Interactions of bacteria and fungi on decomposing litter: differential extracellular enzyme activities.

Fungi and bacteria are key agents in plant litter decomposition in freshwater ecosystems. However, the specific roles of these two groups and their interactions during the decomposition process are unclear. We compared the growth and patterns of degradative enzymes expressed by communities of bacteria and fungi grown separately and in coexistence on Phragmites leaves. The two groups displayed both synergistic and antagonistic interactions. Bacteria grew better together with fungi than alone. In addition, there was a negative effect of bacteria on fungi, which appeared to be caused by suppression of fungal growth and biomass accrual rather than specifically affecting enzyme activity. Fungi growing alone had a high capacity for the decomposition of plant polymers such as lignin, cellulose, and hemicellulose. In contrast, enzyme activities were in general low when bacteria grew alone, and the activity of key enzymes in the degradation of lignin and cellulose (phenol oxidase and cellobiohydrolase) was undetectable in the bacteria-only treatment. Still, biomass-specific activities of most enzymes were higher in bacteria than in fungi. The low total activity and growth of bacteria in the absence of fungi in spite of apparent high enzymatic efficiency during the degradation of many substrates suggest that fungi provide the bacteria with resources that the bacteria were not able to acquire on their own, most probably intermediate decomposition products released by fungi that could be used by bacteria.

Ergosterol as a measure of living fungal biomass: persistence in environmental samples after fungal death.

The membrane lipid ergosterol is found almost exclusively in fungi, and is frequently used by environmental microbiologists as an indicator of living fungal biomass, based on the assumption that ergosterol is labile, and therefore rapidly degraded after the death of fungal hyphae. We studied the degradation of ergosterol in environmental samples without living fungi. Under the conditions used in this study, ergosterol was very stable both when added as a pure compound and when associated with dead fungi. The decrease of ergosterol was at most 34% during 2 months when protected from sunlight. Presence of a natural bacterial assemblage did not enhance degradation over this time period, as compared to sterile controls. However, photochemical degradation was significant, and led to a 43% decrease of in ergosterol content during 24 h. These results suggest that ergosterol should be used cautiously as a biomarker for living fungi.