No abstract

No abstract

Traditional studies of microbial communities are incomplete because of the inability to identify and quantify all contributing populations. In the present study, we directly determine the abundance and dbution of sulfate-reducing bacterial populations in a microbial mat community by using hybridization probes complement to the 16S-lHke rRNAs of major phylogenetic groups. Most of the major groups were found in this single community, distributed for the most part in nonoveriapping depth Intervals of the mat. The reflection of the phylogenetic structure in the community strture suggests that those species making up the maor phylogenetic groups perform specific interrelated mebic functions in the community. Comparison of pouation profiles to previously observed rates of sulfate reduction suggests there are additional populations of sulfate-reducing bacteria both within the photooxic zone and deeper in the mat.Phylogenetic relationship is an inseparable part of ecological study. In the macroecological world, the phylogenetic dimension is taken for granted. Animals and plants are obviously different, and their complex morphologies make distinction among orders, families, and genera relatively straightforward. In contrast, descriptions of the microbial world are not of this nature. Despite their incredible metabolic and phylogenetic diversity, most bacteria (prokaryotes) cannot be classified in a phylogenetically meaningful way on this basis. Even if an environment contains bacteria less closely related to each other than animals are to plants, this is not apparent from direct microscopic examination. The problem is compounded by the fact that generally only a small fraction of the species in a microbial community can be cultured, although culturing has traditionally been the prerequisite for identification. Worse yet, the phenotype expressed in pure culture does not necessarily reflect the phenotype(s) expressed in the natural setting. Since microbial ecology developed in the complete absence of a phylogenetic framework, the ecology of a community such as a microbial mat could be defined in only incomplete and superficial terms.Over the past decade and a half, reliable methods for determining microbial phylogenies based on gene sequence (1) and methods for recovering genes directly from environmental samples (2, 3) have been developed. Probes based on these sequences are capable of identifying and classifying the members of a microbial community in situ (4, 5). Thus, the microbial ecologist is now in a position comparable to the macroecologist; organisms in a given community can be readily identified, related, and compared to microorganisms in other communities or in comparable niches. In this regard, the use of rRNA sequences to characterize natural community structure is increasingly common (3, 6, 7). However, most studies have used this phylogenetic dimension primarily to place the organism (i.e., sequence) within a phylogenetic framework. This leaves outstanding the question of how the phylogeny is reflect...