Picturing the world of ocean microbes

February 5, 2010

Streaming matter! Exploding organisms! Sinking particles! These are some of the things that populate the world of oceanic bacteria, providing the microorganisms with nutrition and perhaps exercise as they race for food, according to CEE Professor Roman Stocker, who is among those who have challenged the traditional view that ocean bacteria rarely swim, but passively wait for food to reach them.

In order to illustrate this concept, Stocker, who studies the fluid mechanics and ecology of ocean microorganisms; artist Glynn Gorick; and former postdoctoral associate Justin Seymour, now a senior research fellow at the University of Technology Sydney, created an image of this dynamic seascape.

The image was selected by Science magazine to grace the cover of its Feb. 5 issue, which highlights the Gordon Research Conferences, a 75-year-old nonprofit organization that will run 400 conferences on different topics in 2010. CEE science writer Denise Brehm interviewed the scientists and artist about their collaboration.

What are we looking at in this image?

Justin: We are looking at the world experienced by a bacterium living in the ocean. For a long time it was assumed that the microscale world experienced by these microbes was fairly homogeneous. This image shows that this microscale world is in fact extremely patchy, structured and dynamic, and highlights that marine bacteria, just like larger foraging organisms, can exploit discrete resource islands. This has implications for overall bacterial growth and production, competition between different microbial populations, and the cycling of carbon and other important chemicals in the ocean.

Roman: Up to 20 years ago, the ocean was described as a "homogeneous soup" at small scales (meters and below). In that scenario, everything was well mixed at these small scales, and small organisms would not have had reason to be motile. Recent research, however, has painted a different picture: the ocean is highly structured and heterogeneous even at minute scales. That is, even marine microbes experience a very dynamic seascape. The image shows a few examples of processes that contribute to create this patchy, dynamic seascape.

The trails and patches represent dissolved organic matter, which serves as nutrients for bacteria. At the top left we show excretions of dissolved organic matter by larger organisms, like zooplankton. At the top left center and bottom right we show the release of such matter by two phytoplankton cells, which creates a high-concentration region to which bacteria are attracted. On the top right, we show the death of a phytoplankton by lysis, possibly due to viral infection. The lysis creates a violent burst of dissolved organic matter. At the center of the image, extending along its height, we show the plume of dissolved organic matter created by a settling marine snow particle. The main actors in the image are motile bacteria, which are swimming by use of their helical flagella.

And is that spherical object at top center meant to be a single phytoplankton?

Roman: Yes, it's called a Coccolithophore, because it carries — phore — little scales — coccoliths. We chose it as a representative phytoplankton; we could have chosen others.

How big would these microbes and this cube be in reality?

Roman: The cube represents 1 milliliter of seawater. That is, it's 1 centimeter on each side. The bacteria are about 1 micrometer in size; the phytoplankton, about 10 micrometers. The zooplankton and marine snow particle could be 100 micrometers to 1 millimeter.

Why should we care about these microbes?

Roman: These microbes represent a crucial link in the marine food web. They are the main players in the oceans' biogeochemical cycles, for example the carbon cycle. These organisms include the phytoplankton, which are the primary producers that enable all life in the ocean. Then there are the heterotrophic bacteria, which are recyclers; they are the only ones who can take up dissolved organic matter that would otherwise be lost from the food web, thereby exerting a fundamental control on the budgets of the elements — like carbon, nitrogen and phosphorus — and on the productivity of aquatic ecosystems.

Why did you have this image created?

Justin: Because seawater appears clear and empty to the naked eye, it's often difficult to comprehend the level of ecological complexity occurring within each individual drop. So our objective was to illustrate graphically some of the ecological processes occurring within these drops of seawater.

Roman: The world of microbes escapes our intuition, because it is so much smaller than ours and the dominant physical processes are rather different from those that dominate our everyday life. An image makes these microscale processes more intuitive and calls attention to the importance of a world that — in virtue of its size — is poorly known, yet extremely important. Work in our laboratory focuses on visualizing the movement of organisms at these small scales, hence a graphic representation of what this micro-world looks like came as a natural extension of our research.

How did you start working with Glynn?

Roman: Glynn has a bit of a reputation in the field, and already had produced several images of marine processes. I saw one of his images at a conference and decided to get in touch. I thought there was great potential in linking his amazing skills in making the ocean come to life with our research on marine microbial ecology.

Glynn, your artwork is beautiful! How do you make each microbe's appearance accurate?

Glynn: Well thanks a lot, Denise! I search for photographs of the larger organisms such as the 1 millimeter size shrimp-like animal — Calanus finmarchicus — and electron micrographs of the very small members of the plankto