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

Abstract

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