Determinants Of Microbial Community Structure Research Articles

Spatial Heterogeneity of Bacterial Assemblages in Marine Sediments: The Influence of Deposit Feeding byBalanoglossus aurantiacus .

Densities and substrate utilization patterns of the bacterial assemblages of three intertidal sandflats were contrasted to investigate spatial heterogeneity. Possible influence of deposit-feeding Balanoglossus aurantiacus (Enteropneusta: Ptychoderidae) was also studied by comparing samples taken from the faecal mounds with surrounding sediments. While there were no significant differences in total bacterial abundance, carbon source utilization patterns, determined using Biolog GN microtitre plates, differed among the sandflats and within the sandflats for samples from grids more than 10m apart. No consistent quantitative or qualitative differences were detected among clusters of ambient sediment samples taken less than 0·5m apart. Qualitative differences in microbial assemblages were found between faecal mounds and ambient (>10cm apart) sediment within a single intertidal flat. These differences were in part due to digestive removal of bacteria, as heterotrophic plate counts and response in Biolog plates (average well colour development, AWCD) were significantly higher for the surrounding sediment. However, when Biolog profiles were normalized to AWCD, it became clear that gut passage resulted in structural shifts in bacterial assemblages. These changes were due to digestion of a subset of ingested bacteria with the concomitant stimulation (or inoculation) of other groups, which were largely unculturable under the conditions employed. These findings illustrate that deposit feeding effects spatial heterogeneity in sedimentary bacterial assemblages. Given the ephemeral nature of these disturbed patches, however, it is not yet clear that deposit feeding is a primary determinant of microbial community structure in this sedimentary landscape.

Seasonal, soil type, and alternative management influences on microbial communities of vegetable cropping systems

Microbial community responses to alternative management may be indicative of soil quality change. In this study, soils were collected from research plots over 2 years and from commercial grower fields over 1 year. Treatments at the sites included 1–9 years of either winter cover cropping or winter fallow practices. Soils were assayed for microbial fatty acid methyl esters (FAMEs), direct count microscopy and Biolog substrate utilization potentials to assess management and environmental influences on soil communities. The strongest influence was season. Soils in early spring (prior to termination of the cover crop) utilized fewer carboxylic acids and generally were enriched in eukaryotic FAMEs, whereas proportionally more bacterial FAMEs were detected in soils at canopy closure and harvest of the summer vegetable crop. Within a season, community FAME and Biolog patterns were related to field properties. FAME profiles from grower fields in early spring and harvest were correlated significantly with soil texture, cation exchange capacity, and carbon content. Changes in community structure and Biolog potential occurred in some soils in response to winter cover crops, although effects were not observed until cover crop incorporation. Greater amounts of fungal and protozoan FAME markers were detected in some cover-cropped soils compared to winter fallow soils. Cover crop residues increased FAME diversity at one research station and Biolog diversity at two research stations and the grower fields. Although seasonal and field-dependent factors are major determinants of microbial community structure, shifts can occur as soil physical and chemical properties change in response to alternative practices, as demonstrated by this study.

Impact of carbon and flooding on the metabolic diversity of microbial communities in soils

The assumption that carbon and soil water content are major determinants of microbial community structure and function is rarely questioned because of substantial evidence of the impacts of these variables on specific populations and functions. The significance of carbon and water for metabolic diversity at the microbial community level was tested on the field scale in agricultural plots varying in carbon inputs and in whether they were flooded. Surface soils in which rice straw was incorporated or burned and which were flooded or unflooded were sampled at monthly intervals three times during the flooded winter period (January to March) and again 1 month postdraining. Biomass carbon and nitrogen were not affected by treatments, active bacterial counts showed slight increases, and respiration rates were increased by carbon inputs and flooding. Biolog microplates were inoculated with soil extracts to quantify the metabolic diversity of the soil microbial community. Canonical correspondence analysis and the Monte Carlo permutation testing showed that differences in substrate utilization patterns were significantly related (P < 0.001) to carbon and flooding treatments. Biolog substrates whose metabolism was altered by the treatments were consistent across dates and tended to be positively related (utilization enhancement) to carbon inputs and negatively related to winter flooding. The importance of carbon as an environmental variable increased over time after straw treatment, whereas the importance of water became evident after flooding and decreased after drainage. The effect of long-term rice straw incorporation on substrate utilization patterns at another field site was consistent with these results despite the dissimilarities of the two soils.