My research interest is microbial biogeochemistry with a focus on trace gas cycling including methane, nitrous oxide, hydrogen, carbon monoxide, and dimethylsulfide. Many of these gases are supersaturated in the surface ocean with respect to the overlying atmosphere. Their ambient concentrations are a result of physical, chemical, and biological factors. I study their distributions in the context of wider elemental cycling and the role of marine microorganisms in governing their net concentration.
Methane and nitrous oxide time-series measurements The Hawaii Ocean Time-series programme has been sampling Station ALOHA, located 60 miles to the north of Oahu, since October 1988. The near-monthly cruises have provided invaluable insight into the structure and function of the oligotrophic ocean gyre over timescales ranging from months to decades. One of the most well-known and valuable datasets to derive from the time-series program are the increasing concentrations of carbon dioxide and the decreasing values of pH in the surface waters. But what about other greenhouse gases? The ocean is generally considered to be slightly supersaturated with methane and nitrous oxide, so how do their concentrations in the upper water-column vary seasonally and from year-to-year? Almost ten years of measurements show critical changes in the inventory of methane in the near-surface ocean and reveal it to be more dynamic than previously realized. In contrast, nitrous oxide behaves almost as predicted with a small seasonal oscillation in dissolved concentrations.
Station ALOHA in the North Pacific, methane and nitrous oxide in surface waters from 2008-2016
Inter comparison of methane and nitrous oxide The time-series analysis of methane and nitrous oxide at Station ALOHA represent a small fraction of their measurements throughout the global oceans which occur at time-series locations, hydrographic survey lines, and discrete oceanographic expeditions. Knowing the extent to which these independent measurements of methane and nitrous oxide agree for a common set of samples is a primary objective of SCOR Working Group #143. The group formed in January 2014 and conducted an inter comparison exercise by distributing replicate seawater samples collected at Station ALOHA to 12 laboratories across the globe. Based on the outcome of this initial intercomparison, it was decided to produce common gas standards and this effort was led by John Bullister at NOAA PMEL. These have now been distributed to 10 labs around the globe. The second inter comparison is now underway with 20 lab groups now involved as we anticipate seeing the benefit of having common gas standards.
Gases supplying the analyzer and gas standards ready to be shipped, and sampling for gases at the CTD rosette
Trace gas production by marine microorganisms A continuing research theme throughout my career has been the microbial production of climate-relevant trace gases. This began with my PhD research on the measurement of dimethylsulfide (Hatton and Wilson, 2007) and now includes methane, nitrous oxide, hydrogen, and carbon monoxide. The research includes both laboratory-based studies on cultured microorganisms (e.g Wilson et al., 2012) and field-based studies in the North Pacific Subtropical Gyre (e.g. Varaljay et al., 2012; Blomquist et al., 2012; Wilson et al., 2013). A long-term goal is the evaluation of the physical and biogeochemical influences governing the net concentrations of these trace gases in the oligotrophic open ocean.
The cyanobacterium Trichodesmium, phytoplankton cultures, and the analysis of cultures (Wilson et al., 2010)
Nitrogen fixation and hydrogen production Biological nitrogen fixation is a critical metabolic process in the oligotrophic ocean as it represents a source of new nitrogen to the nutrient-starved ecosystem. An interesting component to nitrogen fixation is that the nitrogenase enzyme produces hydrogen gas when it converts atmospheric nitrogen gas to ammonia. For every molecule of nitrogen that is reduced one molecule of hydrogen is produced although only a small amount is released into the ambient environment as the majority of the hydrogen gas tends to be recycled by the microorganism. Nitrogen fixation research has included method development (Wilson at al., 2012); its role in the oligotrophic gyre (Bottjer et al., 2017; Wilson et al., 2017) while research into hydrogen gas production associated with nitrogen fixation has focused on the efficiency of hydrogen cycling in relation to nitrogen fixation (Wilson et al, 2013).
Schematic of hydrogen cycling associated with biological nitrogen fixation (Wilson et al., 2010)
Ecosystem structure and function Since 2007, almost one year of my life has been spent at sea (or getting ready for the expeditions) in the oligotrophic NPSG. An inevitable outcome of spending this much time in the same place is consideration of how the ecosystem organizes itself in space and time. The temporal dynamics that have been the focus of recent expeditions from timescales of days-to-weeks in 2012 (Wilson et al., 2015) to diel patterns in microbial metabolism in 2014 and 2015 (Ferron et al., 2015; Wilson et al., 2017). The spatial scales are most frequently considered with respect to mesoscale eddy features as they propagate westward across the NPSG and influence community structure and activity along the way.
Oceanographic sampling and observations, plans for eddy field analysis, hydrocast data