Award: OCE-1948042

The chemical element phosphorus is a natural resource that we depend on in our daily lives. For example, it is a key building block for life in the ocean, where phytoplankton require it to produce half the oxygen that we breathe. However, phosphorus is relatively scarce over wide areas of the ocean surface where phytoplankton live. Some of this scarcity is natural, while some of it is due to human activities, which may exacerbate phosphorus scarcity in the future. As a result, phytoplankton have developed innovative strategies to acquire phosphorus from seawater. These strategies may become more and more important to safeguarding global oxygen supply and planetary health, if the future oceans experience heightened levels of phosphorus stress, as predicted. In this project, we have studied the strategies that marine microbes use in order to acquire phosphorus from the pool of complex phosphorus-containing molecules in seawater called dissolved organic phosphorus (DOP). DOP is composed of several major types of phosphorus, depending on the chemical bonds that are present. In P-esters (~80% of DOP), phosphorus and carbon are bound together through an oxygen atom (P-O-C), while in P-anhydrides (~10%), oxygen helps bind multiple phosphorus atoms to each other (P-O-P). Based on experiments with live microorganisms in the laboratory and natural ocean settings, we discovered that phytoplankton may prefer the P-anhydrides, or polyphosphates, over the P-esters as a nutritional source of phosphorus for growth. This preference could contribute to the much lower prevalence of P-anhydrides in DOP. In contrast, bacteria do not seem to share this preference for P-anhydrides and instead utilize them just as much as P-esters. This observation may be driven by the high requirement that bacteria have for organic carbon. Indeed, our results from the California Current Ecosystem point to bacterial carbon demand as a key factor underlying paradoxically high rates of DOP cycling that exist there, despite sufficient levels of phosphorus that are available in seawater. Our results also demonstrate that marine microbes acquire DOP through a diversity of enzymes that depend on metals such as iron, zinc, and manganese. Overall, our project shows that microbial cycling of phosphorus in the oceans is highly dynamic, regardless of overall phosphorus availability, and intricately linked with the cycling of carbon and trace elements. If phosphorus stress increases in the future oceans due to human activity, our results suggest that the microbial demand for trace elements, which are already in short supply, may increase as well. This project provided training and educational opportunities for two postdocs, two PhD students, and multiple undergraduate students, including individuals from underrepresented backgrounds in environmental science. Through this work, we have developed preliminary components of an educational app to teach K-12 students about the marine phosphorus cycle. Results have been communicated to scientific and general audiences through publications, conference presentations, seminars, outreach events, and videos. Last Modified: 01/23/2023 Submitted by: Julia M Diaz