Heavy Metal Tolerance Among Free-living Fungi Isolated from Soil Receiving Long Term Application of Wastewater

In this study fungal profiles of agricultural field soil irrigated with industrial wastewater and sewage containing varying concentrations of heavy metals (Chromium, Nickel, Cobalt, Copper and Cadmium) have been investigated. The impact of long term heavy metal contamination on emergence of heavy metal tolerant soil fungal population, changes in morphological diversity and metal tolerance limits among isolated fungi was studied. The agricultural field soil received long term (>20 years) wastewater application showed metal accumulation compared to the untreated soil. The viable count of soil fungal population from three different agricultural field soil was found in order of 10(5) to 10(-4) CFU gm(-1) of soil indicating a normal viable count with little variations. Viable plate count of fungal population on metal amended plates decreased with increasing concentration of tested metals (Cr6+, Cd++. Cu++, Co++ and Ni++) from 100 to 400 mu gml(-1). The decrease was higher on cadmium amended plates and lower against Chromium. The control site, which did not receive wastewater application showed relatively less metal tolerant fungal viable count on Cd++ and Ni++ plates when compared at 100 mu gml(-1) as compared to contaminated sites. Similarly, presence of metal tolerant fungal population was also observed from wastewater sample. The common soil fungi isolated and characterized from metal amended plates belong to 18 genera and 15 unidentified species. Occurrence of different fungal genera from site A B and C indicated different patterns of decrease on different metal amended plates with increasing concentration. Among these 73 isolated fungal species a high level of tolerance was recorded to Cr6+ followed by Cu++, Co++ and Ni++ while the lowest level of tolerance was for Cd++. The minimum inhibitory concentration (MIC) values of 73 metal tolerant fungal isolates, was ranged from 200 to 2000 mu gml(-1) against one or more heavy metals. The level of tolerance to heavy metals also varied even among the isolates of single genus. Aspergillus was the predominant genus recovered from contaminated soils where the MIC values are highly varied among different isolates of Aspergillus. The current study found that long term release of wastewater has not disturbed the fungal population dynamics in contaminated sites as compared to uncontaminated sites. However, it tends to exert selective pressure on fungal populations of soil, leading to the development of increased level of metal tolerance in fungal species.