AUTHOR=Landa Marine , Turk-Kubo Kendra A. , Cornejo-Castillo Francisco M. , Henke Britt A. , Zehr Jonathan P. 

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

JOURNAL=Frontiers in Microbiology

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.666739

DOI=10.3389/fmicb.2021.666739

ISSN=1664-302X

ABSTRACT=<p>The unicellular N<sub>2</sub>-fixing cyanobacteria UCYN-A live in symbiosis with haptophytes in the <italic>Braarudosphaera bigelowii</italic> lineage. Maintaining N<sub>2</sub>-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<sub>2</sub> 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<sub>2</sub> 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<sub>2</sub> fixation (single cell N<sub>2</sub> fixation rates), <italic>nifH</italic> gene transcription, and UCYN-A2 cell division (fluorescent <italic>in situ</italic> hybridization and <italic>nifH</italic> gene abundances). The results showed that the light period is critical for maintenance of regular patterns of gene expression, N<sub>2</sub> 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.</p>