The prevalence in bacterioplankton genomes of specialized genes for the metabolism of volatile organic compounds is a "smoking gun" that points to a hidden VOC cycle potentially of significant magnitude. With funding provided through this EArly-concept Grant for Exploratory Research (EAGER), researchers at Oregon State University and the University of Colorado at Boulder will gather new evidence about the VOC cycle in the ocean photic zone by: 1) measuring the turnover rates of VOC compounds by plankton communities on a Surface Ocean Lower Atmosphere (SOLAS) cruise, and 2) identifying the organisms and biochemical machinery that mediate VOC oxidation. This research is potentially transformative because quantitative evidence indicating significant VOC cycling would cause an overhaul of carbon cycle models and focus attention on specialized metabolic processes that have received little attention and are poorly characterized. This research is high risk for similar reasons: while mounting evidence points to a significant "hidden carbon cycle", its importance will not be known until its magnitude is measured, and the achievement of this goal requires an investment in specialized knowledge and technology.
The project is a collaboration between atmospheric chemists and marine microbiologists who bring together the knowledge and technology needed to solve this problem on a SOLAS cruise and in a laboratory setting. Measurements of the seawater concentrations of VOC compounds (e.g. methanol, formaldehyde, dimethylsulfide, trimethyamine, trimethylamine oxide, acetonitrile, acetone, isoprene, glyoxal, methylglyoxal and acetaldehyde) and turnover rates determined by the incubation of isotopically-labeled compounds with microbial plankton suspensions will provide information about variation in these geochemical processes across a transect that extends from a productive continental shelf to an oligotrophic subtropical gyre. Later the same team will measure the production and oxidation of these compounds by microbial isolates in a controlled setting, focusing on biochemical pathways that oxidize one carbon (C1) units from Oxidized VOC (OVOC) and methylated dissolved organic carbon (MDOC). Comprehensive measurements of microbial diversity in the field and transcriptome responses in the laboratory will set the stage for future research linking VOC cycling to specific organisms, metabolic pathways and genes, and for understanding when, and in response to what selective pressures, the microbial community engages in these processes.
Broader Impacts: VOCs play varied and important roles in atmospheric chemistry, acting as precursors for photochemical formation of ozone and aerosol, i.e. two secondary pollutants that also affect the radiative forcing of climate. Information about biological sources and sinks of VOCs in the ocean surface could result in a better understanding of the underlying causes of variation in air/sea VOC fluxes, and potentially could alter predictions about the impact of climate change on ocean surface ecology and air/sea interactions. Additionally, the project will address biochemical mechanisms that underlie VOC cycling and should provide experimental evidence about relevant gene functions. Therefore, revised gene annotations resulting from this work could improve the accuracy of future predictions of VOC metabolism made from genomes and metagenomes. This proposal includes support for postdocs, graduate and undergraduate students and is integrated with the Oregon Institute of Marine Biology's NSF funded Center for Ocean Sciences Education Excellence (COSEE) program, bringing training and experience from this project to community college professors.
This project discovered that new and unknown biochemical processes are being used by marine plankton to cycle organic compounds that form gases easily and can enter the atmosphere from the oceans. These gaseous compounds were previously unrecognized as important components of the marine carbon cycle. These discoveries help answer questions about the sources of many of these molecules that are detected in the atmosphere and impact Earth's radiative budget and climate. These discoveries will also improve the accuracy of carbon cycle models, which are needed to understand the ongoing impacts of climate change. The project was designed to test the hypothesis that ocean plankton can both produce and consume compounds that are known to chemists as "volatile organic carbon" (VOC). VOC compounds include formaldehyde, methanol, acetaldehyde, methylchloride, methylamine, trimethylamine, trimethylamine oxide, dimethylsulfide, and many other low molecular weight organic compounds that readily form gasses. These compounds are able to diffuse from the ocean into the atmosphere, where they can change atmospheric chemistry, and therefore have been studied by atmospheric chemists. The procedures and instruments needed to study VOCs are specialized, so these compounds have not been studied in detail by biochemical and ecological oceanographers, making their biological sources and sinks an unresolved question. This project supported measurements of VOC compounds in surface seawater samples on an oceanographic cruise, and measurements of VOC production and consumption by phytoplankton and bacterioplankton cultures, in a laboratory. An interdisciplinary scientific team composed of atmospheric chemists and physicists and marine microbiologists was formed to perform these measurements. On a NOAA research vessel in the North Atlantic in the summer of 2012, the team measured high rates of methanol and trimethylaminoxide oxidation, thus providing support for the hypothesis that that natural microbial plankton communities have a high capacity for oxidation of these low molecular weight VOCs. In their laboratories at Oregon State University in Corvallis, OR and at NOAA labs in Boulder, Colorado, the team showed that some phytoplankton produce methanol, acetaldehyde, and acetone and that some bacterioplankton can oxidize these compounds to CO2. This was the first time that it had been shown that phytoplankton and bacterioplankton cycle the highly reactive compound actetaldehyde. VOCs are important to Earth science for two reasons: 1) they play varied and important roles in atmospheric chemistry, acting as catalysts of chemical reactions and cloud formation and absorbing light radiation, and 2) the biological sources and sinks of VOCs in the ocean surface, in principle, can change the VOC flux accounting when geochemists attempt to estimate how much carbon is transferred between the Earth's major reservoirs, the atmosphere and the oceans, and what the forms of that carbon are.
