Open ocean and coastal strains of the N2-fixing cyanobacterium UCYN-A have distinct transcriptomes

Although N2 fixation can occur in free-living cyanobacteria, the unicellular endosymbiotic cyanobacterium Candidatus Atelocyanobacterium thalassa (UCYN-A) is considered to be a dominant N2-fixing species in marine ecosystems. Four UCYN-A sublineages are known from partial nitrogenase (nifH) gene sequences. However, few studies have investigated their habitat preferences and regulation by their respective hosts in open-ocean versus coastal environments. Here, we compared UCYN-A transcriptomes from oligotrophic open-ocean versus nutrient-rich coastal waters. UCYN-A1 metabolism was more impacted by habitat changes than UCYN-A2. However, across habitats and sublineages genes for nitrogen fixation and energy production were highly transcribed. Curiously these genes, critical to the symbiosis for the exchange of fixed nitrogen for fixed carbon, maintained the same schedule of diel expression across habitats and UCYN-A sublineages, including UCYN-A3 in the open-ocean transcriptomes. Our results undersore the importance of nitrogen fixation in UCYN-A symbioses across habitats, with consequences for community interaction and global biogeochemical cycles. From 6-15 April 2015 at Station ALOHA in the North Pacific Subtropical Gyre, whole-community water samples were collected from 45m for metatranscriptome analysis of the UCYN-A sublineages A1, A2 and A3. Samples covered a full light-dark cycle with 8 time points and 3 h intervals. In total there were 16 samples (two replicates per time point). Time points were as follows: 9:00-L3, 12:00-L6, 15:00-L9, 18:00-L12, 21:00-D3, 00:00-D6, 03:00-D9, 06:00-2D12, 09:00-2L3 and 12:00-2L6, where L and D stand for light and dark period, respectively, 2L and 2D the second light-dark cycle, and the number the corresponding hours into the light or dark period.