Project: Diel growth and activity of Prochloroccocus in an Oxygen Deficient Zone

NSF abstract:

Marine oxygen deficient zones (ODZs) occupy less than 1% of the volume of the ocean, but are responsible for up to half of the nitrogen moved from the ocean to the atmosphere each year. Nitrogen is a key limiting nutrient for phytoplankton, thus this loss of nitrogen from the ocean has impacts on the total amount of marine photosynthesis, which removes carbon dioxide from the atmosphere and provides the base for marine food webs. As ocean temperatures increase, the size of oxygen deficient zones is predicted to increase. This project focuses on a microorganism that sits at the intersection of the nitrogen, carbon and oxygen cycles within marine ODZs, the cyanobacterium Prochlorococcus. Using water samples acquired on a previous cruise to the Eastern Tropical North Pacific ODZ, the project assesses the growth and activity of Prochlorococcus over the day/night cycle to determine their impacts on other members of the ODZ microbial community. In the lab, cultures of Prochlorococcus are being grown on different forms of nitrogen and their growth rates and oxygen production are being measured. Understanding the functioning of biological communities that reside within these waters is important for predicting global elemental cycles. A central goal of this project is to broaden the pool of individuals who pursue graduate work in environmental sciences by providing undergraduate students from underrepresented minority groups early research experience, sustained financial support, peer mentorship and exposure to the discipline at a national level. Two female graduate students and five undergraduate students that are members of groups underrepresented in earth sciences are participating in the project and are being trained and supported in their STEM careers.

This project investigates the roles of unique strains of Prochlorococcus found in low oxygen and low light regions of the Eastern Tropical North Pacific (ETNP) in the oceanic carbon and nitrogen cycles. In all three major marine ODZs, there are broad regions where the top of the ODZ shoals to within the photic zone, resulting in a secondary chlorophyll maximum entirely within the anoxic zone. This secondary chlorophyll maximum is dominated by cyanobacteria, largely Prochlorococcus, the numerically dominant phototroph in the oligotrophic oceans. Here Prochlorococcus is photosynthesizing under very low light levels, fixing carbon and producing oxygen, with implications for the surrounding microbial community. Although the organic carbon produced by Prochlorococcus within the ODZ may be an additional source of organic matter for heterotrophic denitrifiers, fueling nitrogen gas (N2) production, the concomitant oxygen production may create a favorable habitat for nitrifying archaea and bacteria which together return ammonium (NH4+) to nitrate. Prochlorococcus must also acquire nitrogen in some form, potentially competing with other nitrogen cycling taxa to do so. Importantly, the growth, oxygen production and nutrient uptake in Prochlorococcus are tightly linked to the diel cycle. Thus, diel production of oxygen by Prochlorococcus may shift the balance between nitrification and denitrification over the course of a diurnal cycle, ultimately influencing the degree of net nitrogen loss. This project is 1) establishing the in situ growth rate of Prochlorococcus populations within the ETNP ODZ using flow cytometry; 2) sequencing metatransciptomes from ETNP ODZ secondary chlorophyll maximum over a diel cycle to explore the role of diurnally fluctuating oxygen on the N cycling community; and 3) characterizing novel Prochlorococcus isolates from the ODZ by sequencing their genomes, comparing their growth rates on different N sources, determining their uptake kinetics for ammonia and nitrite, measuring their oxygen evolution and assessing their diel transcriptional response under low oxygen.

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.