Evolution The good , the bad and the lonely

perform this job quite admirably. Although they have not yet been isolated in pure culture, the microbes concerned and the catalytic pathway of the anammox process have been identified, largely through the efforts of Keunen, Jetten and colleagues at the University of Nijmegen. The specifics of the anammox pathway are quite remarkable, and especially startling (Fig. 1) are the highly toxic and reactive intermediates that are formed — hydroxylamine and hydrazine, the second of which finds a very different use as rocket fuel. The location, structure and biophysical properties of the ladderanes suggest that they have a specific function in anammox bacteria. The enzymes that drive the process are localized in a special intracellular compartment, the anammoxosome (Fig. 1). This is the same place in which Damsté et al. find the ladderanes. Presumably, the rigid structure and dense packing of the ladderane membranes help to contain the noxious anammox intermediates. This would be analogous to the way that eukaryotic organelles called lysosomes compartmentalize digestive enzymes, protecting the rest of the cell from damage. To test this hypothesis, Damsté et al. examined the permeability of the ladderane membrane. Unlike other membrane compartments in the cell, the anammoxosomes were completely impervious to a variety of agents. So the unique structure of the ladderanes does seem to play a role in compartmentalizing the anammox intermediates. In evolutionary terms, one might ponder which came first: the ladderanes, or the anammox pathway? Now that we know what to look for, a search for wider distribution of these lipids may shed light on the question. “The cell consists of numerous halfliving chemical molecules suspended in water and enclosed in an oily film. When the whole sea was a vast chemical laboratory the conditions for the formation of such films must have been relatively favourable.” Thus J. B. S. Haldane imagined the early evolution of one of the most defining structures of life, the cell membrane. Given current theories about those promiscuous times, when lateral genetic transfer is thought to have exceeded vertical transmission, the invention of a defined cell barrier and the emergence of the cell as ‘self ’ was certainly a turning point in the evolution of cellular life. The variety of membrane lipid structures existing in those early times could well have been quite diverse. But innovations in membrane lipid structure as dramatic as that described by Damsté et al. appear to be generated rarely today. These new observations will certainly help in understanding the physiological processes that occur in anammox bacteria, and the mechanisms of subcellular compartmentalization in prokaryotes. They may well also provide insight into evolution and innovation in membrane lipid structures in general. ■