Diatoms are an enigma. Neither plant nor animal, they share biochemical features of both. Though simple single-celled algae, they are covered with elegant casings sculpted from silica. Now researchers have sequenced the genome of a frequently studied diatom species, taking a big step toward resolving the paradoxical nature of these odd microbes.
Diatoms occupy vast swaths of ocean and fresh water, where they play a key role in the global carbon cycle. They're a source of food for many organisms, as well as the cause of ocean "blooms" that are sometimes toxic. "This is a group of organisms that has amazing importance in global ecology," says Deborah Robertson, an algal physiologist at Clark University in Worcester, Massachusetts.
Since 2002, Daniel Rokhsar, a genomicist at the DOE Joint Genome Institute in Walnut Creek, California, and his colleagues have been unraveling the genome of the saltwater diatom Thalassiosira pseudonana. The draft genome consists of 34 million bases and about 11,500 genes, Rokhsar and 44 colleagues report in the 1 October issue of Science.
Analyses of these genes and the proteins they encode confirm that diatoms have had a complex history. Like other early microbes, they apparently acquired new genes by engulfing microbial neighbors. Perhaps the most significant acquisition was an algal cell, which provided the diatom with photosynthetic machinery. Some biologists hypothesize that diatom ancestors branched off from an ancestral nucleated microbe from which plants and animals later arose. As diatoms, plants, and animals evolved, each must have shed different genes from this common ancestor. As a result, diatoms were left with what looks like a mix of plant and animal DNA.
The newly analyzed genome has also begun to shed light on how a diatom constructs its intricately patterned glass shell. So far, Rokhsar and his colleagues have uncovered a dozen proteins involved in the deposition of the silica and expect to find more. Such progress could be a boon to materials scientists, says Robertson. With the genome in hand, interest in diatoms is going to expand, predicts Edward Theriot, a diatom systematist at the University of Texas, Austin: "It will help put diatoms on everyone's radar."