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Many diverse microbes have been detected in or isolated from glaciers, including novel taxa exhibiting previously unrecognized physiological properties with significant biotechnological potential. Of 29 unique phylotypes isolated from Victoria Upper Glacier, Antarctica (VUG), 12 were related to the poorly studied bacterial genus Hymenobacter including several only distantly related to previously described taxa. Further study of these microorganisms revealed genotypic, phenotypic, morphological and chemotaxonomic divergence from named species and suggested that they likely represent novel Hymenobacter species. These studies also indicated, however, that the systematics of Hymenobacter and related microorganisms is more complex than previously realized, and may exhibit poorly defined species boundaries due to cosmopolitan dispersal, significant rates of horizontal gene transfer and reintroduction of archived genotypes, e.g., from glacial ice. These processes are reflected in the carotenoid composition of Hymenobacter and related organisms, which includes several novel methyl- and xylosyl-derivatives of 2'-hydroxyflexixanthin with distributions indicative of horizontal gene transfer or differential gain and/or loss of terminal biosynthetic pathway steps. These processes have been previously underappreciated in assessments of microbial carotenoid diversity and suggest the need for fine-scale phylogenetic study of carotenoid distribution in other microbial taxa. Further comparative genomics-based evaluation of microbial carotenoid biosynthesis indicated its wide phylogenetic distribution and diversification, controlled by several lineage-specific modes of evolution including horizontal transfer, de novo enzyme evolution followed by differential gene loss, co-evolution with biochemically associated structures and elevated mutation rates. The latter especially interacts with horizontal transfer depending on metabolic pathway topology, exemplified by the evolution of purple bacterial carotenoid biosynthesis. Exploration of VUG microbial diversity, therefore, not only revealed novel taxa and biotechnologically interesting compounds but also spurred broader evaluation of the mechanisms of metabolic pathway evolution applicable to many other taxa and biochemical pathways. / Microbiology and Cell Biotechnology