= Microbial community
A \b[microbial community] is an assemblage of <microorganisms> occupying a defined habitat or sampled environment. Its description includes which organisms occur, their abundances and their interactions with one another and the environment. The boundary matters: the organisms on one <leaf>, in a patch of <soil> or in a defined experimental mixture answer different questions in <microbial ecology>.
A controlled example is the2011 experiment by <Gerd Innerebner> and colleagues with <Arabidopsis thaliana>. They compared plants with defined bacterial colonization histories before introducing a <Pseudomonas syringae> pathogen, then measured both symptoms and pathogen abundance. Many plant-derived <Sphingomonas> isolates reduced disease, whereas other isolates did not, even at comparable colonization densities. Presence and abundance alone therefore did not specify the protective function.<Protection of Arabidopsis thaliana against leaf-pathogenic Pseudomonas syringae by Sphingomonas strains in a controlled model system by Gerd Innerebner and others>[Abstract; Figures1–5: controlled colonization and pathogen challenge]{ref}
Composition and activity are also distinct measurements. In an invented two-member community, counts of900 and100 cells give relative abundances90% and10%. If both double, those percentages remain unchanged although the total cell count doubles. A <DNA sequencing> survey supplies another kind of observation: marker counts can reflect gene-copy number and sampling effects, so they are not automatically counts of organisms or measurements of their metabolic rates.<Divergence and redundancy of 16S rRNA sequences in genomes with multiple rrn operons by Silvia G. Acinas and others>[Abstract: copy-number and sequence heterogeneity affect diversity estimates]{ref}
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