The Correlation of Digestive Vacuole pH and Size with the Digestive Cycle in Paramecium caudatum¹

Abstract

ABSTRACT. The temporal changes in the size and pH of digestive vacuoles (DV) in Paramecium caudatum were reevaluated. Cells were pulsed briefly with polystyrene latex spheres or heat‐killed yeast stained with three sulfonphthalein indicator dyes. Within 5 min of formation the intravacuolar pH declined from ∼7 to 3. With the exception of a transient and early increase in vacuolar size, vacuole condensation occurred rapidly and paralleled the acidification so that vacuoles reached their lowest pH and minimal size simultaneously. Neutralization and expansion of vacuole size began when vacuoles were GT8 min old. No labeled vacuoles were defecated prior to 21 min after formation but almost all DV were defecated within 1 h so that the digestive cycle of individual vacuoles ranged from 21 to 60 min. Based on these size and pH changes, the presence of acid phosphatase activity, and membrane morphology, digestive vacuoles can be grouped into four stages of digestion. The DV‐I are GT6 min old and undergo rapid condensation and acidification. The DV‐II are between 4 to 10 min old and are the most condensed and acidic vacuoles. The DV‐III range in age from 8 to ∼20 min and include the expanding or expanded vacuoles that result from lysosomes fusing with DV‐II. The DV‐IV are GD21 min old, and since digestion is presumably completed, they can be defecated. The rise in intravacuolar pH that accompanies vacuole expansion suggests that lysosomes play a role in vacuole neutralization in addition to their degradative functions. The acidification and condensation processes in DV‐I appear to be unrelated to lysosomal function, as no acid phosphaiase activity has been detected at this stage, but may be related to phagosomal functions important in killing food organisms, denaturing proteins prior to digestion, and preparing vacuole membrane for fusion with lysosomes.

Remineralization and recycling of iron, thorium and organic carbon by heterotrophic marine protists in culture

“…Small protist grazers are a primary sink for bacteria and similarly sized algal cells (0.1-1 µm) in marine systems (Azam et al 1983, Fenchel 1987, and such particles are known to constitute a major reservoir of particlereactive trace metals (Cho & Azam 1990, Lee et al 1993. Phagotrophic digestion of particles by protists is likely to be accompanied by lowered pH (Fok et al 1982), intense enzymatic activity (Fok et al 1984), reactive oxygen species (Stossel 1974), and high concentrations of reduced organic material. This chemical environment can cause changes in the redox state and coordination chemistry of ingested trace metals, which can have significant impacts on the mobility and bioavailability of trace metal species.…”

“…(Fig. 4E), despite the fact that in the former case Fe was almost entirely intracellular in the prey, and about 60% extracellular in the latter.In intracellular (phagotrophic) digestion, the initial pH can drop as low as 2 (Fok et al 1982), and during subsequent steps hydrolytic enzymes operative at pH 5 attack bacterial membrane components (Fok et al 1984). Such processes might solubilize Th, associated non-specifically with external membranes.…”

The regeneration of highly bioavailable iron by meso‐ and microzooplankton

“…The overall gut pH of copepods appears to be less acidic than the digestive vacuoles of protists (Fok et al 1982;Pond et al 1995;Tang et al 2011). Although a pH as low as 3, as observed during the fusion of digestive vacuoles with lysosomes, could contribute potentially to the stabilization and release cellular ferrous Fe, the subsequent pH rise to 7 during the active digestion stage, which is characterized by the release of digestive enzymes (Fok et al 1982), makes the egestion of Fe(II) less likely. In contrast, there is a higher probability of egestion of Fe(II) from copepod guts because the urosome-metasome transition zone of the copepod gut becomes anoxic during the digestion process (Tang et al 2011).…”

“…The activity and type of digestive enzymes are dependent on pH, temperature, and diet, which vary significantly among digestive vacuoles of microzooplankton and the gut of mesozooplankton (Fok et al 1982;Solgaard et al 2007;Freese et al 2012). For example, in ciliates during the initial fusion of digestive vacuoles with lysosomes the pH is lowered to 3, killing the ingested microorganisms (Fok et al 1982).…”

“…For example, in ciliates during the initial fusion of digestive vacuoles with lysosomes the pH is lowered to 3, killing the ingested microorganisms (Fok et al 1982). Subsequently, the pH in the digestive vacuoles rises from 3 to 7, accompanied by acid phosphotase activity, which suggests active digestion (Fok et al 1982). In contrast to the temporal pH changes inside digestive vacuoles, pH in the gut of copepods varies spatially between 5.4 and 9.35 and can vary with the chemical composition of the ingested prey (Pond et al 1995;Tang et al 2011).…”