Community structure of microbes in natural environments

Abstract

Abstract Community structure refers to the taxonomic types of microbes and their relative abundance in an environment. This chapter focuses on bacteria with a few words about fungi; protists and viruses are discussed in Chapters 9 and 10. Traditional methods for identifying microbes rely on biochemical testing of phenotype observable in the laboratory. Even for cultivated microbes and larger organisms, the traditional, phenotype approach has been replaced by comparing sequences of specific genes, those for 16S rRNA (archaea and bacteria) or 18S rRNA (microbial eukaryotes). Cultivation-independent approaches based on 16S rRNA gene sequencing have revealed that natural microbial communities have a few abundant types and many rare ones. These organisms differ substantially from those that can be grown in the laboratory using cultivation-dependent approaches. The abundant types of microbes found in soils, freshwater lakes, and oceans all differ. Once thought to be confined to extreme habitats, Archaea are now known to occur everywhere, but are particularly abundant in the deep ocean, where they make up as much as 50% of the total microbial abundance. Dispersal of bacteria and other small microbes is thought to be easy, leading to the Bass Becking hypothesis that “everything is everywhere, but the environment selects.” Among several factors known to affect community structure, salinity and temperature are very important, as is pH especially in soils. In addition to bottom-up factors, both top-down factors, grazing and viral lysis, also shape community structure. According to the Kill the Winner hypothesis, viruses select for fast-growing types, allowing slower growing defensive specialists to survive. Cultivation-independent approaches indicate that fungi are more diverse than previously appreciated, but they are less diverse than bacteria, especially in aquatic habitats. The community structure of fungi is affected by many of the same factors shaping bacterial community structure, but the dispersal of fungi is more limited than that of bacteria. The chapter ends with a discussion about the relationship between community structure and biogeochemical processes. The value of community structure information varies with the process and the degree of metabolic redundancy among the community members for the process.