Primary production in a large fraction of the surface ocean communities is limited by the availability of nitrogen (N). Heterocyst-forming cyanobacteria associated with diatoms of the genera Hemiaulus, Rhizosolenia, and Chaetoceros can dominate surface communities throughout the global oceans, and are an important source of N to these communities. Additionally, these unique associations are directly linked to a highly efficient carbon export system. In order to study the nature and evolution of these symbioses, the genomes of representatives of symbionts from the three common diatom associations were sequenced. The genomes revealed the evolutionary pressure a symbiont undergoes as a result of long-term associations with unicellular diatoms. The implications of the genome streamlining, specifically in the N metabolism pathways, extend to mechanisms for maintenance of the association as well as possible symbiont regulation mechanisms by the diatom. The genomes could additionally be implemented in an unbiased examination of diatom symbiont transcription through an extensive environmental RNA sequence data set. This revealed significant reduction in photosystem II gene expression and possible regulation on the cyclic electron transport of the most abundant diatom symbiont population. The environmental sequences also showed very low diversity amongst the diatom symbiont populations. Lastly, the examination of interruption elements in heterocyst-forming cyanobacteria expanded the framework for which to study these genetic features. Although the majority of the element sequences proved to be highly dynamic, the recombinase sequences on each interruption element gave insights into the evolutionary paths of these unique genetic features. Additionally, the presence of elements in non-N2 fixation genes and in cyanobacteria not capable of cell differentiation implicate the interruption elements in processes beyond heterocyst formation. The genomic sequences of oceanic diatom symbionts have opened a myriad of approaches for which to study the globally significant diatom-diazotroph associations. These analyses have revealed much about the ecology and evolution of diatom symbionts, and carry implications into global oceanic nutrient cycling, as well as plant-microbe associations, in general.
