Adsorption of dissolved free amino acids on colloidal DOM enhances colloidal DOM utilization but reduces amino acid uptake by orders of magnitude in marine bacterioplankton

In an attempt to quantify abiotic adsorption of easily metabolizable dissolved free amino acids (DFAA) to colloidal dissolved organic matter (CDOM) in the sea, laboratory experiments were performed using dextran of various molecular size ranging from 4.4 to 2000 kDa or phytoplankton-derived CDOM (>1 kDa) as model CDOM, and leucine or a DFAA mixture. Abiotic leucine adsorption to dextran was found to be independent of the molecular size of the dextran. Adsorption of leucine to dextran and phytoplankton-derived CDOM was saturated at similar to 7.5 nmol mg(-1) dextran-C; other DFAA species reached higher saturating concentrations (maximum: serine, 48.6 nmol mg(-1) dextran-C), Adsorption of DFAA to dextran and phytoplankton-derived CDOM led to an approximately 3 times more efficient utilization of dextran and CDOM by marine bacterioplankton than colloidal matter to which no DFAA were adsorbed. Bacterial uptake of CDOM-adsorbed leucine, however, was reduced by 2 to 3 orders of magnitude as compared to the uptake of 'truly' free (non-adsorbed) leucine offered simultaneously at the same concentration. This finding might also be relevant for bacterial production measurements if leucine or thymidine are used as a tracer in colloidal-matter-rich micro-environments such as marine snow. All the CDOM-adsorbed DFAA (except glycine and threonine) were detectable without prior hydrolysis by o-phthalaldialdehyde (OPA) derivatization in the HPLC. Adsorption of labile DFAA on otherwise refractory CDOM also has important implications for the biogeochemical flux of CDOM by forming 'semi-labile' molecules which are more readily degradable by bacterioplankton and thereby reducing the flux of CDOM to the deep ocean.