Project: Features and implications of nitrogen assimilation trait variability in populations of Prochlorococcus

NSF Award Abstract:
The marine bacterium, Prochlorococcus, is a central part of the food web in the subtropical open ocean, one of the largest biomes on the planet. Like plants on land, Prochlorococcus and other phytoplankton are capable of photosynthesis, harnessing light to convert carbon dioxide into sugars and other organic matter. This matter feeds all the life in the sea. Akin to terrestrial plants, Prochlorococcus requires additional nutrients or "fertilizer" to grow and photosynthesize. Among these nutrients, nitrogen is often scarce across much of the sunlit ocean. Thus, understanding the means through which nitrogen is obtained and used by Prochlorococcus has important consequences for understanding how the nitrogen and carbon cycles are coupled in the ocean. Not all Prochlorococcus cells have the genetic capacity to use all sources of inorganic nitrogen, i.e. nitrate, nitrite, and ammonium. Some can use all three, some the last two, and some only ammonium. Cells that can use nitrate must sequentially transform it to nitrite and then to ammonium before they can make the building blocks for proteins. The genesis of this project derives from the observation that some Prochlorococcus cells release nitrite into the seawater during growth on nitrate. This project examines this feature of the physiology of these cell lines, and asks whether cells that release nitrite can support the growth of other cells than cannot use nitrate, in effect creating a cross-feeding situation that could make the system more robust. Understanding the drivers behind the coexistence of cells with different ways of obtaining nitrogen, a key currency in the ocean, will provide important insights on the flow of nitrogen in marine ecosystems. This project also sheds light on the structure of interactions between microbes and provides the broader scientific community (for instance, those studying diverse microbiomes related to human health and disease or agriculture) a new perspective on how microbes form beneficial partnerships. This project supports immersive laboratory-based research experiences for undergraduate students, who design and execute experiments directly related to the overall project goals. The project further supports the work of the investigators to engage with the general public on topics related to phytoplankton, photosynthesis, and the ecosystem services provided by these marine organisms.

In the low-light adapted LLI clade of Prochlorococcus, the focus of this project, nearly all cells possess the downstream half of the nitrate assimilation pathway (for the assimilation of nitrite). Only a fraction of LLI cells, however, have the complete nitrate assimilation pathway. Incomplete assimilatory nitrate reduction, with concomitant nitrite release, has been observed for LLI cells during growth on nitrate as the sole nitrogen source. Further, the nitrite released by cells growing on nitrate can support the growth of Prochlorococcus that can use nitrite but not the more oxidized nitrate. Overall, within a group of closely-related Prochlorococcus, there is genotypic and phenotypic diversity related to the production and consumption of nitrite, a central intermediate in the nitrogen cycle. The investigators propose to further develop Prochlorococcusas a model system to explore nitrite cycling within populations and provide new insights on how trait variability and the selection of complementary functions facilitates robustness and/or resiliency in microbial populations. The overarching hypothesis is that the population-level assembly of distinct functional types of Prochlorococcus emerges through interactions that are mediated, in part, by cross-feeding of nitrite. To address this broad hypothesis, the investigators are focusing on the following objectives: 1) assessing the physiological underpinnings of incomplete assimilatory nitrate reduction and nitrite release through in-vitro biochemical characterization of nitrite reductase enzymes and transcription profiling of cells subjected to light and temperature stress, 2) examining the nitrite production and consumption rates of Prochlorococcus strains across environmental gradients such as light, temperature, and nutrient availability in order to constrain the environmental parameters that modulate nitrite cycling, and 3) determining the frequencies and activities of nitrogen assimilation genotypes within laboratory and field populations, under varying environmental conditions and perturbations. Outcomes from objectives 1 and 2 help to constrain the tradeoffs associated with incomplete nitrate reduction and the release of nitrite (a valuable commodity to nitrogen limited cells) to facilitate modelling and interpretation of how partnerships between nitrogen assimilation genotypes are structured. These insights help to direct experiments in Objective 3, which aims to examine controlled laboratory co-cultures and field populations in order to produce quantitative data on the emergent features of Prochlorococcus populations where interactions are mediated by the cross-feeding of nitrite. These data are being used to develop an improved understanding of how interactions mediated by a common public good might give rise to emergent properties of populations, including resilience to perturbation and greater population-wide efficiency in nitrogen assimilation.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.