Acclimation of stream‐bed heterotrophic microflora: metabolic responses to dissolved organic matter

Abstract

SUMMARY. Studies were performed to assess the acclimation of the stream‐bed heterotrophic microflora to sources of dissolved organic matter (DOM) typical of its environment and microfloral responses to pulses of DOM. Microcosm measurements of dissolved organic carbon (DOC) uptake, dissolved oxygen uptake. ATP concentration and epitluorescence microscopic counts (EMC) were performed using stream‐bed sediments and heterogeneous dissolved organic matter (DOM) sources. Three study sites included an undisturbed woodlot spring seep, a small stream traversing a cattle pasture and a larger stream draining a catchment used for silage crops, pastures and woodlands. The DOM sources were cold water extracts of forest floor leaf litter, bovine manure, the green alga Ulothrix and jewel weed (Impaliens capensis L.) leaves. DOC uptake occurring in 2.5–5.0 h incubations indicated an acclimation of the microflora at each site to DOM sources generated by surrounding land use. The sediment microflora from the larger stream did not readily metabolize bovine manure DOM and the latter was used in an acclimation experiment. A minimum of 48 h of cumulative exposure to bovine manure DOM at 15–20°C were required to yield measurable changes in sediment microbial activity of sediment microbial biomass. The same microflora retained an ability to readily metabolize the added DOM source after 72 h of exposure to unamended stream water. The time frame of microfloral responses during acclimation indicated that changes leading to the metabolism of a DOM source were initially enzymatic and eventually involved growth and selection for specific decomposers within the microbial community. We conclude that in order to utilize naturally occurring pulses of carbon and energy, stream‐bed heterotrophs must be already enzymatically prepared, induced, when the pulse occurs.