Interesting things come in small packages

In life, it's said that few things are certain except death and taxes. Similarly, in biology, while there are exceptions to many 'rules', there do seem to be a few certainties that have stood the test of time. One of these is the general organization and structure of the photosynthetic apparatus in chlorophyll-containing green plants and cyanobacteria, in which the photosynthetic electron-transport chain consists of two photosystems, one of which generates oxygen. However, a study by Jonathan Zehr and colleagues recently published in Nature (Tripp et al. [1]) presents an unprecedented exception to the general concept of what constitutes the essential core genome of free-living chlorophyll-containing cyanobacteria.The story begins with molecular surveys of nitrogen-fixing microorganisms in ocean surface waters. Biological nitrogen fixation is carried out only by certain species of bacteria (for example, cyanobacteria and rhizobia) and archaea (for example, some methanogens), and represents a crucial component of the global nitrogen cycle, converting atmospheric nitrogen into biologically available nutrients. Nitrogen fixation is of special interest to oceanographers and biogeochemical modelers, as it helps feed vast nutrient-poor regions of the open ocean. Microscopic surveys of ocean plankton had initially indicated that only a few species - in particular large filamentous cyanobacteria such as Trichodesmium - were responsible for the bulk of open-ocean nitrogen fixation [2]. Later, however, molecular surveys for nitrogenase genes (which encode the enzymes responsible for nitrogen fixation) suggested that microorganisms other than filamentous cyanobacteria might be important in open-ocean nitrogen fixation [3]. Along with measurements of open-ocean nitrogen fixation in cell fractions of less than 10 μm [4], these data indicated that unicellular cyanobacteria [3] are also important players in marine biological nitrogen fixation.The first hurdle that Tripp et al. [1]