The occurrence of fungi in drinking water has received increased attention in the last decades, and fungi are now generally accepted as drinking water contaminants. The knowledge about the occurrence and diversity of fungi in water has increased considerably from a low knowledge base. However, the relevance of waterborne fungi for water quality and human health is poorly understood and still conflicting. Scientific reports on effective treatment against fungi in water are also few. This article presents a review of the literature on fungal water studies, including some general results, and considerations of significance, limits, contradictions, precautions, and practical consequences.

In order to determine the occurrence, distribution, and significance of mold species in groundwater-and surface water-derived drinking water in Norway, molds isolated from 273 water samples were identified. Samples of raw water, treated water, and water from private homes and hospital installations were analyzed by incubation of 100-ml membrane-filtered samples on dichloran-18% glycerol agar. The total count (number of CFU per 100 ml) of fungal species and the species diversity within each sample were determined. The identification of mold species was based on morphological and molecular methods. In total, 94 mold species belonging to 30 genera were identified. The mycobiota was dominated by species of Penicillium, Trichoderma, and Aspergillus, with some of them occurring throughout the drinking water system. Several of the same species as isolated from water may have the potential to cause allergic reactions or disease in humans. Other species are common contaminants of food and beverages, and some may cause unwanted changes in the taste or smell of water. The present results indicate that the mycobiota of water should be considered when the microbiological safety and quality of drinking water are assessed. In fact, molds in drinking water should possibly be included in the Norwegian water supply and drinking water regulations.

Seventeen strains of Saprolegnia spp. were examined for morphological and physiological characteristics, and seven were examined for their pathogenicity to Atlantic salmon, Salmo salar L. Two of the Saprolegnia strains tested caused 89 and 31% cumulative mortality in challenged salmonids and were significantly more pathogenic than the other strains tested. The positive control (Saprolegnia parasitica ATCC 90213) caused 18% mortality, but this was not significantly higher than non-pathogenic strains (0-3% cumulative mortality). All the pathogenic Saprolegnia strains and two non-pathogenic strains had secondary cysts with long, hooked hairs, a characteristic which is claimed to be typical of S. parasitica. This characteristic is apparently necessary, but does not in itself determine the ability to cause mortality in Atlantic salmon. However, all the pathogenic Saprolegnia strains in the present study showed a significantly higher initial growth rate of cysts in sterilized tap water than did non-pathogenic strains. The results of the present study suggest that initial growth rate of germinating cysts in pure water, together with the presence of long hooked hairs on the secondary cysts, may be indicators of pathogenicity of Saprolegnia strains to Atlantic salmon.

Animals and plants are increasingly suffering from diseases caused by fungi and oomycetes. These emerging pathogens are now recognized as a global threat to biodiversity and food security. Among oomycetes, Saprolegnia species cause significant declines in fish and amphibian populations. Fish eggs have an immature adaptive immune system and depend on nonspecific innate defences to ward off pathogens. Here, meta-taxonomic analyses revealed that Atlantic salmon eggs are home to diverse fungal, oomycete and bacterial communities. Although virulent Saprolegnia isolates were found in all salmon egg samples, a low incidence of Saprolegniosis was strongly correlated with a high richness and abundance of specific commensal Actinobacteria, with the genus Frondihabitans (Microbacteriaceae) effectively inhibiting attachment of Saprolegniato salmon eggs. These results highlight that fundamental insights into microbial landscapes of fish eggs may provide new sustainable means to mitigate emerging diseases.

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