The physiology and ecology of a novel, obligate mycophagous flagellate

Abstract To determine if conidia of the nematophagous fungus Drechmeria coniospora are subject to predation by soil protozoa, several sandy soils were enriched with 10⁹ conidia of this fungus per g dry soil. After incubation of the samples at 20°C for three weeks, a flagellate was detected as the most dominant mycophagous protozoan. Conidia of several fungi, with minimum diameters between 2 and 16 μm, supported growth of this flagellate, irrespective of pigmentation. Bacteria however could not be used for growth, although bacteria and also latex beads of the same size were ingested. This is, to our knowledge, the first report of an obligate mycophagous soil-borne flagellate. The flagellate was able to grow at the expense of the conidia of D. coniospora in liquid culture, with a specific growth rate of about 0.1 h⁻¹; the optimum temperature was 20–24°C. Approximately 10 D. coniospora conidia were required for one flagellate division. In sterilized soil, enriched with 10⁸ D. coniospora conidia per g dry soil, the specific growth rate was 0.014 h⁻¹, when the soil was at 50 or 65% of its water-holding capacity (WHC). In drier soil, i.e. 25% WHC, no growth took place. During growth of the flagellate in soil, the number of D. coniospora was reduced by about 20%, which was in the same order of magnitude as expected on the basis of the requirement of 10 D. coniospora conidia for one flagellate division. Since many conidia remained in the soil after growth of the flagellate, we concluded that although the flagellate is an interesting organism, it does not play a very important role in the survival of D. coniospora conidia in the soil.