Armbrust.indd MH

microscopic, eukaryotic phytoplankton known as diatoms. These photosynthetic workhorses are found in waters worldwide, wherever there is sufficient light and nutrients. Their name is derived from the Greek diatomos, meaning ‘cut in half ’, a reference to their distinctive two-part cell walls made of silica (Fig. 1). Each year, diatom photosynthesis in the sea generates about as much organic carbon as all the terrestrial rainforests combined. But unlike much of the carbon generated by trees, the organic carbon produced by diatoms is consumed rapidly and serves as a base for marine food webs. In coastal waters, diatoms support our most productive fisheries. In the open ocean, a relatively large proportion of diatom organic matter sinks rapidly from the surface, becoming food for deep-water organisms. A small fraction of this sinking organic matter escapes consumption and settles on the sea floor, where it is sequestered over geological timescales in sediments and rocks and contributes to petroleum reserves. Given the crucial role of diatoms in the global carbon cycle, plans have been made, controversially, to reduce atmospheric levels of the greenhouse gas carbon dioxide by fertilizing large regions of the ocean with iron to generate huge blooms of diatoms. Fresh insight into the mechanisms underlying the global impact of diatoms came from the availability of the roughly 34 megabases (Mb) of DNA sequence for the nuclear, plastid and mitochondrial genomes of the cosmopolitan diatom Thalassiosira pseudonana. Whole-genome sequence of a second model diatom, Phaeodactylum tricornutum (27 Mb), soon followed, and draft sequence is now available for the polar species Fragilariopsis cylindrus (80 Mb) and the toxigenic coastal species Pseudo-nitzschia multiseries (300 Mb). One of the more intriguing outcomes of the sequencing projects thus far is a recognition of the unique combination of genes and metabolic pathways that distinguish diatoms from the evolutionarily distinct plant and animal lineages. Enormous amounts of diversity are encapsulated within diatoms. For example, T. pseudonana and P. tricornutum probably diverged from one another only about 90 million years (Myr) ago, yet their genomes are about as different as those of mammals and fish, which diverged about 550 Myr ago. Here I explore the intersection of diatom ecology, biogeochemistry and genomics, with a focus on the roles of diatoms in past and contemporary oceans. What emerges is a genomics-based reflection of the complex interactions that define marine ecosystems, in which metabolites and capabilities are shared across different kingdoms of organisms. The goal is twofold: first, to provide a window into the fascinating world of this unusual group of organisms that has such a crucial role in regulating the stability of our planet; and second, to gain a deeper understanding of how diatoms may fare under future ocean conditions. This is crucial because alterations in diatom populations resulting from climate change could have a dramatic effect on Earth’s atmosphere.