Assessment of airborne fungi is important due to their potential impacts on health and on global climate. Conventional culture-based methods often lead to underestimation of airborne fungal levels and the results are often not reproducible. Ergosterol, a primary sterol in cell membrane of fungi, had been proposed as a biomarker for assessing ambient fungal prevalence. To serve such purpose, reliable conversion factors of ergosterol to number concentrations of fungal spores should be established....[ Read more ]

Assessment of airborne fungi is important due to their potential impacts on health and on global climate. Conventional culture-based methods often lead to underestimation of airborne fungal levels and the results are often not reproducible. Ergosterol, a primary sterol in cell membrane of fungi, had been proposed as a biomarker for assessing ambient fungal prevalence. To serve such purpose, reliable conversion factors of ergosterol to number concentrations of fungal spores should be established.

The prerequisite of reliable conversion factors is the understanding of the ergosterol content in individual species of fungal spores or yeast cells that is commonly found in the ambient environment. Therefore, gas chromatography-mass spectrometry (GC-MS) was used to determine ergosterol contents of the dominant fungal spores/yeast cells collected from the ambient aerosols and cultivated in the laboratory. The ergosterol contents of the 12 common airborne species studied in this thesis ranged from 0.047 to 9.919 pg/spore or yeast cell. It was found that the larger the surface area of the spore, the higher the ergosterol content. Ergosterol contents of the spores also varied with culturing age of the fungal vegetative growth and became steady when the fungi growth became mature. Decomposition but not all of ergosterol in some dead species was found and change of ergosterol contents under the effects of cold storage was observed.

With the known spore ergosterol contents of the common ambient species determined, a weighted average conversion factor, 0.197 pg/spore, was established with respect to the relative contributions of the common and dominant ambient species. This weighted average conversion factor of ergosterol to the number concentration of fungal spores and yeast cells were applied to field filter-based measured ergosterol content. The geometric mean of estimated number concentrations was 1757 spores/m³ (geometric standard deviation: 2.85), matched in the order of magnitude with the geometric mean of culturable fungal concentrations, which was 475 CFU/m³ (geometric standard deviation: 2.18), taking the general consensus of the underestimation in viable sampling. Apart from testing the applicability of this weighted average conversion factor to the field samples in this study, the conversion factor was also applied to archived ergosterol dataset. The results verified that the conversion factor is applicable in assessing the ambient airborne fungal prevalence.

As the origin of a large portion of atmospheric organic carbon remains unidentified, the study of fungal contribution to the atmospheric organic carbon becomes important. With similar approach, a weighted average conversion factor for organic carbon to number concentration, and vice versa, was established. The fungal contribution to atmospheric organic carbon was found to be as high as 7.43% in one of the aerosol samples.

This thesis highlighted the significance and feasibility of the establishment of proper conversion factors for biomarkers in assessing ambient fungal prevalence. Preliminary applicability of the established conversion factors was verified with archived data as well.