Histopathology of the thymus of coho salmon Oncorhynchus kisutch experimentally infected with Renibacterium salmoninarum
To identify the biochemical response of heterotrophic bacterioplankton to changing environmental conditions, seasonal and diel cycles of bacterial protein and DNA synthesis rates were estimated in temperate estuarine habitats from H-3-leucine (Leu) and H-3-thymidine (TdR) incorporation rates. Several short-term temperature manipulation experiments (5 to 35 degrees C) and 2 mesocosm experiments were performed to examine the effects of temperature and substrate supply on the ratio of Leu:TdR, respectively. The molar ratio of Leu to TdR varied about 5-fold (5.6 to 29.5) in the field and the values of the ratio were lower and more constant during high temperature (>25 degrees C) and high chlorophyll a (>8.0 mu g l(-1)) periods. In the temperature manipulation experiments, the Leu:TdR ratio decreased as temperature increased. In the mesocosm experiments, the Leu:TdR ratio was negatively correlated with chlorophyll a concentrations and bacterial specific growth rates. We propose that changes toward less favorable environmental conditions (e.g. reductions in temperature or substrate supply in temperate estuaries) might reduce bacterial protein and DNA synthesis rates simultaneously. However, the former process may be favored to maximize survival and this might lead to a higher Leu:TdR ratio. Conversely, when environmental conditions turn favorable, both processes could be enhanced and bacteria might optimize DNA duplication over protein metabolism to maximize reproduction, resulting in lower Leu:TdR ratios. Our results further indicate the complementariness of H-3-thymidine and H-3-leucine incorporation measurements for understanding processes controlling bacterial production since the ratio of these 2 tracer methods varied independently with temperature and substrate supply.