Dimethylsulfide (DMS for short), is a sweet smelling sulfur gas found globally in the upper surface ocean. If you’ve ever opened a can of corn, you know what DMS smells like!
What is it? Where does it come from?
DMS is an anti-greenhouse gas and contributes up to 40% of the global atmospheric sulfur flux (Lana et al. 2011). When DMS volatilizes from the surface ocean to the atmosphere, it promotes cloud formation, which blocks radiation from the sun and thereby cools sea surface temperatures. DMSP, the precursor compound to DMS, is originally formed inside of phytoplankton cells. When phytoplankton die or a zooplankton comes along and munches on them, DMSP is released into the water where hungry bacteria are waiting for this yummy carbon and sulfur source to convert it into energy. About 10% of the time, when bacteria consume DMSP, they form a by-product of DMS.
So, can DMS reverse anthropogenic climate change?
In 1987 James Lovelock and his colleagues proposed that phytoplankton are actually controlling their sea surface temperatures by creating more DMSP to eventually make its way into the atmosphere and form clouds. More recently, Lovelock and his colleague Chris Rapley suggested a somewhat extreme solution to climate change and rising sea surface temperatures: place giant pumps in the ocean that bring deep waters full of nutrients to the surface to induce a phytoplankton bloom (Lovelock and Rapley 2007). Not only would a bloom remove carbon dioxide (CO2) from the atmosphere (phytoplankton breathe in CO2, whereas we breathe in O2), it could also promote DMS production and therefore cool sea surface temperatures. Seems like an easy solution?
The unknowns of ocean fertilization
Oceanographers have already conducted multiple large scale studies investigating the effects of addition of nutrients (particularly iron) to the surface ocean, also known as ‘ocean fertilization’ (Williamson et al. 2012). The results highlight two potential flaws of ocean fertilization: 1. Creating a phytoplankton bloom and removing CO2 from the atmosphere does not mean the CO2 will be sequestered out of the surface ocean (and buried in the deep ocean). A lot of processes determine the magnitude of sequestration, including grazing. A phytoplankton bloom is associated with grazers (zooplankton and bacteria) that consume phytoplankton and simultaneously respire CO2, which can then re-enter the atmosphere. 2. DMS has shown different responses (both increases and decreases) in different ocean fertilization experiments. There are still too many unknowns about DMS cycling to truly understand how its production will be altered with ocean fertilization.
A case study in the complexities of DMS and ocean fertilization
Despite these uncertainties from multiple scientific investigations, private firms have made plans to conduct large scale ocean fertilization in order to profit from carbon credits. Most recently, a huge bloom (~10,000 km2) was seeded by dumping 100 tonnes of iron into the waters off the western coast of Canada (Tollefson 2012). A lot of controversy surrounds this unsanctioned experiment, and it directly violates international treaties that have banned ocean fertilization until we have a better handle on environmental impacts and actual carbon sequestration rates (Schiermeier 2007). The $2.5 million project was funded by the Haida Nation, an indigenous people of the Pacific Northwest Coast of North America, with promises of improving dwindling salmon stocks. While the project was conducted by the Haida Salmon Restoration Corporation (HSRC), it seems it was spearheaded by US entrepreneur, Russ George (former executive of Planktos another ocean fertilization firm), who acted as chief scientist onboard the chartered fishing boat. In addition, the National Oceanic and Atmospheric Administration(NOAA) provided 20 data buoys to the project. After reports of the illegal dumping of iron, NOAA made a statement saying they were misled with the intentions of the project. Certainly some interesting data was collected (and freely available to the public with granted permission from the HSRC), but the experiment was not designed to maximize scientific output and therefore we will never know if DMS production increased in this bloom. Perhaps most interesting are reports that salmon stocks doubled in 2014. However, a significant amount of research would be needed to actually correlate the fertilized bloom to increases in salmon stocks.
Clearly, there is still a lot to learn about ocean fertilization and whether or not the anti-greenhouse properties of DMS can be harnessed to mitigate climate change.
Erin McParland is a graduate student at USC in the department of Marine and Environmental Biology. Her thesis aims to understand why phytoplankton produce DMSP, the primary source of DMS in the oceans. She recently returned from a cruise where she investigated DMSP production in the equatorial Pacific, an area of the ocean greatly understudied in terms of DMS(P) cycling.