Although fungi, bacteria, and specific bacterial taxa, such as the actinomycetes, have been studied extensively in various habitats, few studies have examined them simultaneously, especially on decomposing leaves in streams. In this study, sugar maple and white oak leaves were incubated in a stream in northeastern Ohio for 181 days during which samples were collected at regular intervals. Following DNA extraction, PCRdenaturing gradient gel electrophoresis (DGGE) was performed using fungus-, bacterium-, and actinomycetespecific primers. In addition, fungal and bacterial biomass was estimated. Fungal biomass differed on different days but not between leaves of the two species and was always greater than bacterial biomass. There were significant differences in bacterial biomass through time and between leaf types on some days. Generally, on the basis of DGGE, few differences in community structure were found for different leaf types. However, the ribotype richness of fungi was significantly greater than those of the bacteria and actinomycetes, which were similar to each other. Ribotype richness decreased toward the end of the study for each group except bacteria. Lack of differences between the two leaf types suggests that the microorganisms colonizing the leaf biofilm were primarily generalists that could exploit the resources of the leaves of either species equally well. Thus, we conclude that factors, such as the ecological role of the taxa (generalists versus specialists), stage of decay, and time of exposure, appeared to be more important determinants of microbial community structure than leaf quality.

Mg(2+) can potentially influence bacterial adhesion directly through effects on electrostatic interactions and indirectly by affecting physiology-dependent attachment processes. However, the effects of Mg(2+) on biofilm structure are largely unknown. In this study, Pseudomonas fluorescens was used to investigate the influence of Mg(2+) concentration (0, 0.1 and 1.0mM MgCl(2)) on biofilm growth. Planktonic and attached cells were enumerated (based on DAPI staining) while biofilm structures were examined via confocal laser scanning microscopy and three-dimensional structures were reconstructed. Mg(2+) concentration had no influence on growth of planktonic cells but, during biofilm formation, Mg(2+) increased the abundance of attached cells. For attached cells, the influence of Mg(2+) concentration changed over time, suggesting that the role of Mg(2+) in bacterial attachment is complex and dynamic. Biofilm structures were heterogeneous and surface colonization and depth increased with increasing Mg(2+) concentrations. Overall, for P. fluorescens, Mg(2+) increased initial attachment and altered subsequent biofilm formation and structure.

Quality of dissolved organic C (DOC) from main‐stem and tributary sites along the Ogeechee River was compared to that of chemically defined organic C substrates in terms of biological availability. Availability was determined by measuring bacterial growth on natural river water (collected at high and low discharges) and defined organic substrates (humic standards and high‐quality sources of C). Availability of DOC to the natural (native) stream bacterial assemblage was compared to that of a defined bacterial inoculum, selected for ability to readily utilize high‐quality substrates. Availability of DOC declined downriver, and DOC in highly colored tributaries was of lower quality than DOC from other sites. In most cases, quality of DOC decreased at high discharge.

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Dissolved organic matter (DOM) and inorganic nutrients may affect microbial communities in streams, but little is known about the impact of these factors on specific taxa within bacterial assemblages in biofilms. In this study, nutrient diffusing artificial substrates were used to examine bacterial responses to DOM (i.e., glucose, leaf leachate, and algal exudates) and inorganic nutrients (nitrate and phosphate singly and in combination). Artificial substrates were deployed for five seasons, from summer 2002 to summer 2003, in a northeastern Ohio stream. Differences were observed in the responses of bacterial taxa examined to various DOM and inorganic nutrient treatments, and the response patterns varied seasonally, indicating that resources that limit the bacterial communities change over time. Overall, the greatest responses were to labile, low-molecular-weight DOM (i.e., glucose) at times when chlorophyll a concentrations were low due to scouring during significant storm events. Different types of DOM and inorganic nutrients induced various responses among bacterial taxa in the biofilms examined, and these responses would not have been apparent if they were examined at the community level or if seasonal changes were not taken into account.Biofilms play important roles in ecosystem processes in streams (35), and factors responsible for the abundance and distribution of microorganisms in such communities have often been studied (e.g., references 12, 16, 17, 33, 45, and 48). Inorganic nutrients (11, 18) and dissolved organic matter (DOM) (17,23,44,45) have been shown specifically to influence microbial abundance in aquatic biofilms (31,48,51).The source, quality, and type of DOM, as well as the quantity, may influence the abundance and distribution of bacteria in stream ecosystems (20,21,24). In addition, other aspects of microbial community function such as respiration, biomass, and extracellular enzyme activity may be limited by dissolved inorganic nutrients and organic matter in streams (49, 51). Also, epilithic bacterial populations can be affected indirectly by inorganic nutrients via the influence of nutrients on algal biomass (45,50).In spite of the number of studies that have looked at responses of microorganisms in streams to DOM and inorganic nutrients, little is known about the influence of nutrients and DOM on the composition and distribution of different bacterial taxa in stream biofilms. This is because earlier investigations were typically based on assemblage-level responses, such as examining total bacterial numbers (e.g., references 16, 24, 31, and 45), with few examining spatial and temporal changes in specific bacterial populations (4,20,32). In this study, we examined how different bacterial taxa in biofilms responded to DOM and inorganic nutrients by using nutrient diffusing artificial substrates (clay flowerpots), an approach commonly used to demonstrate nutrient limitation in streams (e.g., references 11 and 43). Responses to different treatments (leaf leachate [LL], glucose, algal exudate...