In aquatic ecosystems, heterotrophic bacteria use dissolved organic carbon to meet their metabolic needs, and their physiological activity determines the fate of carbon. Bacterial growth efficiency (BGE), or the proportion of assimilated carbon used to produce biomass, reflects the balance between energy yielding catabolic processes (i.e., respiration) and biomass yielding anabolic processes (i.e., production). However, these processes are not independent and theoretical and physiological studies predict metabolic tradeoffs. To test these predictions, we measured bacterial respiration and production in lakes, rivers, and beaver ponds across Quebec. In addition, we collected a suite of physical and chemical measurements from each water body to explore the drivers of metabolism and metabolic tradeoffs.
Results/Conclusions
We found a strong positive relationship between BGE and production. This relationship supports a predicted tradeoff between growth rate and maintenance energy requirements. That is, when growth is slow, maintenance energy requirements dominate net bacterial metabolism. Next, we used physical, chemical, and biological variables to find predictors of these metabolic rates. We found that this relationship was in part driven by variation in nutrient and organic matter availability. Furthermore, we found that ecosystem type was a major factor explaining variation in production. Specifically, beaver ponds had the most productive — and efficient — microbial community. Together, these data suggest that most aquatic microbial communities in Quebec are limited by their ability to produce biomass. As result, they primarily respire organic matter to meet their maintenance energy requirements. However, in these energy limited systems, processes that increase productivity will also increase the efficiency at which these communities use organic matter.