Aquatic actinomycete-fungal interactions and their effects on organic matter decomposition: A microcosm study

The role of fungi in the decomposition of organic matter in streams has bee n well examined, although the role of bacterial antagonists in such process es has gained little attention. To examine bacterial-fungal interactions, e xperiments involving pairwise combinations of four actinomycete isolates (A 1+ and A2+ could remove chitin from chitin-containing media, and A1- and A2 - could not) and two fungal isolates (F+ a true fungus, F- an oomycote) wer e conducted. For each bacterial-fungal combination, 250-ml microcosms were sampled at 8 day intervals for 32 days. Microbial biomass and organic matte r, as well as the activities of five extracellular enzymes, were measured. Each experiment consisted of a control group and four treatment groups. Con trols comprised sterilized stream water and macrophytes. The first treatmen t was inoculated with only actinomycetes (similar to 10(3) cells ml(-1)), t he second treatment was inoculated with only fungi (similar to 10(2) cells ml(-1)), the third group was inoculated simultaneously with actinomycetes a nd fungi, and the fourth group was inoculated with actinomycetes 2 days aft er fungal establishment. For all combinations, the lowest rates of organic matter decomposition were expected in the controls, as a result of only phy sical degradation. In contrast, the greatest rates of organic matter decomp osition were predicted in treatments inoculated with F+ 2 days prior to A1- or A2-. Greater than 50% of the organic matter was decomposed in each of t he fungal treatments. Fungal-actinomycete interactions resulted in reduced fungal biomass relative to the fungal-only treatments. However, when inocul ated 2 days apart, combinations of F- and actinomycetes resulted in enhance d rates of organic matter decomposition, as well as greater levels of extra cellular enzyme activities. These results demonstrate that actinomycete-fun gal interactions and their colonization dynamics affect the accumulation of biomass, extracellular enzyme activities, and rates of organic matter deco mposition.