Pronounced\nDiel Cycling of Dissolved Carbohydrates\nand Amino Acids in the Surface Ocean and across Diverse Regimes

The metabolism of\nphytoplankton cells is synchronized with the\ndiel light cycle. Likewise, associated heterotrophic bacteria adjust\ntheir diel expression of transporter- and catabolism-related genes\nto target the dissolved organic matter released by the phytoplankton\ncell. Dissolved combined carbohydrates (DCCHO) and dissolved amino\nacids (DAA) are major phytoplankton products and bacterial substrates.\nHere, we show that diel variations of DCCHO and DAA concentrations\naccounted for a significant turnover of the total organic carbon (TOC)\npool (up to 5.0%, at a rate of 0.37 μM C h<sup>–1</sup>) and total organic nitrogen (TON) (up to 5.5%, 0.04 μM N h<sup>–1</sup>) across diverse oceanic regimes (sub-Antarctic to\nsubtropical waters of the Southwestern Pacific Ocean). Glucose contributed\nmost to the observed carbon turnover, while polar amino acids dominated\nthe nitrogen turnover. DAA concentration and composition correlated\nwith viral abundance, suggesting that viral lysis may have caused\nthe the highest DAA concentration at night. Our finding of diel cycling\nof major dissolved organic phytoplankton products supports the notion\nof universally synchronized ecosystem dynamics. Such periodicity may\nenhance nutrient cycling and thus primary production and constrains\nparts of the yet uncharacterized labile organic carbon flux fueling\nthe microbial carbon pump.