Pathogenic Yet Environmentally Friendly? Black Fungal Candidates for Bioremediation of Pollutants

Abstract

A collection of 163 strains of black yeast-like fungi from the CBS Fungal Biodiversity Center (Utrecht, The Netherlands), has been screened for the ability to grow on hexadecane, toluene and polychlorinated biphenyl 126 (PCB126) as the sole carbon and energy source. These compounds were chosen as representatives of relevant environmental pollutants. A microtiter plate-based culture assay was set up in order to screen the fungal strains for growth on the selected xenobiotics versus glucose, as a positive control. Growth was observed in 25 strains on at least two of the tested substrates. Confirmation of substrate assimilation was performed by cultivation on closed vials and analysis of the headspace composition with regard to the added volatile substrates and the generated carbon dioxide. Exophiala mesophila (CBS 120910) and Cladophialophora immunda (CBS 110551), both of the order Chaetothyriales and isolated from a patient with chronic sinusitis and a polluted soil sample, respectively, showed the ability to grow on toluene as the sole carbon and energy source. Toluene assimilation has previously been described for C. immunda but this is the first account for E. mesophila. Also, this is the first time that the capacity to grow on alkylbenzenes has been demonstrated for a clinical isolate. Assimilation of toluene could not be demonstrated for the human opportunistic pathogen Pseudoallescheria boydii (CBS 115.59, Microascales), but the results from microtiter plate assays suggest that strains of this species are promising candidates for further studies. The outstanding abilities of black yeast-like fungi to thrive in extreme environments makes them ideal agents for the bioremediation of polluted soils, and for the treatment of contaminated gas streams in biofilters. However, interrelations between hydrocarbonoclastic and potentially pathogenic strains need to be elucidated in order to avoid the possibility of biohazards occurring.

Keywords: Biofilter; black fungi; toluene.

Figure 1.

Graph of toluene microtiter plate-screening of Cladophialophora immunda (strain 17, CBS 110551). Optical density (700 nm) is plotted against time (days). The graph is derived from the average data points of duplicates. Three growth curves are represented: medium plus hydrocarbon (H), medium plus glucose (G), medium plus glucose, vitamins and trace elements (GVT).

Figure 2.

Graph of gas chromatography screening for toluene consumption and CO₂ production by Cladophialophora immunda (strain 17, CBS 110551).

Figure 3.

Graph of toluene microtiter plate-screening of Exophiala mesophila (strain 64, CBS 120910). Optical density (700 nm) is plotted against time (days). The graph is derived from the average data points of duplicates. Three growth curves are represented: medium plus hydrocarbon (H), medium plus glucose (G), medium plus glucose, vitamins and trace elements (GVT).

Figure 4.

Graph of gas chromatography screening for toluene consumption and CO₂ production by Exophiala mesophila (strain 64, CBS 120910).

Figure 5.

Toluene degradation pathway in fungi. Enzymes that are differently present in the five genomes are highlighted (modified from Parales et al. 2008).