Diatoms are the single most important drivers of the oceanic silicon biogeochemical cycle. Due to their considerable promise in nanotechnology, there is tremendous interest in understanding the mechanism by which they produce their intricately and ornately decorated silica‐based cell wall. Although specific proteins have been implicated in some of the key steps of silicification, the exact mechanisms are poorly understood.Silicon transporters, identified in both diatoms and silicoflagellates, are hypothesized to mediate silicon uptake. Recently, macropinocytosis, the non‐specific engulfment of extracellular fluid, (...) was proposed as a more energetically favorable uptake mechanism, which can also explain the long‐observed effect of salinity on frustule morphology. We explore the bioenergetic, membrane recycling, and vacuolar volume requirements that must be satisfied for pinocytosis‐mediated silicon uptake. These calculated requirements contrast starkly with existing data on diatom physiology, uptake kinetics, growth, and ultrastructure, leading us to conclude that pinocytosis cannot be the primary mechanism of silicon uptake. (shrink)

Fluid-phase endocytosis (pinocytosis) kinetics were studied inDictyostelium discoideum amoebae from the axenic strain Ax-2 that exhibits high rates of fluid-phase endocytosis when cultured in liquid nutrient media. Fluorescein-labelled dextran (FITC-dextran) was used as a marker in continuous uptake- and in pulse-chase exocytosis experiments. In the latter case, efflux of the marker was monitored on cells loaded for short periods of time and resuspended in marker-free medium. A multicompartmental model was developed which describes satisfactorily fluid-phase endocytosis kinetics. In particular, it (...) accounts correctly for the extended latency period before exocytosis in pulse-chase experiments and it suggests the existence of some sorts of maturation stages in the pathway. (shrink)