Bacterial growth efficiency in natural aquatic systems

Heterotrophic bacteria perform two major functions in the transformation of organic matter: They produce new bacterial biomass (bacterial secondary pr oduction [BP]), and they respire organic C to inorganic C (bacterial respir ation [BR]). For planktonic bacteria, a great deal has been learned about B P and its regulation during the past several decades but far less has been learned about BR. Our lack of knowledge about BR limits our ability to unde rstand the role of bacteria in the carbon cycle of aquatic ecosystems. Bact erial growth efficiency (BGE) is the amount of new bacterial biomass produc ed per unit of organic C substrate assimilated and is a way to relate BP an d BR: BGE = (BP)/(BP + BR). Estimates of BGE for natural planktonic bacteri a range from <0.05 to as high as 0.6, but little is known about what might regulate this enormous range. In this paper we review the physiological and ecological bases of the regulation of BGE. Further, we assemble the litera ture of the past 30 years for which both BP and BR were measured in natural planktonic ecosystems and explore the relationship between BGE and BP. Alt hough the relationship is variable, BGE varies systematically with BP and t he trophic richness of the ecosystem. In the most dilute, oligotrophic syst ems, BGE is as low as 0.01; in the most eutrophic systems, it plateaus near 0.5. Planktonic bacteria appear to maximize carbon utilization rather than BGE. A consequence of this strategy is that maintenance energy costs (and therefore maintenance respiration) seems to be highest in oligotrophic syst ems.