Bacterial utilization of dissolved glucose in the upper water column of the Gulf of Mexico

Several aspects of bacterial glucose assimilation and bacterial production (EP) were investigated over 2 d in the upper 300 m of the Gulf of Mexico. G lucose concentrations ranged from 2 to 15 nM in surface seawater; glucose u tilization rates were 1-3 nM d(-1) in the upper 150 m of the water column a nd 0.02-0.8 nM d(-1) in deeper water (150-300 m). Turnover of glucose and b acteria were similar and were in the range of 0.01-0.4 d(-1). Measured gluc ose assimilation rates imply that glucose supports similar to 5-10% of the observed BP in surface waters. The limiting factors for bacterial glucose u tilization varied with depth. Glucose assimilation seemed to be Limited by availability of inorganic N in the chlorophyll maximum. Additions of nitrat e or ammonium had similar effects on glucose assimilation. BP was limited b y C below and above the chlorophyll maximum. Additions of high-molecular-we ight (HMW) dissolved organic matter (DOM) decreased bacterial glucose uptak e rates, indicating that substances competing with glucose assimilation wer e derived from HMW DOM extracellularly. Thus, the measurement of free gluco se assimilation appears to integrate bacterial utilization of combined gluc ose and indicates that many of the free sugars in seawater may be derived f rom polymers. The higher glucose assimilation rates achieved with inorganic nutrient additions indicate that bacterial degradation of C-rich organic m atter (OM) could vary between ocean regions with varying inorganic nutrient concentrations.