Contrasting Short-Term Antibiotic Effects on Respiration and Bacterial Growth Compromises the Validity of the Selective Respiratory Inhibition Technique to Distinguish Fungi and Bacteria

Abstract

The selective inhibition (SI) technique has been widely used to resolve fungal and bacterial biomass. By studying bacterial growth (leucine/thymidine incorporation) and respiration simultaneously, this study demonstrates that the inhibitors the SI technique is based on do not efficiently or specifically resolve fungal and bacterial contributions to respiration. At concentrations that completely inhibited bacterial growth, the bactericide streptomycin had no influence on the SI technique's respiration measurement, and complete inhibition of bacterial growth using oxytetracycline resulted in marginal respiration reductions. The fungicides captan and benomyl severely inhibited non-target bacterial growth. Cycloheximide did not reduce bacterial growth at moderate concentrations, but the cycloheximide respiration reduction was no higher in a soil with more fungal biomass, casting doubt on its ability to discriminate fungal respiration contribution. Conclusions regarding bacteria and fungi based on the SI technique using these inhibitors are thus compromised. The inhibition of glucose-activated respiration by the bactericide bronopol appeared to correlate with bacterial growth inhibition, however. Bronopol, combined with growth-based techniques, could aid development of a new framework to resolve decomposer ecology in soil.

Contrasting Soil pH Effects on Fungal and Bacterial Growth Suggest Functional Redundancy in Carbon Mineralization

“…Using this technique, increases in the fungal biomass/bacterial biomass ratio of 4.5-fold between pH 7 and 3 (12) and of twoto sixfold between pH 6 and 3 (17) have been reported. However, the partitioning of potential respiration by using antibiotics to estimate fungal and bacterial biomasses has repeatedly been challenged (56,64). Still, it is noteworthy that this biomass technique, which inherently relies on active microorganisms, has high responsiveness to pH effects.…”

Dissimilar bacterial and fungal decomposer communities across rich to poor fen peatlands exhibit functional redundancy

“…Other recent studies provide evidence in support of the concept of functional redundancy. Rousk et al (2009) concluded that functional redundancy in carbon mineralization was exhibited in fungal and bacterial communities in upland soils along a pH gradient, as respiration was relatively unchanged despite significant differences in microbial growth. An overlap in biogeochemical function and ecology of bacteria and fungi was also proposed by Strickland and Rousk (2010) to account for a lack of correlation between changing trends in cycling of key elements and shifts in fungal:bacterial dominance.…”

Amino acid cycling in plankton and soil microbes studied with radioisotopes: measured amino acids in soil do not reflect bioavailability

“…The authors concluded that the various biomass methods agreed and that fungi usually dominated the biomass of soil microbes. However, the widely used method of selective inhibition of respiration (Bååth and Anderson 2003) has recently been shown to overestimate fungal respiration, since respiration of many bacteria is also limited by supposedly fungal-specific inhibitors (Rousk et al 2009). …”