The trophic tapestry of the sea.

O ceanic ecosystems are the largest habitats on Earth, and living biomass in these systems is dominated by planktonic microorganisms. Microbial ecologists have long sought to understand the role of environmental selection in shaping the diversity of planktonic microorganisms. The oceans have proven a dauntingly complex arena to conduct such studies. Marine ecosystems are immense and remote and hence chronically undersampled. Moreover, the enormous diversity of planktonic microorganisms challenges our most sophisticated computational capabilities. Only recently, through creative applications of new technologies have oceanographers begun to overcome some of these hurdles. In this issue of PNAS, Lauro et al. (1) use a comparative genome approach to provide insight into how environmental pressures have shaped microorganism lifestyles. The study highlights key differences in the genomes of two oceanic bacteria: Sphingopyxis alaskensis, an abundant -proteobacterium adapted to low-nutrient oceanic environments, and Photobacterium angustum, a -proteobacterium that grows optimally in nutrient-enriched seawater. By examining the genetic underpinnings that differentiate these microorganisms the study uses the lens of molecular biology to view lifestyle features of ocean microbes.