon. Collectively, these studies suggest that certain types of carbon-intensive algae are flourishing and will play increasingly prominent roles as carbon pumps, removing carbon dioxide from the atmosphere. Using the isotopic signature of phytoplankton amino acids embedded in skeletons of deep water soft corals, Kelton McMahon and colleagues determined how plankton dominance changed in the North Pacific over the past millennium. Their analysis reveals that there was a transition from dominance by non-nitrogen-fixing cyanobacteria to that by eukaryotic microalgae. What's more, around the beginning of the industrial age, another transition occurred to a stronger nitrogen-fixing cyanobacterial community. The two transition periods were found to be markedly different; whereas the first transition took more than 600 years, the second, more recent transition occurred over less than 200 years. Since some bacteria of the more recent transition act as very efficient carbon pumps, the authors suggest that the ongoing trend might lead to a more efficient carbon pump system in the world's oceans.

A second study by Sara Rivero-Calle et al. found a dramatic increase in calcium carbonate-coated (coccolithophore) algae in the North Atlantic, from 2% to more than 20%, between 1965 and 2010. Their analysis suggests this increase was largely driven by carbon dioxide levels and the Atlantic Multidecadal Oscillation. The researchers used survey data of plankton combined with a model that considered more than 20 biological and physical factors. They found a clear link between coccolithophore algae and carbon dioxide increases, and a secondary link between AMO patterns and the increase in algae. Similar to the study by McMahon et al., this dramatic increase was found in a phytoplankton group that is important for carbon cycling, since it incorporates carbon in its scaly exterior.

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