Transcriptional patterns of Emiliania huxleyi in the North Pacific Subtropical Gyre reveal the daily rhythms of its metabolic potential.

Emiliania huxleyi is a calcifying haptophyte, contributing to both the organic and inorganic marine carbon cycles. In marine ecosystems, light is a major driver of phytoplankton physiology and ultimately carbon flow through the ecosystem. Here, we analysed a Lagrangian time-series of metatranscriptomes collected in the North Pacific Subtropical Gyre (NPSG) to examine how in situ populations of E. huxleyi modulate gene expression over day-night transitions. Many E. huxleyi contigs had a diel expression pattern, with 61% of contigs clustering into modules with statistically significant diel periodicity. Contigs involved in processes that build up energy stores, like carbon fixation and lipid synthesis, peaked around dawn. In contrast, contigs involved in processes that released energy stores, like respiration and lipid degradation, peaked mid-day and towards dusk. These patterns suggest an orchestrated cycle of building, then consuming energy stores in E. huxleyi populations in the NPSG. Selected contigs related to the cell cycle also exhibited significant diel periodicity consistent with phased modulations of division observed in culture. Overall, these patterns of gene expression suggest a daily metabolic cascade that could contribute to both organic and inorganic carbon flow in this nutrient depleted ecosystem.