Bacterial stalks are nutrient-scavenging antennas

Abstract

Environmental inorganic phosphorus is frequently the growth-limiting nutrient for aquatic microorganisms, and there is fierce interspecies competition for the available supply. What are the limiting factors in effective bacterial scavenging of environmental phosphorus? It depends on the concentration. When the supply is relatively high, the limit is determined by the number of phosphate transporters on the surface times their individual throughput. At very low concentrations, however, the physics of diffusion at small scales comes into play. A relatively sparse distribution of transporters on the cell surface can capture every molecule that approaches the surface and phosphate influx becomes diffusion-limited. The prosthecate (stalked) bacteria, such as Caulobacter crescentus, have evolved an effective fitness strategy for these low-phosphorus environments that are dissected by Wagner et al. (1) in this issue of PNAS. Long, slender stalks extend the effective surface area of the cell, acting as a sort of nutrient-collecting antenna while adding little to the cell volume (Fig. 1). Thus, the cell’s phosphate collection effectiveness is greatly increased at low cost. This hypothesis for the stalk’s function was suggested by observations 40 years ago that the prosthecate bacteria grow much longer stalks in phosphate-limited media (2, 3).