Mangroves, Microbes, and Greenhouse Gases

For five weeks spanning October and early November, Brett Jameson, a doctoral candidate at the University of Victoria in British Columbia, Canada, is working with BIOS biogeochemical oceanographer Damian Grundle on a project investigating the microbial production of nitrous oxide (N2O) in low oxygen marine environments.

Nitrous oxide is an important and increasingly abundant trace gas which, in the troposphere—the lower layer of the Earth’s atmosphere where nearly all of the weather takes place—has a warming potential that is more than 300 times that of carbon dioxide (CO2) over a 100-year timescale. N2O also survives transport to the stratosphere where it contributes to the depletion of the ozone layer.

Jameson is building on work that he, Grundle—who is also one of Jameson’s doctoral co-advisors—and a team of scientists recently published in the journal Limnology & Oceanography Letters. The paper details a new method of measuring N2O production in the sediments of outer continental margins, the transitional regions on the Earth between continental (land) and oceanic crust. Using a combination of newly developed microsensors and mathematical modeling, the team was able to accurately measure vertical N2O profiles from sediment cores underlying the Northeast Subarctic Pacific Ocean (NESAP), an area off the coast of British Columbia, Washington, and Oregon. With these profiles, Jameson and colleagues produced the first estimates of net N2O production and release from offshore sediments of the outer continental margin.

For this new research project, which will form the basis for his doctoral thesis, Jameson is interested in working to improve estimates of N2O production rates and fluxes (movements) in under-described sediment-based ecosystems, and connecting these variables to microbial community diversity and activity.

“Most of the research efforts so far have been focused on intertidal or nearshore sediments and have been limited to the use of methods that require considerable disturbance to the chemical gradients that occur in sediment surface layers,” Jameson said. “Now that I have validated a new method with the offshore sediments, I’m trying to see if I can adapt it to improve estimates from mangrove ecosystems.”

The role that mangrove forests play in the global N2O cycle is not well understood: scientists still do not know if they act as an overall source (place that produces N2O) or sink (place that consumes N2O). However, it is known that both production and consumption of N2O takes place within the mangrove sediments by microbes—bacteria and archaea (single-celled organisms similar to bacteria)—through the nitrogen cycle.

“Whether or not a system is a net source of N2O to the atmosphere becomes a question of the balance between production and consumption processes,” Jameson said. Scientists also know that areas of maximum N2O production occur in areas of low oxygen concentration, as this has been observed within water column oxygen minimum zones in the ocean.

To begin his investigation, Jameson is taking small sediment core samples from a mangrove stand located along Ferry Reach near the BIOS campus on the east end of Bermuda. The cores are then incubated in the mesocosm facility at BIOS during light and dark cycles. This mirrors the changes that occur over the course of a natural day, which lets him see how N2O production and consumption rates vary over time in response to changes in both light and temperature.

Jameson is utilizing the same microsensors that he used during the NESAP project, which gives him the ability to measure N2O gradients within the upper 2mm of the sediment core, where they happen most rapidly. His goal is to use data gathered from these profiles, combined with mathematical modeling, to produce an estimate of N2O production and consumption rate with sediment depth, as well as flux across the sediment-water interface.

“Ideally, these data might be more broadly applicable to other mangrove ecosystems that occupy oligotrophic (low-nutrient) waters,” Jameson said. “There are always new revisions to the global N2O budget, including what areas are producing and consuming N2O and how these influence concentrations in the atmosphere. With this improved method we can get more accurate estimates of inshore and nearshore sediments, as well as offshore and deep ocean sediments.”

Jameson first came to BIOS in the summer of 2015 as a Canadian Associates of BIOS (CABIOS) intern, during which time he conducted his undergraduate thesis research on the photosynthetic efficiency of coral reef communities under the guidance of BIOS reef systems ecologist Eric Hochberg. He returned after graduating from Dalhousie University (Halifax, Nova Scotia, Canada) in the fall of 2016 to work as a teaching assistant for the Marine Invertebrate Zoology course, then worked with microbial oceanographer Rachel Parsons in the Microbial Ecology Laboratory.

“BIOS was my very first introduction to hands-on ocean research and really solidified my passion for ocean science,” he said. “After leaving BIOS in 2016, I wasn’t certain if I’d ever make it back, so returning this year has felt like a homecoming of sorts. I have so much love for this institution and the opportunities it has given me.”