Regulation of bacterial assemblages in oligotrophic plankton systems: results from experimental and empirical approaches.

Bacteria are relevant members of planktonic food webs, both in terms of biomass and production share. The assessment and comprehension of the factors that control bacterial abundance and production are, thus, necessary to understand how carbon and nutrients circulate in planktonic food webs. It is commonly believed that bacterial abundance, activity and production are either determined by the available nutrient levels ('bottom-up' control) or by the effect of predators ('top-down'). These factors have also been shown to regulate the internal structure (the physiological and phylogenetic structure) of the bacterioplankton black box. We present here different empirical and experimental ways in which the factors that control bacterial communities are assessed, among them, the direct comparison of the rates of bacterial growth and losses to grazing. Application of several of these methods to open ocean data suggests that bacteria are regulated by resources at the largest scales of analysis, but that this overall regulation is strongly modulated by predators in all types of systems. In the most oligotrophic environments, bacterial abundance and growth are regulated by predators, while in the richest environments it is bacterial (phylogenetic, size, activity) community composition that is most affected by protist predators, while abundance can be influenced by metazoans. Because changes in bacterial community composition require that bacteria have enough nutrient supply, the overall effect of these regulations is that bacterial growth appears to be top-down regulated in the most nutrient-poor environments and bottom-up regulated in the richer ones.