They might be tiny, but microorganisms play a vital role in our ecosystems. They are found throughout the ocean, with over 100,000 of these tiny cells in each millilitre of surface seawater. Some marine microbes can transform nitrogen from the atmosphere into ammonia, which serves as fertilizer for the ocean. These nitrogen-fixing microbes account usually for around 1% of the total microbes in seawater but they have a big impact on the overall nitrogen cycle in the ocean. In a new study first-authored by Dr. Brent Robicheau, a recent PhD graduate, the LaRoche lab at Dalhousie University is researching one of these nitrogen-fixing microbes, nicknamed UCYN-A, to better understand their distribution in the Northwest Atlantic Ocean.
Water sampling in the Bedford Basin
The LaRoche lab used seawater samples collected from the Bedford Basin to retrieve microbial cells. By sequencing the DNA of these cells, researchers were able to create a time series of microbes in the region. Pairing microbial data with oceanographic measurements collected by the Bedford Institute of Oceanography further allowed Brent and his co-authors to better understand the seasonal ecology of the nitrogen fixers within the microbial community. This included data on temperature and nutrients that tend to change with the seasons.
Brent’s research showed that the occurrence of UCYN-A in the Bedford Basin peaks from summer into fall, largely agreeing with prior research on this topic. However, an interesting finding was that this microbe also shows a strong weekly temporal difference between two of its ecotypes (or species ‘variants’) over multiple years. One of these ecotypes, which is thought to be more associated with the open ocean, was recovered in the basin during the hottest weeks of the summer.
Overall, these results demonstrate the importance of carrying out repeated weekly sampling in coastal areas to find new microbial occurrence patterns.
The importance of UCYN-A
The DNA sequence for UCYN-A was first detected over 20 years ago. “The specific sequence for UCYN-A was found all over the world's ocean but nobody knew what organism it belonged to”, says Dr. Julie LaRoche, head of the LaRoche lab.
Years later cells belonging to this microbe were captured and sequenced, identifying it as a cyanobacteria living in symbiosis with phytoplankton cells . Between these organisms, carbon and nitrogen are mutually exchanged, both essential elements for life on Earth. This microbial relationship supports both carbon and nitrogen fixation, critical biogeochemical processes in the ocean that help support the base of the ocean food web.
Interestingly, nitrogen fixation has generally been understudied in coastal areas, and so future research from the Bedford Basin could potentially lead to new information about nitrogen fixers and their overall diversity and ecology within the coastal Northwest Atlantic.
“There is a lot of interest in figuring out the full fate of the UCYN-A-phytoplankton symbiosis in the oceans. For example, others have shown that it is grazed by copepods, corals, and possibly dinoflagellates. And others have also found it on sinking particles leaving the sunlit layer of the ocean, hence it can also contribute to carbon export,” says Brent.
To date, no reliable laboratory cultures presently exist for this globally important microbe. A better understanding of the microbes’ occurrences in the Bedford Basin can provide insights into the microbe’s global distribution and function in a real-world environment.
Future insights into marine microbial ecology
The recent publication of this research in Science Advances is providing new insights into the range and nature of diversity that exists within the globally important UCYN-A. At the moment, eight sub-types of the microbe have been proposed.
“The Bedford Basin time series provides an excellent historical dataset with ample potential for helping to develop better models of when certain nitrogen fixing microbes are likely to be observed under certain environmental conditions”, says Brent.
Continued time-series observations are needed in the Bedford Basin to detect any future shifts within the nitrogen-fixing community in the Northwest Atlantic region, this would include for example changes arising due to climate change.
Looking towards future research, this better understanding of UCYN-A in a natural environment can help to improve its survival in laboratory settings, which would hopefully open the doors for further exploration into its life history.
