Phosphoenolpyruvate Carboxylase: Allosteric Regulation at the Ecosystem Level by Dissolved Organic Matter

Abstract

In natural unpolluted aquatic ecosystems the carbon sources that are available for heterotrophic microbial life are limited. Readily utilizable dissolved organic matter (UDOM) is present only in very small amounts with a high substrate diversity, and frequently the supply is discontinuous (Münster and Chróst 1990; Chapter 2, this volume). The qualitative and quantitative composition and metabolic activity of heterotrophic bacterial assemblages in aquatic ecosystems is to a great extent determined by the availability of DOM, although abiotic factors are also important. Because of the fundamental significance of dissolved organic substrates, very effective and rapid regulatory mechanisms that enable bacteria within a genetically determined range to adapt continuously to different and often growth-limiting levels of DOM are to be expected. Compared with the very beginning of aquatic microbiology in the early 1960s, adequate methods for measurements of activity and production of aquatic bacteria are now available. The different “activities” we are measuring are the expression of catabolic (production of low-molecular-weight intermediates and energy) and anabolic (biosynthetic) pathways (production of biomass). Behind the data stands the bacterial metabolism with its adaptive capabilities for life and survival in continuously changing environments (e.g., substrate, temperature, diurnal cycles, pressure). However, our knowledge of regulatory properties of enzymes of ecological importance in aquatic bacteria is very limited.