AME 33:127-136 (2003)  -  doi:10.3354/ame033127

ABSTRACT: Biofilms growing on gravel surfaces representative of hyporheic sediments were exposed to inorganic nitrogen or amino acid additions in experimental systems to examine potential responses in bacterial growth, oxygen removal, extracellular enzyme activities and bacterial community structure. Additions of ammonium and a mixture of amino acids increased oxygen removal by >50% in small perfusion cores, with a stronger response to amino acids than equimolar NH₄ addition. Some extracellular enzyme activities were affected by a significant increase in a- and b-glucosidase activities in amino acid relative to NH₄ additions, while peptidase activities were only weakly affected by either NH₄ or amino acid addition. Community structure assayed via RAPDs did not change in parallel with responses of metabolic variables, and there was no concordance between RAPD clusters and experimental treatment. In flow-through mesocosms mimicking hyporheic flowpaths, similar additions of NH₄ and amino acids yielded significant differences in patterns of oxygen decline along the flowpaths. Differences in bacterial growth and extracellular enzyme activity were restricted to the sampling locations closest to the head of the flowpath (point of N injection) with a general convergence of values nearer the end of mesocosms. As for the perfusion core experiment, there was no concordance between metabolic and community structural response as measured by clustering of RAPD data. Nitrogen supply clearly altered biofilm metabolic activity but there was no parallel change in community structure. We suggest that the ability to acquire inorganic nitrogen or amino acid nitrogen is sufficiently distributed across bacterial taxa so that shifts in function do not necessarily require shifts in bacterial community structure.

KEY WORDS: Hyporheic · Metabolism · Dissolved inorganic nitrogen · Amino acids · Bacteria · Production · Respiration · Nitrification · Enzymes