The effect of silver nanoparticles on Trichoderma harzianum, Rhizoctonia spp., and fungal soil communities

Abstract

Silver nanoparticles (AgNPs) have known anti-microbial properties and are applied in many industrial applications ranging from medicine to fabric preservation. Recently, researchers have proposed the use of AgNPs in agriculture to control plant pathogenic fungi. The use of AgNPs to control plant pathogen fungi does raise concerns as they may potentially affect the activity of beneficial soil microorganisms. In this work the effects of AgNPs on the plant pathogenic fungus, Rhizoctonia spp., and their biocontrol agent, Trichoderma harzianum, and fungal soil communities were investigated. The results showed that T. harzianum mycelium growth was very tolerant to high levels of AgNPs (up to 600 mg L-1) while Rhizoctonia spp. were more sensitive (mycelial growth was affected at 20 mg L-1). Nevertheless, AgNPs effect on reproductive stage of T. harzianum, e.g. spore production, was not clear as it only showed on one concentration. Despite the decrease in spore production of T. harzianum after AgNPs exposure, the spores successfully germinated when cultivated on fresh growth medium (more than 60%). Following up these findings, T. harzianum and AgNPs were combined to examine the synergistic potential of these chemical and biological controls on growth of Rhizoctonia spp. Interestingly, the combination of AgNPs and Trichoderma did not appear to act synergistically to reduce Rhizoctonia growth in vitro. In subsequent work the effect of AgNPs contamination on soil fungal communities was assessed by Illumina MiSeq Next Generation Sequencing (NGS) and processed using the UPARSE pipeline run with USEARCH. The soil contamination experiments were carried out over a period of 2 years as previous studies have only examined effects of AgNPs contamination over a few months. Before analysing the metabarcoding data from the Illumina sequencer, a method was developed to find a suitable technique to process the data. It was found that single forward read sequences produced more operational taxonomic unit (OTU) than single reverse and paired end sequences. Therefore, single forward read sequences were used to investigate the effect of AgNPs on soil fungal communities in this study. Soil contamination by AgNPs reduced fungal species richness, evenness, and changed the community structure. For example, species such as Cryptococcus terreus was the most abundant in controls but these were replaced by other species including Trichocomaceae sp. in AgNPs contaminated soil. Tolerant species, such as T. spirale were identified in highly contaminated soil (660 mg kg-1 of AgNPs) and this species has been found in previous studies examining metal contamination. Overall the findings from this thesis suggest that more intensive study will be required when considering AgNPs as an alternative to synthetic fungicides to control plant pathogenic fungi as they have a negative impact on the fungal community in soil even at lower levels e.g. 3 mg kg-1 of AgNPs.