AME 58:55-66 (2009)  -  DOI: https://doi.org/10.3354/ame01350

ABSTRACT: Bacterial production of humic-like fluorescent dissolved organic matter (FDOM_H) 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 FDOM_H, which was determined by Excitation-Emission Matrix Spectroscopy (EEMS), was observed by Day 20, showing the production of FDOM_H by bacteria. This increase lagged behind the increase in bacterial numbers, suggesting that the production of FDOM_H 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 FDOM_H (BAC-FDOM_H). High-performance size-exclusion chromatography with fluorescence detection (Excitation/Emission = 335/410 nm) revealed that most of BAC-FDOM_H 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-FDOM_H 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-FDOM_H resistant to solar irradiance constituted a part of FDOM_H in ambient seawater.

KEY WORDS: Dissolved organic matter · Fluorescent dissolved organic matter · Humic-like substance · Bacteria · Excitation-Emission Matrix Spectroscopy · High-performance size-exclusion chromatography