Effect of Temperature on Respiration of a Mesophilic and a Thermophilic Fungus

Abstract

The respiratory rates of mycelia of the esophHic fungus, Aspergilas niger, and the thermophilc fungus, Thernomyces kninosus, were comparable at their respective temperature optima for growth. The respiratory rate of A. niger was independent of changes in temperature between 15 and 40 C. The respiratory rate of T. lnuginosus increased with increase in temperature between 25 and 55 C. There are very few studies on the effect of environmental variables on the respiration of fungi. This appears to be due to difficulty in obtaining homogenous mycelial suspensions for use in manometry (1, 3). We have studied the effect of temperature on respiration of a mesophilic fungus (Aspergillus niger) and a thermophilic fungus (Thermomyces lanuginosus) using mycelial suspensions from shaker-grown cultures. We report that the intact cells of A. niger, in contrast to those of T. lanuginosus, show a region of temperature-compensated respiration.. T. lanuginosus RM-B was isolated from horse dung (8). Both fungi were grown from spore inocula on a rotary shaker in a glucose-asparagine medium (8) supplemented with 0.01% Difco yeast extract. A. niger was cultured at 30 to 33 C and T. lanuginosus at 50 C. Under these conditions, the average growth rates calculated from the linear portions of the growth curves were 16 mg dry weight/h for A. niger, and 22 mg dry weight/h for T. lanuginosus. Mycelia were harvested by filtration 12 to 16 h after spores were added, washed twice with distilled H20, suspended in 50 mm Na-K phosphate (pH 5.6), and used for manometry without delay. 02 uptake was measured with an Aminco Warburg apparatus. The main compartment of the Warburg flasks contained mycelial suspension (4-8 mg dry weight) +2% glucose in a total volume of 3.0 ml of 50 mm Na-K phosphate (pH 5.6). The center well contained 0.2 ml of 6 N KOH. At least three flasks were used for each determination and each experiment was repeated at least twice unless mentioned specifically. RESULTS The respiratory rates of both fungi as measured by the uptake of 02 remained uniform (Fig. 1) at all experimental temperatures 'Supported by Council of Scientific and Industrial Research, New Delhi, India. except at those close to the upper limit of growth. Exogenous glucose caused a 2.5-fold stimulation of respiratory rate of A. niger at all temperatures studied (15-40 C). In contrast, glucose had no effect on respiration of T. lanuginosus at 30 C, but it caused a 1.3-fold stimulation at 40 C and a 1.7-fold stimulation at 50 C. At their respective temperature optima of growth, the average Qo2 (Al 02 taken up/mg dry weight-h) of A. niger (30 C) was 53.1 ± 1.6, and of T. lanuginosus (50 C) was 42.4 ± 6.4. This showed that the metabolic rate of the thermophilic fungus adapted for growth at elevated temperature was not higher than that of the mesophilic fungus. The temperature coefficient of the rate of respiration (Qlo) ofA. niger from 20 to 35 C, in the absence of exogenous glucose, was 1.1. In the presence of glucose, the respiratory rate between 15 and 40 C also was essentially constant (Fig. 2); the Qlo remained close to 1. In contrast, the Qlo of endogenous respiration of T. lanuginosus varied considerably with temperature. For example, between 30 and 40 C the Qlo was 2.4, between 40 and 50 C it was 1.9. In the presence of glucose, the Qio between 30 and 40 C was 3.3, and between 40 and 50 C it was 2.4. Respiratory rate exhibited a steep decline above 55 C. A small increase in temperature near the minimum temperature of growth of the thermophilic fungus produced a marked effect on its metabolism. Figure 2 shows an Arrhenius plot of the data on total 02 uptake in the presence of glucose for A. niger and T. lanuginosus from several experiments. Although the absolute values of Qo, differed somewhat with different samples of mycelial suspension, it is clear that the respiration of A. niger was independent of temperature between 15 and 40 C. To confirm that Qo2 of mycelial suspension of A. niger remains constant between 15 and 40 C, respirometry was done at an arbitrarily chosen reference temperature (30 C) and at a test temperature (T C) using samples from the same suspension.