UCYN-A is a globally abundant nitrogen fixing cyanobacterium with a very unusual metabolism and lifestyle. These microorganisms express nitrogenase genes with maximum transcript abundances during the light period (Zehr et al., 2007).Previous results in the group of Jonathan Zehr found that UCYN-A cyanobacteria are genetically incapable of oxygenic photosynthesis because the entire PSII apparatus is missing (although the PSI apparatus appears to be intact). Moreover, PSII isn´t the only component that is lacking in this genome: RuBisCo (ribulose-1,5 bisphosphate carboxylase-oxygenase that fixes CO2) and the tricarboxylic acid cycle (TCA) that usually are present in the cyanobacteria, are missing in UCYN-A.The lack of biosynthetic pathways suggested that this organism depend on other organisms, either in close association or in symbiosis, for critical nutrients.Thanks to flow cytometry and the metagenomic studies, this group obtained a complete genome sequence from UCYN-A and observed by quantitative isotopic analysis that UCYN-A has a symbiotic association with a unicellular Prymnesiophyte (Thompson et al., 2012).In this project we will determine whole-genome transcription over natural diel cycle in order to get a better understanding of the interplay of host and cyanobacterium metabolisms, and how it relates to supply of organic matter from the host eukaryote to the cyanobacterium.The transcriptome work will help to determine the global importance of N2 fixation by this cyanobacterial group in order to better constrain the global ocean nitrogen budget.- Thompson, A., R. Foster, A. Krupke, B. Carter, N. Musat, D. Vaulot, M. Kuypers & J.P. Zehr. Science 337:1546 (2012).- Zehr, J., S. Bench, B. Carter, I. Hewson, F. Niazi, T. Shi, H. Tripp & J. Affourtit. Science 322: 1110 (2008).- Zehr, J., J. Montoya, I. Hewson, E. Mondragon, C. Short, A. Hansen, B. Jenkins, M.Church & D.M. Karl. Limnol. Oceanogr. 52: 169 (2007).