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Impacts of nitrogen limitation on the sinking rate of the coccolithophorid Emiliania huxleyi (Prymnesiophyceae)

Pantorno A., Holland D.P., Stojkovic S. and Beardall B. 2013. Impacts of nitrogen limitation on the sinking rate of the coccolithophorid Emiliania huxleyi (Prymnesiophyceae). Phycologia 52: 288–294. DOI: 10.2216/12–064.1 The biological carbon pump in the ocean plays an important role in controlling atmospheric CO2 levels. Approximately 1% of the yearly 50–60 Pg C of marine primary production settles in the deep ocean, where it is effectively sequestered for centuries to millennia. Central to the strength of the pump is the sinking velocity of phytoplankton cells and other organic debris. Stokes' Law indicates that the sinking velocity of a spherical cell will depend on its size and density, where larger, heavier cells will sink at a faster rate. Given that growth conditions can result in changes in cell size and macromolecular composition of phytoplankton, it might be expected that such changes could cause alterations in sinking velocity and carbon drawdown via the biological carbon pump. In the future, phytoplankton cells in the open ocean are predicted to be more subject to nutrient limitation. In this study we examined the effects of nitrogen limitation on the sinking velocity of Emiliania huxleyi, a coccolithophore responsible for significant phytoplankton blooms and biological drawdown of carbon. Nitrogen limitation caused changes in macromolecular composition, especially lipid content and coccosphere thickness. However, the overall density of the cells remained similar, and, as a consequence, cell size was the major determinant of sinking rate with N-limited cells in exponential phase sinking more slowly than N-replete cells. Cells in stationary phase showed the reverse trend with N-limited cells sinking faster, although not as fast as N-replete cells in exponential phase. N-limited cells produced more transparent exopolymers, suggesting an increased capacity for marine snow formation. These observations have implications for the efficiency of the biological carbon pump in areas that have increased nutrient stress caused by global climate change.