Abstract
Sunlight is the dominant control on phytoplankton biosynthetic activity, and darkness deprives them of their primary external energy source. Changes in the biochemical composition of phytoplankton communities over diel light cycles and attendant consequences for carbon and energy flux in environments remain poorly elucidated. Here we use lipidomic data from the North Pacific subtropical gyre to show that biosynthesis of energy-rich triacylglycerols (TAGs) by eukaryotic nanophytoplankton during the day and their subsequent consumption at night drives a large and previously uncharacterized daily carbon cycle. Diel oscillations in TAG concentration comprise 23 ± 11% of primary production by eukaryotic nanophytoplankton representing a global flux of about 2.4 Pg C yr−1. Metatranscriptomic analyses of genes required for TAG biosynthesis indicate that haptophytes and dinoflagellates are active members in TAG production. Estimates suggest that these organisms could contain as much as 40% more calories at sunset than at sunrise due to TAG production.
Highlights
Sunlight is the dominant control on phytoplankton biosynthetic activity, and darkness deprives them of their primary external energy source
DGCC is expected to scale nearly linearly with the number of cells[21], and the relatively stable concentrations of DGCC at Station ALOHA indicates that the number of cells was invariant throughout the diel cycle of sunlight, i.e. production of cells was balanced by grazing and other mortality factors
The diel cycle in TAG concentrations we observed in the North Pacific Subtropical Gyre (NPSG) is evidence that the biochemical composition and caloric content of eukaryotic phytoplankton changes profoundly throughout the course of a day
Summary
Sunlight is the dominant control on phytoplankton biosynthetic activity, and darkness deprives them of their primary external energy source. Diel cycles in sunlight represent an important source of environmental variability for plankton communities in the oligotrophic subtropical gyres of the world’s ocean. This is true for phytoplankton, which depend on sunlight to drive the biosynthesis of macromolecules essential for growth. Field studies on diel periodicity of phytoplankton biochemical composition are rare[16,17,18] To this end, we investigated the lipid composition of the planktonic community (i.e., the metalipidome) at high temporal resolution over multiple day/night cycles during two research cruises to the North Pacific Subtropical Gyre (NPSG). Comparison to measurements of primary productivity showed that diel synthesis and consumption of TAGs account for a large fraction of the flux of carbon and energy through the ecosystem
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