Microbial community at MOLA 2019-2021. Seasonality of amino acid enantiomers and microbial community at MOLA time series in the NW Mediterranean

Seasonality in light, temperature, and nutrient availability are well-known to regulate phytoplankton blooms and the bacterioplankton community. During the spring bloom, phytoplankton release biomolecules as part of the dissolved organic matter (DOM) pool that controls bacterioplankton. Here, we investigated the seasonal variability of phytoplankton biomass, enantiomers of dHAA that are the most abundant biomolecule class, bacterial abundances, and bacterioplankton community composition at the Microbial Observatory Laboratory Arago (MOLA) in the NW Mediterranean Sea from 2019 to 2021. Phytoplankton biomass estimated from pigment biomarkers suggests a spring bloom succession from cryptophytes, haptophytes, and prasinophytes in March to diatoms in April. The spring bloom coincided with a 50% increase in l-amino acid (l-dHAA) enantiomers and increased high-nucleic acid (HNA) bacterial abundance by 60%. After the spring bloom, elevated concentrations of d-amino acid (d-dHAA) enantiomers and gamma-aminobutyric acid, suggesting that bacterial remineralization of labile biomolecules contributed to the seasonal accumulation of dissolved organic carbon (DOC). Linking biochemical compounds with the bacterioplankton community showed a seasonal succession of niches and substrate regimes. The parallel analysis of DOM and bacterioplankton community provides an important baseline for bacteria-substrate relationships over the seasonal cycle in the northwestern Mediterranean Sea.