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Abstract

Recent research has indicated that susurface runoff (groundwater) discharging into headwater streams can contribute significant quantities of dissolved organic matter (DOM) to the stream-bed microbiota. Bacterial utilization of the immobilized DOM (mainly associated with sediment biofilms) is potentially an important foundation to the stream trophic structure. The current study focused on the immobilization of dissolved free amino acids (AAs) along this pathway. Using ¹⁴C-labelled isotops, immobilization was traced by measuring the loss of AAs from solution, and their incorporation into sediment biofilms or their evolution as CO. At natural concentrations, an AA mixture was turned over rapidly (>90% in 2 h), with similar immobilization rates per unit volume of sandy or stony sediment. Some 25% of the immobilized AAs were respired as CO₂. AAs were utilized in proportions similar to the above in experiments simulating groundwater percolation through cores of sandy stream-bed sediment. At artificially elevated concentrations (up to 10⁴· natural levels), proportions of AAs immobilized were steadily reduced (from 94% to 61%), but this still represented a 5 orders of magnitude increase in the AA immobilization rate. At the same time, the proportion of CO₂released increased from 28 to 67 %. While this represented a decrease in the proportional immobilization of the added organic matter, the net result was still a massive increase in AA retention, as well as a probable stimulation of microbial activity. The latter was reflected in bacterial biomassestimates, which had approximately doubled within 48 h at the highest AA enrichment, compared with the unenriched control. This demonstrates an efficient utilization by the stream-bed sediments of a pulsed increase in labile DOM availability. With glycine enrichments alone, similar increases in immobilization rates were observed. However, the proportion of CO₂evolved from the immobilized AA decreased with increasing glycine concentration, from around 50% to 8%. This could mean that glycine alone did not stimulate microbial activity in the same way as the AA mixture apparently did (at least in the short term), and that the elevated immobilization rates were largely due to abiotic adsorption og glycine to the biofilm surfaces. These data suggest that, at least under certain conditions, abiotic processes may be an important mediator in the microbial utilization of DOM. Further work is required to determine the exact nature of such conditions. V_max for AA immobilization by the sediments was estimated to occur at concentrations in excess of 5 · 10⁵ µg C · l^-1. To explain why such a high immobilization potential should be maintained by the sediment biofilms, it would perhaps be more pertinent to consider their chemical environment in terms of the groundwater DOM loading (i.e. DOM concentration · groundwater discharge rate) rather than the groundwater DOM concentration alone. While the latter may rarely more than double in headwater catchments, variabilities by at least a factor of 10⁵ in DOM loading would not be uncommon during storm events. Thus sediment biofilms would appear to be ideally suited for capitalizing on such ephemeral increase in DOM loading, and the flux of low molecular weight DOM within the stream-bed sediments may be extremely high. The complete results will be published later elsewhere.