The Life Cycle and Growth Kinetics of an Anaerobic Rumen Fungus

SUMMARY: The life cycle of an anaerobic fungus isolated from the rumen of sheep was studied and was found to be similar to that of the chytrids. The organism was monocentric. At 39 °C the duration of the life cycle varied from about 26 to 32 h. The zoospores were spherical or oval in shape and were polyflagellate (8 to 17 flagella per zoospore). During the first 6·5 h of growth there was a rapid development of an extensive, non-septate, highly branched rhizoidal system; during this period the ‘main’ rhizoid increased in length exponentially with a doubling time of 2·49 h. Between 6·5 to 9·5 h after inoculation, the rate of extension of the main rhizoid declined, and no further extension occurred after 9·5 h. The main rhizoid increased in width at its base from 2·2 to 15·0 μm during the first 13 h after inoculation, indicating that intercalary wall growth occurred. Nuclei were occasionally observed in the rhizomycelium using DAPI (4′,6-diamidino-2-phenylindole) staining. Zoosporangia varied in shape from spherical to columnar, and some columnar zoosporangia were observed to become spherical. The zoosporangium initially increased in volume at an exponential rate with a doubling time of 1·56 h. Between 14 to 20 h after inoculation, growth of the zoosporangium decelerated and little growth occurred after 20 h: the zoosporangium had a final volume of 2·5 × 105μm3. At about 21 h after inoculation, a septum was formed at the base of the zoosporangium, delimiting it from the rhizoidal system. The formation of this septum was correlated with the cessation of zoosporangial growth and the onset of zoosporogenesis. After zoosporogenesis, zoospores (about 88 zoospores per zoosporangium) were liberated through a pore formed in the zoosporangial wall opposite the main rhizoid. About 3 h after zoospore release the rhizoidal system became less refractile, suggesting that autolysis had occurred. Growth of the isolate was inhibited by nikkomycin (an inhibitor of chitin synthase), but not by amphotericin B or nystatin (antibiotics which bind to sterols).