Award: OCE-2048470

INTELLECTUAL MERIT

The oceans are an important global resource that provide numerous ecosystem services supporting life on our planet. Earth’s major biogeochemical cycles are mediated in part by the actions of more than an octillion microorganisms that populate the oceans. That number of microbes is several orders of magnitude greater than the numbers of stars in the observable universe. Many of these microorganisms live in the surface waters of the expansive open ocean and some are capable of photosynthesis. These photosynthetic organisms, also known as phytoplankton, are intricately linked to the cycling of nitrogen, carbon, and other elements in the oceans. Using sunlight for energy, they can convert carbon dioxide into the building blocks for new cells. Like plants on land, these organisms form the base of the food web and ultimately support fisheries as well as the biological carbon pump that helps to control the amount of carbon dioxide in the atmosphere. Ocean phytoplankton do approximately as much photosynthesis annually as all the plants on land.

Among the phytoplankton in the global ocean, Prochlorococcus is by far the most numerous. It has efficiently colonized the surface of the nutrient poor tropical and subtropical open ocean. Along with its close relative, Synechococcus, Prochlorococcus is responsible for a sizeable fraction of annual primary production in the oceans (i.e., the global amount of carbon dioxide converted into new living biomass). Although Prochlorococcus needs sunlight as an energy source, it also needs other key elements like nitrogen, phosphorus, and iron. In many of the regions where Prochlorococcus dominates, nitrogen is in particularly short supply. In past work, we have assessed the distribution of Prochlorococcus that can use nitrogen sources that we previously thought them incapable of using (e.g., nitrate). Through the exploration of Prochlorococcus genomes, we also identified the potential for some Prochlorococcus to participate in the cycling of important nitrogen sources in the ocean.

In our project, we have examined the role that Prochlorococcus plays in the production and consumption of nitrite. This type of nitrogen is made and used by several biologically mediated reactions that support the energy requirements and nutritional needs of diverse microbes. Because nitrite sits at the center of these different reactions, understanding the inputs and outputs to nitrite can help us further understand how microbial communities efficiently use nitrogen, as well as how microbes interact in ways that could propagate up the food chain. The project resulted in a seminal peer-reviewed study on the phenomenon of incomplete assimilatory nitrate reduction by an abundant group of Prochlorococcus that is typically found in the vicinity of elevated nitrite concentrations in the water column. These cells appear to release nitrite due to a bottleneck in the pathway by which they convert nitrate to amino acids that results in a buildup of nitrite in the cell. This phenomenon maps onto a subset of this group of Prochlorococcus that have a specific set of nitrate and nitrite assimilation genes. While some phytoplankton can use nitrate reduction as a safety valve for excess photochemically generated electrons during periods where they get too much light, we found that this does not occur in Prochlorococcus. Rather, the degree to which Prochlorococcus cells release nitrite appears to be related to their physiological acclimation to nitrogen-limiting conditions. In this physiological state, the cells are primed to release substantial quantities of nitrite when exposed to nitrate. These exposures would typically happen during short-term and intermittent upwelling events that bring nitrate rich waters from the deep to the sunlit surface where Prochlorococcus are abundant. Thus, sharp increases in nitrate availability in upper water column of nitrogen-poor open ocean ecosystems are likely to lead to cascading effects, partly driven by Prochlorococcus, in the availability of nitrogen cycle intermediates such as nitrite.

 

BROADER IMPACTS

The project has focused on elucidating Prochlorococcus’ role in nitrogen transformations and has the potential to benefit society by advancing our knowledge of interactions between an important primary producer and an important intermediate in the nitrogen cycle. Prochlorococcus, along with other phytoplankton, sit at the base of the food web and thus ultimately support healthy oceans and sustainable fisheries. Nitrogen availability often limits the growth of these organisms. The project outcomes include the discovery of enhanced nitrite production potential by nitrogen-limited Prochlorococcus. These data have the potential to be used by modelers to understand how perturbations in nitrate availability might impact primary production, fluxes of inorganic nitrogen in marine ecosystems, and the broader food web in the open ocean. The PI has participated in outreach activities with the Boston Public Library to introduce science concepts to elementary school age children. The project has supported a technician as well as an undergraduate researcher who has used his experience to transition to graduate school in a related field.

 

Last Modified: 06/25/2025
Modified by: Paul Mark Berube