What is Deep DOM?

by Winn Johnson, Woods Hole Oceanographic Institution

Catherine extracts DOM by pumping seawater through a
cartridge to which DOM sticks. (Winn Johnson, WHOI)

As the DeepDOM cruise draws to a close, it is high time to talk about deep DOM itself. Marine dissolved organic matter (DOM) is a vast reservoir of carbon, containing 660 petagrams of carbon, which is as much as is held in the atmosphere.

A single petagram is approximately equivalent to the mass of the entire population of Brazil—if each person was an elephant. Despite the size of this carbon reservoir, relatively little is known about the structure of the organic molecules in it. That’s what we’re trying to find out.

Currently, geochemists classify organic matter in the ocean based on how quickly it is removed from the ocean. Organisms can remove DOM by converting it to carbon dioxide through respiration or DOM can stick to particles sinking into the sediments of the ocean floor. The oldest organic matter in the ocean is thousands of years old meaning that it has been circulating in the ocean for longer than a full ocean circulation cycle (~1000 years).

Winn and Liz filter water for an experiment  using
isotopically labeled carbon to study how marine microbes
change DOM. (Krista Longnecker, WHOI)

At the other extreme there are organic compounds that are easily used by the microbial populations of the ocean, making them difficult to even measure, as they exist only fleetingly before being consumed. In the middle of the spectrum are molecules that organisms can use, but that require a specialized enzyme or more energy to degrade, or that are broken down by a physical mechanism, such as light. We are particularly interested in understanding the role marine microbes play in transforming and degrading DOM, as well as how the microbial community is shaped by the DOM that is available.

As we approach Barbados we have sampled the ocean down to 5,500 meters, dangling the CTD rosette a mere 20 meters above the ocean floor. We have traveled 5,000 miles collecting about 200 samples along the way. These samples will allow us to see how DOM is transformed on the molecular level as it is transported within different water masses, such as Antarctic Bottom Water, North Atlantic Deep Water, and Antarctic Intermediate Water (see the CTD video to learn more about these water masses). Not only will this give us a picture of how DOM varies spatially in the ocean, but it will allow us to compare the molecular make-up of DOM as it changes with time and while traveling in these water masses.

With this information we can learn more about the processes that transform DOM and the factors that control what types of DOM persist in the ocean and what is removed. We will also combine our results with the biological analyses that have been described in the blog to identify connections between biological activity and the make-up of DOM.  

Catherine and Krista filter seawater to remove particles so that
we can analyze the dissolved matter. (Liz Kujawinski, WHOI)

How do we analyze DOM? 

Back in our lab back at WHOI we have an instrument called a Fourier transform ion cyclotron resonance mass spectrometer or FT-ICR-MS, to make it a little less of a mouthful. This instrument can detect molecules present in very low abundances and distinguish between molecules of different masses out to approximately four decimal places. These characteristics make it well suited to analyze the mixture of molecules that comprise marine DOM. Our analysis of a single sample typically yields only about 10,000 molecules, illustrating the truly incredible complexity of working with marine DOM.