Limnol. Oceanogr., 44(7), 1999, 1625–1633

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 d21 in the upper 150 m of the water column and 0.02–0.8 nM d21 in deeper water (150–300 m). Turnover of glucose and bacteria were similar and were in the range of 0.01–0.4 d21. 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. OM in seawater consists of dissolved, colloidal, and particulate components. Of these, the dissolved reservoir is the largest and is considered the major source supporting heterotrophic bacterial metabolism in the ocean (e.g., Azam et al. 1983; Kirchman 1990; Kirchman et al. 1991). However, the reactivity of relatively few individual components of DOM has been investigated because of analytical difficulties in determining very dilute concentrations of organic compounds in seawater. Recent studies indicate that dissolved carbohydrates are important bacterial substrates in seawater (Billen and Fontigny 1987; Jørgensen et al. 1993; Rich et al. 1996). Large changes in total combined carbohydrate concentrations with depth indicate high reactivity (Benner et al. 1992; Pakulski and Benner 1994; Skoog and Benner 1997), pointing to a potentially important role of carbohydrates in bacterial growth and respiration. A highly sensitive technique for molecular-level determinations of neutral aldose concentrations (Rocklin and Pohl 1983; Johnson and LaCourse 1990) has recently been adapted for seawater samples (Mopper et al. 1992), making direct, sensitive determinations of free and combined aldoses possible in marine samples. Glucose has 1 Present address: Department of Marine Science, University of Connecticut at Avery Point, 1084 Shennecossett Road, Groton, Connecticut 06340. 2 Present address: Department of Ecology, University of Minnesota, 1987 Upper Buford Circle, St. Paul, Minnesota 55108.