Actinomycetes as antagonists of litter decomposer fungi

The actinomycete communities of North American forests and impacts of their activities on decomposer fungi have not been thoroughly investigated. In the current study, the community size and species richness of the forest floor actinomycete communities in cool temperate aspen poplar and lodgepole pine forests of Alberta, Canada, was investigated. Based on the isolations conducted on chitin agar, both aspen and lodgepole forests had relatively large actinomycete communities (104–105 CFU g−1 soil). One hundred and fifty-six different actinomycete strains were isolated and the genus Streptomyces predominated in both forests. The community size as well as the number of different actinomycete strains was higher in aspen forests than in pine forests, but the exact reasons for these differences are not clear at this stage. It may be that favorable conditions created by endogeic earthworms (e.g. burrowing and mixing of organic material, dispersal of actinomycetes) increase the density and diversity of actinomycetes in aspen litter. It is also possible that pine litter anti-microbial compounds, waxy material and phytotoxic products of pine litter decomposition may have negative affects on actinomycetes in pine forests. Both in vitro and microcosm studies showed that the frequently isolated actinomycetes could act as antagonists for some common leaf litter and wood decomposer fungi. Of the litter decomposer fungi, fast growing genera (e.g. Mucor, Penicillium, Trichoderma) were more tolerant of actinomycete antagonism than slow and moderately slow-growing genera (e.g. Cladosporium, Mortierella). The colonization of organic substrates by some actinomycetes did reduce the degree of subsequent colonization by susceptible decomposer fungi.