Biological nitrogen (N2) fixation is an important source of fixed nitrogen for ecosystems. However, it can only be performed by certain Bacteria and Archaea, since the oxygen evolved by photosystem II inhibits the nitrogenase enzyme that catalyzes N2 fixation. Most N2-fixing cyanobacteria avoid oxygen inhibition by temporally separating the activities such us Crocosphaera and Cyanothece (which fix N2 during the dark period) or by spatially localizing nitrogen-fixing activity in specialized cells (heterocysts). The marine nonheterocyst-forming Trichodesmium fixes N2 during the day without specialized cells, probably due to a unique combination of spatial and temporal separation of activities. Candidatus Atelocyanobacterium thalassa (UCYN-A) is a recently discovered unicellular cyanobacterium that may be one of the most abundant N2-fixing cyanobacteria in the ocean. UCYN-A is unusual since it lacks photosystem II, does not have the genes for carbon fixation, and is a symbiont of a unicellular eukaryotic haptophyte alga. It appears to be also capable of N2 fixation during the light. We used a whole genome array to determine if UCYN-A, with its greatly streamlined genome and lack of 2 kai genes, still maintains the diel cycle typical of closely related unicellular cyanobacteria. We also wanted to determine if the two different sublineages, UCYN-A1 and UCYN-A2, maintain the same cycles. Moreover, we also compared UCYN-A transcription data to previously published data for Prochlorococcus sp. MED4 (a non- N2-fixer), Cyanothece ATCC 51142 and Crocosphaera watsonii WH 8501(unicellular N2-fixers). Moreover, we also analyzed the transcription patterns of cultures of the N2-fixing cyanobacteria Trichodesmium erythraeum IMS101 during diel cycles. The gene transcription patterns of UCYN-A1 and UCYN-A2 were highly similar. Furthermore, our results showed the PSI activity may be important in the energetics of N2 fixation in the symbiosis. Finally, the transcription patterns showed possible differences with other N2 fixing cyanobacteria that may be linked to symbiosis with the haptophyte host.
Maria del Carmen Muñoz Marin is a Postdoctoral Scholar at the University of Córdoba, Spain. She did her PhD at the same University under the supervision of Prof. José Manuel García Fernández and Prof. Jesús Díez Dapena studying the carbon and nitrogen metabolism of the non-N2 fixer cyanobacterium Prochlorococcus. One month after she defended her PhD she got the Marie Curie Outgoing (IOF) Fellowship starting a postdoc at the University of Santa Cruz (California) for two years and a half under the supervision of Prof. Jonathan Zehr. During the postdoc, she carried out transcriptomic analysis of the uncultured N2 fixer cyanobacterium UCYN-A. Another focus of her research has been the identification of new sublineages of this symbiotic cyanobacterium.