Benjamin Meaker Visiting Professor Ann Pearson, Harvard University, USA

Marine Ecology: Modelling a Theory of the Redfield Ratio

15 Oct - 15 Dec 2018

Biography

Professor Ann Pearson is Professor of Environmental Sciences in the Department of Earth and Planetary Sciences at Harvard University. Her research focuses on applications of analytical chemistry, isotope geochemistry, and molecular biology to biochemical oceanography and Earth history. Recent work has focused on the carbon and nitrogen cycles, with emphasis on how biological communities respond to changing environmental conditions. Her work advances the analytical chemistry techniques used to interpret chemical signatures – specifically lipid biomarkers and their isotopic compositions – left by ancient sea-dwelling microorganisms, and also looks at theoretical frameworks for interpreting these signatures.

Professor Pearson received her Ph.D. in Chemical Oceanography from the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution Joint Program in Oceanography in 2000 and joined the Harvard University faculty in 2001. She received a Fellowship for Science and Engineering from the David and Lucille Packard Foundation in 2004, a Radcliffe Institute Fellowship in 2009, the Paul Gast Lectureship of the Geochemical Society in 2015, and is an Investigator of The Gordon and Betty Moore Foundation’s Marine Microbiology Initiative.

During her stay Professor Pearson will be hosted by Dr. Fanny Monteiro (Geographical Sciences) and Professor Richard Pancost (Chemistry).

Project Summary

The Redfield Ratio (RR) is the observation that the elemental ratio of nitrogen to phosphorous in marine
organisms is similar to the ratio of nitrogen and phosphorous nutrients dissolved in seawater. Understanding what controls the N:P ratio in the ocean is a key question in the oceanographic community that has been investigated for more than 50 years (Redfield, 1963 on “The influence of organisms on seawater composition”), yet the fundamental regulation of N:P is still not well understood. This represents a critical gap in knowledge, as the RR is fundamentally important to the regulation of nutrients in the ocean. These nutrients control the base of the food chain as well as how much atmospheric CO2 is taken up by the ocean under a changing climate.

The goal of this project is to develop a quantitative feedback model to establish the biological basis of the oceanic RR. The model will describe the coupling between bacterial and algal processes
that control the size of the N inventory relative to the availability of P, thereby exerting a self-regulating control on the RR. The approach will be implemented in the Earth system model known as GENIE, using a framework known as ecosystem resource competition theory. The results will be used to predict marine nutrient states, to re-examine oceanographic records of algal species succession, and to predict changes in the ocean nutrient state under conditions of future climate change. The implications of the project include the possibility to more accurately forecast future changes at the base of the marine ecosystem, impacting marine food webs and fisheries.

Events

• Molecular paleontology: The ancient fossils we cannot see
  Public lecture, Wed 5 December 6pm - 7pm, Peel Lecture Theatre, School of Geographical Sciences.
  
  Multicellular, complex animals that leave skeletal hard fossils are a feature of the Phanerozoic.  Beginning with the Cambrian explosion 540 million years ago, the Phanerozoic represents biology’s mature age, yet life on Earth certainly began more than 2 billion years earlier and was largely microbial.  Lipid biomarkers preserved in ancient sedimentary rocks – otherwise known as molecular fossils – provide one of the few available windows to this record of early microbial life.  Lipid biomarkers are found at least to the mid-Proterozoic, with the oldest assemblages dating to around 1.6 and 1.1 billion years ago.  The compounds present in these samples shed light on the nature of early ecosystems.  When studied in tandem with modern analogue environments or in comparison to important environmental events of the Phanerozoic, the types of organisms and physiological and geochemical functions of past ecosystems often can be determined.  Earth and its biosphere have evolved together; here I will present highlights from several recent case studies that illustrate the power of investigating molecular fossils.
  
  
• Lipids of marine Archaea - a new paleobarometer for pCO2?
  Department BRIDGE group lecture, Wed 7 November 12pm - 1pm, Seminar Room 1, School of Geographical Sciences. 
   
  Marine planktonic Thaumarchaeota oxidize ammonia and fix inorganic carbon to biomass.  The pathway for carbon fixation in these taxa is known as the 3-hydroxypropionate/4-hydroxybutyrate (3HP/4HB) pathway, and unique among all known autotrophic carbon metabolisms, this pathway exclusively uses HCO₃^-, rather than CO₂, as the substrate for carbon fixation.  To date it has been assumed that growth in the ocean at pH ≥ 7.5 (high [HCO₃^-]) necessarily implies that Thaumarchaeota are never carbon-limited. However, recently we measured carbon isotope ratios (d¹³C) of archaeal lipids (GDGTs) in a marine water column transect, showing that the expressed isotope effect (e) scales directly with dissolved CO₂ concentrations, implying that d¹³C_GDGT values hold promise as a pCO₂paleobarometer.  The same relationship appears to hold also with a limited set of modern sediment core-tops. Sensitivity of this new proxy is similar to other geochemical methods such as boron isotopes and the alkenone paleobarometer, although additional calibration and testing is needed.   
  
  This seminar is co-advertised by Cabot (Environmental Change Theme).
  
  
• Geochemically distinct influence of carbon source on the distribution of carbon isotopes in bacteria.
  Graduate student seminar, Tues 12 Dec, 3-4pm, Folk House Cafe, Park Street
  To be given at one of the bi-weekly Marine Ecology and Climate (MEC) meetings. MEC is a reading group ran by Dr. Monteiro which gathers PhD students, postdocs and faculty members of the schools of Chemistry, Earth Sciences, and Biological Science, working on different aspects of Marine Ecology and Climate.