Award: OCE-1434914

Phosphorus is an element that is essential for all life, and is an important nutrient for the growth of the organisms that are at the base of the food chain in the ocean: marine microbes, both phytoplankton and bacteria. In many areas of the ocean, phosphorus is found at extremely low concentrations, such that the abundance and activity of microbes may be controlled by the availability of this scarce resource. Large areas of the ocean with particularly low concentrations of phosphorus include the Sargasso Sea, in the western North Atlantic ocean, and the Gulf of Mexico (away from the coasts). In these low phosphorus marine environments, the phytoplankton community is dominated by very small cells, from the group cyanobacteria, and non-photosynthetic bacteria—even smaller than the phytoplankton—which are the most abundant microbes of all. In these area, the concentrations of dissolved inorganic phosphorus are so low that any available nutrients are taken up extremely rapidly, with the turnover rate of dissolved phosphorus (or the renewal rate of the available nutrients) as rapid as just an hour or two. In these phosphorus-depleted environments, we hypothesized that the phytoplankton and bacteria would employ different strategies for acquiring and using this scarce resource, and that these strategies may shed light on the factors controlling their abundance and activity in low-nutrient environments. To investigate this hypothesis, we conducted field experiments in the Sargasso Sea and in the Gulf of Mexico to quantify: 1) the rate of phosphorus uptake by different microbial groups, from different sources of dissolved phosphorus, 2) how quickly the microbes recycle this phosphorus within their cells, and 3) the strategies of different microbial groups for how they use this scarce resource within their cells. We found that the small, non-photosynthetic bacteria are responsible for the majority of phosphorus uptake, both dissolved inorganic and organic forms of phosphorus, though for an individual cell the bacteria rely less heavily on organic phosphorus than the phytoplankton. Additionally, these bacteria recycle the phosphorus within their cells more rapidly than the phytoplankton, and may be responsible for helping fuel the very rapid turnover of the dissolved phosphorus pool by consuming phosphorus, transforming it in their cells, and rapidly returning it to the dissolved pool. We quantified the distribution of phosphorus within these microbial cells to see how the microbes allocate their phosphorus between critical biochemicals including lipids, which make up the membrane of cells, DNA (deoxyribonucleic acid), which encodes genetic information in cells, and polyphosphate, which is a phosphorus storage compound and may be used as an energy reserve. We found that the phosphorus-rich, non-photosynthetic bacteria allocate more of their cellular phosphorus into lipids compared to the phytoplankton, and that one type of phytoplankton allocates more of their cellular phosphorus to polyphosphate compared to the other microbial groups. In summary, we demonstrated that bacteria are playing a distinctly different, and dominant, role in phosphorus cycling compared to phytoplankton in these low nutrient environments. During the course of this project, the two early-career scientists leading the project worked closely with two undergraduate students, providing training and mentorship in scientific research. One of the students participated in the 9-day field experiments at sea aboard the research vessel the R/V Atlantic Explorer, gaining extensive hands-on experience with ship-based research. Both students presented the results of their research at a national conference, and one of them is leading the preparation of a paper for publication in a peer-reviewed scientific journal. To reach a younger, broader audience, we also created a workshop for middle school students. Working with the non-profit writing workshop 826NYC, over the course of four evenings, a group of middle school students explored the world of marine microbes and created five unique board games based on the marine microbial plots. Last Modified: 09/29/2017 Submitted by: Solange Duhamel