Pronounced Diel Cycling of Dissolved Carbohydrates and Amino Acids in the Surface Ocean and across Diverse Regimes

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