Transcriptomic Analysis and Microscopic Observations in the Cyanobacterium UCYN-A during Diel Cycles

Candidatus Atelocyanobacterium thalassa (UCYN-A) is a nitrogen-fixing marine cyanobacterium recently recognized for its widespread distribution and significant contributions to oceanic nitrogen (N₂)-fixation. UCYN-A is a group of related cyanobacteria that are symbiotic with a single-celled eukaryotic phytoplankter, the haptophyte Braarudosphaera bigelowii. UCYN-A fixes N₂ and expresses nitrogenase during the day. Since the nitrogenase is inactivated by oxygen evolved through photosynthesis, most cyanobacteria use temporal or spatial separation of photosynthesis and N₂ fixation. Genomic studies revealed that UCYN-A lacks the entire PSII apparatus (photosystem II). The lack of oxygenic photosynthesis at least partially explains why they can fix nitrogen during the day, although the host is a photoautotroph. However, UCYN-A has retained photosystem I (PSI), and PSI activity may be important in the energetics of N₂ fixation in the symbiosis.

Because UCYN-A lacks photosystem II, which normally supplies electrons to photosystem I from water, UCYN-A needs alternative electron donors if it uses photosystem I to make the reductant NADPH. In order to determine if UCYN-A expresses photosynthetic genes and which other proteins may be involved with energy metabolism, we developed a whole genome array to examine gene transcription over the diel cycle in two strains.

Our results show that there is a temporal separation of the expression of photosynthesis genes from the expression of nitrogenase genes. Moreover, the transcription profile of NADH dehydrogenases and hydrogenases suggest they may be involved as alternative electron donors for the N₂ fixation.

In addition, we used a double-CARD-FISH (Catalyzed Reporter Deposition-Fluorescence in situ Hybridization) assay to study cell division of the host and symbiont during diel cycles in relation to UCYN-A gene expression carried out during the transcriptomic analysis.

These results help us move toward a deeper understanding of the metabolism of this unusual cyanobacterium and the differences in environmental adaptations between these two strains.