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Taxonomic composition and cell size of marine eukaryotic phytoplankton

Final Activity Report Summary - TASIO (Taxonomic composition and cell size of marine eukaryotic phytoplankton)

Phytoplankton are central to the oceans' ecological and biogeochemical services, as they constitute the basis of the marine food web and in addition are major drivers in the cycling of elements. Phytoplankton take CO2 up in the surface ocean to produce organic carbon through the process of photosynthesis. In the contemporaneous ocean, they are responsible for roughly half of the total carbon fixation on Earth. Part of this carbon sinks out and is sequestered in the deep ocean for long periods. In this way phytoplankton contribute to the regulation of atmospheric CO2 levels and therefore the climate. In the project we studied the mechanisms that control the distribution of phytoplankton in the oceans and its impact in the regulation of atmospheric CO2 levels.

% Using extant and fossil assemblages of marine phytoplankton we have demonstrated that marine phytoplankton possess global dispersal ranges. In larger species such as terrestrial plants and animals, the spatial range of distribution spreads from the centre of the species' origin. The farther away from this centre, the fewer individuals of the species will have in common. The study showed that the dispersal of marine planktonic microbes is not limited in the same way as that of larger animals.

Most scientists believe that allopatric speciation, where different species arise from an ancestral species only after breeding populations have become geographically isolated from each other, is the dominant mode of speciation. Physical barriers to dispersal restrict interbreeding between populations so that, given enough time, such populations diverge and form different species. The lack of barriers to passive dispersal of plankton suggests that species' distributions primarily are controlled by environmental selection rather than by dispersal limitation.

Of particular interest to the project were diatoms and coccolithophorids, two groups of marine phytoplankton that play key roles in the regulation of atmospheric carbon dioxide. Carbon uptake by marine phytoplankton, and its export as organic matter to the ocean interior lowers the partial pressure of CO2 in the upper waters and facilitates the drawdown of atmospheric CO2 (the biological pump). Conversely, precipitation of calcium carbonate by coccolithophorids increases the partial pressure of CO2 and promotes outgassing to the atmosphere.

We were working with data we already had from a database in the Atlantic Ocean and observed that the distribution of these two groups of phytoplankton is controlled by the rate of nutrient input to the system. If nutrients enter the upper oceans very quickly diatoms dominate, but if nutrients are supplied slowly, then coccolithophorids are selected.

When our field results were projected into a coupled atmosphere-ocean general circulation model, the project's model predicted a dramatic reduction in the nutrient supply to the photic layer as a result of increased thermal stratification. Moreover, by altering phytoplankton community composition, increased ocean stratification led to a decreased efficiency of the biological pump in sequestering atmospheric CO2, implying a positive feedback in the climate system.

Over the past 250000 years, Earth's climate has undergone profound and cyclical changes (i.e. glacial/interglacial episodes with 104- to 105-year cyclicity), which offer an excellent framework to study the dynamics of microbial plankton communities through long-term climate perturbations. We found striking cycles of community departure and recovery tightly coincident with the temporal evolution of Earth's climate. The explanation for this recovery pattern is linked to the great dispersal abilities of marine planktonic microbes. Dispersal allows species to track changes in global environmental conditions and slows down evolutionary processes.