Unexpected Presence of the Nitrogen-fixing Symbiotic Cyanobacterium UCYN-A in Coastal Monterey Bay Waters

In the last decade, the biogeography of nitrogen fixation processes in the ocean has been expanded to colder and nitrogen rich coastal environments. Phylogenetic studies based on the nifH gene, which encodes the nitrogenase enzyme, have shown that marine diazotrophs are comprised of highly diverse cyanobacterial, and heterotrophic (or photohereotrophic) bacterial and archaeal taxa. Among them, the symbiotic cyanobacteria UCYN-A has been revealed as one of the most abundant and widespread nitrogen-fixers, and includes several sublineages that live associated with genetically distinct but closely related prymnesiophyte hosts. For instance, the UCYN-A1 sublineage is associated with an open ocean picoplanktonic prymnesiophyte, whereas UCYN-A2 is associated with the coastal nanoplanktonic coccolithophore Braarudosphaera bigelowii, suggesting that different sublineages may be adapted to different environments. Here, we study the diversity of nifH genes present at the Santa Cruz Municipal Wharf in the Monterey Bay (California) and report for the first time the presence of UCYN-A. Sequence and quantitative PCR data over an 8-year time-series (2011-2018) suggests that UCYN-A has recently invaded the Bay, is present during summer-fall months, and is dominated by UCYN-A2. The presence of UCYN-A in the cold and upwelling-influenced waters of the Monterey Bay underscores the need to further explore the geographic extent of this partnership and provides additional support to the hypothesis that UCYN-A1 and UCYN-A2 sublineages are different ecotypes.