Combined pigment and metatranscriptomic analysis reveals synchronized diel patterns of phenotypic light response across domains in the open ocean

Sunlight is the most important environmental control on diel fluctuations in phytoplankton activity, and understanding diel microbial processes is essential to the study of oceanic biogeochemical cycles. Yet, little is known about the in situ frequency of phytoplankton metabolic activities and their coordination across different populations. We investigated the diel orchestration of phytoplankton activity involved in photosynthesis, photoacclimation, and photoprotection by analyzing the pigment and quinone distribution in combination with metatranscriptomes in the surface waters of the North Pacific Subtropical Gyre (NPSG). We found diel cycles in pigment abundances resulting from the balance of their synthesis and consumption. The night represents a metabolic recovery phase to refill cellular pigment stores, while the photosystems are remodeled towards photoprotection during the day. Transcript levels of genes involved in photosynthesis and pigment metabolism had highly synchronized diel expression patterns among all taxa, suggesting that there are similar regulatory mechanisms for light and energy metabolism across domains, and that other environmental factors drive niche differentiation. Observed decoupling of diel oscillations in transcripts and related pigments in the NPSG indicates that pigment abundance is modulated by environmental factors extending beyond gene expression/regulation, showing that metatranscriptomes may provide only limited insights on real-time photophysiological metabolism.