Identification of ice nucleation sources from marine phytoplankton
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Phytoplankton are a significant source of organic matter to atmospheric aerosols and may provide effective ice nucleating particles (INP). INP catalyze the freezing of water at temperatures warmer than homogeneous freezing (-38 °C), thereby widening the range of conditions under which atmospheric ice (and therefore ice-containing clouds) form. INP from phytoplankton are poorly understood in terms of composition, the conditions required to form them, and the temperatures at which they catalyze freezing. The objectives of this research are: 1) Identify sources of INP produced by phytoplankton and characterize their ice nucleation properties. 2) Identify how phytoplankton physiology (particularly growth rate) affects the production of INP. These objectives will be realized through a series of laboratory experiments using representative globally important phytoplankton grown under highly controlled conditions, primarily using continuous cultures. We will focus on the hypothesis that nucleic acids and proteins are sources of effective INP.
According to the last two Intergovernmental Panel for Climate Change (IPCC) assessments, aerosols and
the clouds that grow on them remain one of the major sources of uncertainty in calculating the Earth’s radiative budget and predicting future climate change. Much of this uncertainty arises from our poor understanding of atmospheric cloud formation, and the processes affecting the lifetime and properties of clouds. Covering 71 % of the Earth’s surface, the ocean is a significant, but poorly constrained, source of aerosol in the form of sea spray. Both field campaigns and controlled laboratory experiments have shown that sea spray containing organic matter from marine phytoplankton acts as INP. The proposed research will build on our recent work, which showed that fast growing phytoplankton produce INP that catalyze freezing at warmer temperatures than slower growing or dying phytoplankton. While it is not known which cellular structures or chemicals derived from phytoplankton form the effective INP; nucleic acids and proteins are elevated during rapid growth and thus are logical initial candidate INP. Experiments will be conducted using continuous culture systems in the laboratory, which enable growth conditions to be controlled, repeatable and defined. This novel approach will provide an unprecedented degree of experimental control over phytoplankton growth and physiology, which is not possible from samples collected at sea or in batch culture systems. By growing continuous cultures of specific taxa of phytoplankton at defined growth rates and physiological status, the relationship between phytoplankton growth and INP production will be characterized at the process level. A new array system will be used to screen the immersion ice nucleation temperatures of potential INP. Once INP are identified, detailed further work (including chemical characterization during ice nucleation) will be conducted using an optical microscope coupled to a cooling stage and Raman spectrometer. These data will be used to develop a mechanistic understanding of the factors that affect INP production by phytoplankton.
Linking phytoplankton growth and physiology to INP properties will produce a mechanistic understanding of the coupling between ocean biology and cloud processes. Results will be disseminated through publication in peer-reviewed journals and presentation at major conferences and archived in the Texas Data Repository. The research will be used as a framework to instruct a high-impact, hands-on capstone learning experience through GEOS 405 Environmental Geosciences for seniors in the College of Geoscience environmental BSc programs at Texas A&M University. The class will take the form of a student-driven research project, which will be approached like an interdisciplinary field campaign. Students will collaborate in the design, sampling, data analysis, and write up of a marine aerosol reference tank (MART) experiment. In addition, a graduate student will be trained in interdisciplinary geoscience research in the Department of Oceanography, while a postdoctoral researcher will be mentored in the Department of Atmospheric Sciences.