Award: OCE-1635369

All organisms on earth need nitrogen to survive. Human beings get their nitrogen from the food they eat. Microscopic organisms in the ocean, like phytoplankton and bacteria, take in nitrogen compounds dissolved in the water around them through their cell membranes. In many parts of the ocean, the amount of nitrogen available to microbes will determine how fast they can grow and the productivity of the food web in the area. For example, the more nitrogen available in a region, the more phytoplankton cells, and the more fish that rely on those phytoplankton cells that can be harvested. Historically, research on nitrogen forms in the ocean and their impact on the ocean food web has focused on inorganic nitrogen ? ammonium, nitrate, and nitrite. The largest pool of nitrogen, however, other than dissolved nitrogen gas, is the dissolved organic nitrogen (DON) pool. We know that this pool, or group of compounds, contains forms of organic nitrogen that microbes can use, like urea or amino acids; most of the compounds in this pool, however, are unknown. In this project, based on recent findings in molecular biology, we proposed that creatine is an important component of the DON pool and that microbes in the ocean can use it to grow. There has been a revolution in molecular biology that reached a high point with the sequencing of the human genome, which is the complete set of DNA in humans. These same tools have been used to sequence other organisms including a marine diatom, a type of phytoplankton. Looking at the genome suggests that this organism can produce creatine and other studies suggested that some microbes could take it up. The overarching goals of this project were to use a new technique to measure concentrations and rates of uptake of creatine in the ocean and to then to determine what environmental conditions influence and control creatine production and use. The specific part of the project we report on here is the rate of uptake of creatine in the ocean. We performed a series of incubations using creatine labeled with 15N so that we could monitor the flow of creatine into cells over time. We were funded to do two cruises off the east coast of the US, which seemed doomed. Thanks to delays due to two hurricanes, engine failure, and a large storm, we were only able to do a few uptake experiments in the Atlantic off of Delaware as planned. Instead, we took advantage of opportunities to do creatine experiments on two other research cruises ? one in the Arctic off of Alaska and one in the Pacific off of California. In virtually all incubations conducted, concentrations of creatine were near the limit of analytical detection in the water and uptake of 15N-labeled creatine into cells was measurable, occasionally at high rates. These findings raise the possibility that creatine behaves similar to ammonium, an inorganic form of nitrogen. Ammonium concentrations in much of the surface ocean are at or below the limit of analytical detection. Despite this, ammonium uptake is believed to support much of the growth of phytoplankton in many parts of the open ocean. This apparent paradox is due to rates of ammonium regeneration and rates of ammonium uptake being tightly coupled such that as soon as ammonium is released, it is rapidly taken up and so does not accumulate. An important future research question is to determine whether creatine behaves the same way. Last Modified: 04/20/2021 Submitted by: Deborah A Bronk