Axenic Paramecium caudatum. III. Biochemical and physiological changes with culture age

As Paramecium caudatum passes through the lag, log and stationary phases of the culture cycle, cellular protein content, polar (PL) and neutral (NL) lipid contents, marker enzyme activities, rate of digestive vacuole formation and cellular viability undergo characteristic changes. Maximal protein content (60 ng/cell) and enzyme activities ranging from 7 nmoles for catalase to 0.2 pmoles/cell/min for alkaline phosphatase were observed between days 2 and 4. This active metabolism paralleled the fine structure of 1 to 3 day-old cells which contained extensive foci of rough endoplasmic reticulum (RER) partially bordered by Golgi stacks and the rapid depletion of those lipid fields accumulated during day 1. Decrease in protein content and enzyme activities in late log phase indicated a slowing of cellular synthesis. The lipids in the medium were largely depleted and accounted for the low lipid uptake of 14 ng/cell on day 5 as compared with 615 ng/cell on day 1. Yet a vast amount of protein lysate was still available in the culture medium. During stationary phase, catalase activity remained constant, but activities of alkaline and acid phosphatases and 5'nucleotidase declined gradually to low levels, while those of Ca2+-ATPase and malate dehydrogenase declined precipitously. Only 25% of the maximal activities of the latter two enzymes remained by the end of stationary phase. A ten-fold increase in the cellular PL and NL content was already observed 24 h postinoculation. This accumulation was used for subsequent growth and cell divisions; PL declined exponentially and NL less steeply between days 1 and 6. PL remained level (5 ng/cell) throughout stationary phase while NL declined further to 1 ng/cell by day 11. The rate of digestive vacuole formation was constant (6.3 +/- 0.5 DV/5 min pulse) during the entire log phase, then declined from 4.4 on day 6 to 0.22 on day 11. When early to mid-stationary-phase cells were subcultured, some lag in growth was seen; a definite lag was observed when inoculating with late-stationary-phase cells. When early-death-phase cells were given fresh nutrients, many died; the surviving ones became fully rejuvenated after 48 h. The biochemical and physiological data from this study are correlated with the morphological study of the companion paper.