Intact polar diacyglycerols (IP-DAGs) are the fatty-acid bearing lipid molecules that compose bacterial and eukaryotic cell membranes. As such, they are one of the most abundant classes of lipid molecules in plankton, and play a major role in the marine carbon cycle. However, until very recently, the molecular diversity of IP-DAGs was poorly understood; the structural identity and characteristics of IP-DAGs were inferred almost exclusively from their constituent fatty acids. These non-phosphorus containing IP-DAGs were largely unknown to chemical oceanography. In contrast, phospholipids, which have been the focus of considerable research, compose a disproportionally small fraction of total IP-DAGs. But we still lack even a cursory understanding of biochemical functions and geochemical fates of non-phosphorus IP-DAGs. Given that these molecules are among the most abundant lipid molecules on the planet, this represents a profound and unexpected gap in our understanding the marine carbon and phosphorus cycles.
In this project, researchers at the Woods Hole Oceanographic Institution will launch a pioneering study of these poorly understood compounds. Their approach will be guided by four questions: (1) How do non-phosphorus lipids contribute to variations in the C:N:P of particulate organic matter in the Sargasso Sea? (2) What are the relative degradation rates of phospholipids and non-phosphorus lipids in surface waters? (3) Which groups of microbes utilize the carbon and phosphorus from different IP-DAGs? (4) What are the relative contributions of different IP-DAGs to particulate organic matter export to the deep-sea?
These questions will be answered by using sophisticated HPLC/MS analyses and novel isotope tracing approaches in conjunction with long-standing methods for measuring the C:N:P of plankton and determining the degradation rates of organic molecules. The research team will establish whether these newly-recognized sulfolipids and betaine lipids molecules are a quantitatively important biochemical option for phytoplankton to affect flexible C:N:P stoichiometry in the face of nutrient stress. They will also elucidate the degradation rate, microbial fate, and export potential of the carbon and phosphorus from IP-DAGs. This will shed new light on the broader roles of these molecules in the cycling of these elements by the planktonic community.
Broader impacts: This project contains components that are specifically designed to meet the NSF criteria for "advancing discovery and understanding while promoting teaching, training and learning." The project will support the training of a graduate student and postdoctoral fellow. In addition, the research team will work with the non-profit Zephyr Foundation in Woods Hole to design educational 'units' based on the team's research that will be tailored to student in grades 6 - 12. The Foundation will present these units as part of their hands-on marine science field trip series that is delivered to over 200 students and their teachers per year. The Foundation is ideally suited for this purpose and attracts school groups from Massachusetts and Rhode Island, including underrepresented and disadvantaged groups such as inner-city programs from Boston and New Bedford.
Phytoplankton are microscopic creatures that live in the sunlit surface waters of the ocean. Like plants on land, phytoplankton take up carbon dioxide through photosynthesis. Since carbon dioxide is a greenhouse gas, phytoplankton play a critical role in regulating Earth's climate. Our project focused phytoplankton in the Sargasso Sea, which is region where the nutrients that phytoplankton need are very scarce. One important nutrient, phosphorous, is bound in molecules called phospholipids, which are made by phytoplankton and almost all other forms of life to make the walls of cells. We found the phytoplankton in the Sargasso Sea save phosphorus by making some, but not all, of their cells walls out of non-phosphorus lipids. We hypothesized that when phytoplankton die, their phospholipids rapidly decompose, yielding phosphorus, while non-phosphorus lipid decompose slowly. We went on an oceanographic research ship to the Sargasso Sea and measured the rates of lipid degradation. Our data showed that phospholipids do indeed degrade quickly, in many cases faster than non-phosphorus lipids. This means that phospholipid degradation could be an important source of phosphorus for phytoplankton in the Sargasso Sea. This is important because we now know more about how phytoplankton are able to take up carbon dioxide in regions of the ocean where phosphorus is scarce. These regions are probably going to expand as the earth continues to warm up, so our study will help to predict the future role that phytoplankton will play in regulating Earths climate.
