Regulation of Bacterial Growth Rates by Dissolved Organic Carbon and Temperature in the Equatorial Pacific Ocean

Abstract

The effect of dissolved organic matter (DOM) and temperature on bacterial production was examined in the equatorial Pacific Ocean. Addition of glucose, glucose plus ammonium, or free amino acids stimulated bacterial production ([3H]thymidine incorporation), whereas changes in bacterial abundance were either negligible or much less than changes in bacterial production. The average bacterial growth rate also greatly increased following DOM additions, whereas in contrast, addition of ammonium alone never affected production, bacterial abundance, or growth rates. Since the large glucose effect was not observed in previous studies of cold oceanic waters, several experiments were conducted to examine DOM-temperature interactions. These experiments suggest that bacteria respond more quickly and to a greater extent to DOM additions at higher temperatures, which may explain apparently conflicting results from previous studies. We also examined how temperate affects the kinetic parameters of sugar uptake. Maximum uptake rates (Vmax) of glucose and mannose increased with temperature (Q10= 2.4), although the half-saturation constant (Km) was unaffected; Km+ S was roughly equal to glucose concentrations (S) measured by a high pressure liquid chromographic technique. Bacterial production and growth rates appear to be limited by DOM in the equatorial Pacific, and thus bacterial production follows primary production over large spatial and temporal scales in this oceanic regime, as has been observed in other aquatic systems. Although temperature may not limit bacterial growth rates in the equatorial Pacific and similar warm waters, it could still affect how bacteria respond to changes in DOM supply and help set steady-state DOM concentrations.