Assessing the global phylum level diversity within the bacterial domain: A review

Microbial ecology is the study of microbes in the natural environment and their interactions with each other. Investigating the nature of microorganisms residing within a specific habitat is an extremely important component of microbial ecology. Such microbial diversity surveys aim to determine the identity, physiological preferences, metabolic capabilities, and genomic features of microbial taxa within a specific ecosystem. A comprehensive review of various aspects of microbial diversity (phylogenetic, functional, and genomic diversities) in the microbial (bacterial, archaeal, and microeukaryotic) world is clearly a daunting task that could not be aptly summarized in a single review. Here, we focus on one aspect of diversity (phylogenetic diversity) in one microbial domain (the Bacteria). We restrict our analysis to the highest taxonomic rank (phylum) and attempt to investigate the extent of global phylum level diversity within the Bacteria. We present a brief historical perspective on the subject and highlight how the adaptation of molecular biological and phylogenetic approaches has greatly expanded our view of global bacterial diversity. We also summarize recent progress toward the discovery of novel bacterial phyla, present evidences that the scope of phylum level diversity in nature has hardly been exhausted, and propose novel approaches that could greatly facilitate the discovery process of novel bacterial phyla within various ecosystems.