The unicellular N₂-fixing cyanobacteria UCYN-A live in symbiosis with haptophytes in theBraarudosphaera bigelowiilineage. Maintaining N₂-fixing symbioses between two unicellular partners requires tight coordination of multiple biological processes including cell growth and division and, in the case of the UCYN-A symbiosis, N₂fixation of the symbiont and photosynthesis of the host. In this system, it is thought that the host photosynthesis supports the high energetic cost of N₂fixation, and both processes occur during the light period. However, information on this coordination is very limited and difficult to obtain because the UCYN-A symbiosis has yet to be available in culture. Natural populations containing the UCYN-A2 symbiosis were manipulated to explore the effects of alterations of regular light and dark periods and inhibition of host photosynthesis on N₂fixation (single cell N₂fixation rates),nifHgene transcription, and UCYN-A2 cell division (fluorescentin situhybridization andnifHgene abundances). The results showed that the light period is critical for maintenance of regular patterns of gene expression, N₂fixation and symbiont replication and cell division. This study suggests a crucial role for the host as a producer of fixed carbon, rather than light itself, in the regulation and implementation of these cellular processes in UCYN-A.