Critical Role of Light in the Growth and Activity of the Marine N2-Fixing UCYN-A Symbiosis

The unicellular N-2-fixing cyanobacteria UCYN-A live in symbiosis with haptophytes in the Braarudosphaera bigelowii lineage. Maintaining N-2-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, N-2 fixation of the symbiont and photosynthesis of the host. In this system, it is thought that the host photosynthesis supports the high energetic cost of N-2 fixation, 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 N-2 fixation (single cell N-2 fixation rates), nifH gene transcription, and UCYN-A2 cell division (fluorescent in situ hybridization and nifH gene abundances). The results showed that the light period is critical for maintenance of regular patterns of gene expression, N-2 fixation 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.