Bacterial production of marine humic-like fluorescent dissolved organic matter and its biogeochemical importance

Bacterial production of humic-like fluorescent dissolved organic matter (FDOMH) was examined in a 90 d culture experiment using a bacterial assemblage collected from coastal water with artificial seawater amended by glucose. A rapid decrease in dissolved organic carbon concentration was noticed during Days 1 to 5, and this coincided with an increase in bacterial numbers. The increase in FDOMH, which was determined by Excitation-Emission Matrix Spectroscopy (EEMS), was observed by Day 20, showing the production of FDOMH by bacteria. This increase lagged behind the increase in bacterial numbers, suggesting that the production of FDOMH was accompanied by an alteration of bacterial biomolecules. After Day 30, the fluorescence intensity showed no significant decrease, and 63% of the maximum intensity on Day 20 remained at the end of the experiment. This high remaining percentage likely reflects the recalcitrant properties of bacteria-derived FDOMH (BAC-FDOMH). High-performance size-exclusion chromatography with fluorescence detection (Excitation/Emission = 335/410 nm) revealed that most of BAC-FDOMH is composed of 3 molecular weight (MW) groups having similar fluorescent properties. The MWs of these were all <1.8 kDa relative to standards with known MW. A photodegradation experiment of BAC-FDOMH demonstrated that the photodegradability of fluorophores varied among the MW groups. Measurements of the MW composition of coastal seawater showed that a recalcitrant fraction of BAC-FDOMH resistant to solar irradiance constituted a part of FDOMH in ambient seawater.