Carbo-loaders of the South Atlantic : How does microbial consumption of complex organic matter vary with latitude and depth?

by Adrienne Hoarfrost, University of North Carolina at Chapel Hill

Adrienne puts some seawater into her
autoclave—a machine used to sterilize
materials under very high temperature
and pressure. (Winn Johnson, WHOI)

Ahoy from the high seas! We are currently at 6°N, having traveled all the way from Uruguay at 38°S aboard the R/V Knorr. On this cruise I’m investigating what microbes eat, how much of it, how fast, and how their appetites vary at different latitudes and depths. Specifically, I’m looking at high-molecular-weight polysaccharides (sugars), which make up a large component of dissolved organic matter (DOM) in the ocean. These carbo-loading microbes, and the differences in their activity at different locations, can give us clues as to how biological activity drives organic matter transformations in the ocean.

As Gwenn masterfully explained in her post on the biological pump, phytoplankton at the surface of the ocean convert carbon dioxide into the material that makes up their bodies (organic carbon) via a process called photosynthesis, producing oxygen in the process. When they die, they sink into the deeper ocean, which sequesters carbon dioxide away from the atmosphere, and provides a source of food for organisms living below the surface. As this organic matter sinks, microorganisms at different depths consume and transform it still further. These organisms are called heterotrophs, meaning they use organic matter as a food source. (You and I are also heterotrophs, with the spaghetti, meatballs, and broccoli I had for dinner last night all qualifying as organic matter.

I’m interested in the appetites of these heterotrophs. Different microbes have varying abilities to eat different components of organic matter—not every heterotrophcan consume every organic molecule. Instead, the microbial community as a whole works together, cumulatively breaking down complex organic matter into smaller pieces that are easier to digest by a greater fraction of the community. Despite this communal effort, not every community can break down every component of organic matter.

Adrienne samples her seawater-
polysaccharide incubations.
(Winn Johnson, WHOI)

What they can do, they do by using enzymes—molecules made by the microbes that break down a specific target organic substrate. Because the majority of marine organic matter that microbes eat is large and bulky, these microbes eject their enzymes outside of the cell (extracellularly, we say) to break down their food into manageable pieces before bringing it into the cell to finish eating it. Imagine trying to swallow an orange whole—it just can’t be done. You need to break it apart into smaller, more manageable segments first.

On this cruise, I’m tracking microbial consumption of several high-molecular-weight polysaccharides in seawater from different latitudes and depths. I’m looking for differences in what organic materials get eaten, how much of it gets eaten how fast, and which microbes are doing the eating.

Because different microbes have varying abilities to eat different things, and microbial communities are different depending on latitude and depth, I expect this to be reflected in which and how much of my substrates are eaten. Ultimately, I want to understand what specific features of these substrates make one more tantalizing than the other and why different microbial communities have differing appetites according to latitude and depth. This might help us understand how biological activity, along with latitude- or depth-dependent variations in that activity, contributes to the composition and transformation of organic matter as it moves through the ocean and down the food chain from source to sink.