Oceanography and the Single Cell

It probably should be a Trivial Pursuit question along the lines of "What do we know more about – outer space or the earth's oceans?" Most people might understandably answer the latter. And most people would be wrong. If you do the most casual of Google searches on the question, you will find countless references in the scientific literature quoting any number of sources all of which run along lines of saying we know more about outer space than we do about our own oceans. That paucity of knowledge becomes a little easier to understand when you consider that there are more microorganisms in the oceans of the world than there are stars in the known universe.

That's one of the challenges facing biological oceanographers, such as DFO scientist William Li, working out of the Bedford Institute of Oceanography in Dartmouth, Nova Scotia. As he and his colleagues look at the infinitesimal - in the form of microbes – they have to consider the enormity of the oceans in which they exist.

He puts the dichotomy in context: "Oceanography is science on a big scale. The only way that we can study the ocean in one large, contiguous piece is by satellite. But the parts of the ocean that we need to study for biology can be very small, especially when we talk about algae and bacteria. So we are trying to understand the ocean bit by bit, looking at very, very small organisms. But in a paradoxical way, we are trying to make statements and conclusions about the ocean on a very large scale; in our case, the North Atlantic, the Arctic Ocean, the Labrador Sea, and ultimately, the global ocean."

But as he points out, this in turn can lead to an automatic disconnect. The microbes they study live very short life spans: a day, a few days, up to a week. So when they are living and dying and creating whatever impact they have on the environment, it's very localized, happening over a very short timescale.

And yet the questions that society is most interested in are things that take a long time to happen, over very large areas. Human-caused climate change is a case in point.

Which in turn leads to: Can you extrapolate what is going on at the microbial level in short periods of time and make conclusions on a macro level over much longer time intervals?

Dr. Li describes how his specialty might contribute to the debate. "Cytometry is the study of cells according to their size, their mass—and various other sub-cellular features. The distinguishing feature of cytometry is that measurements are made on individual cells one at a time. There are billions of cells in just one liter of seawater. However, the modern technology of flow cytometry enables us to make these measurements on millions of individual cells all at once."

Marine scientists take their measurements of ocean microbes using the same bio-medical instruments used by hospitals for human blood work, reconfigured for their own purposes. When a cell comes into the sensing zone of the instrument, a short burst of laser light hits the cell that in turn emits two kinds of light in the visible electromagnetic spectrum that the computer picks up.

One is the laser light that is "scattered" off the cell. In many cases, scattered light is blue in colour. The other stems from the laser light that is absorbed. Absorption causes an excitation process within the cell that results in a burst of outward fluorescing light, which can be green, orange or red in colour.

The various pieces of information reveal certain fundamental characteristics of the cell such as its type, size and in some instances its health.

Scientists know enough about ocean microbial cells to understand that there are groups of them that appear very similar. But they also know that within each of these similar-looking groups, there is inherent variability. The flow cytometer can differentiate between "gross" differences - red colored looking phytoplankton, green colored and blue-green colored ones and so on. But it is not discriminating enough to inform researchers of still finer subgroups within a particular group. That is to say the flow cytometers can make distinctions at a coarse level but not at a very fine level.

Dr. Li observes, "Flow cytometry is an important research tool because cytometric cell analysis is very fast, very precise, and very accurate. But generally it does not resolve species identity. In other words, flow cytometry provides a very effective window into the ocean, but sometimes it is necessary to open that window wider using other tools. For example, if we suspect that a certain species is causing a harmful algal bloom in our local waters, cytometry can give us a rapid indication of something different going on from the baseline. But we cannot say whether it is in fact a harmful algal species because generally the cytometer cannot tell us that. So we will have to go to conventional microscopy. Conventional microscopy, for its part, will bring us closer to an answer, but at the expense of a great deal of expert labor and time."

Because scientists are able to collect so much cytometric data so quickly over an extended time period using consistent methods, they are able to develop large databases and have good baselines for things happening around the world. That makes cytometry a good sentinel: but it also is quite a bit more than that, quite a bit more than an early warning system.

Asked how he would like to see this sort of work characterized, Dr. Li replies, "In looking for what the very small can tell us about the very large, we have taken an instrument you can buy off the shelf, and use it in a different way. We are starting to collect information about microbial plankton everywhere in the world's oceans, including our backyard, which for us means, Halifax Harbour. We make these measurements at our doorstep and for such long periods of time that we can build up a library of data that can be used to understand how plankton ecosystems, first of all work in their natural state and secondly, how they may change when they are affected by pollution, contamination, climate change, overfishing, ocean acidification, and all other such pressures that DFO scientists are taking under observation."

Given that there are more microbes in the seas than stars in space, it is work that will continue for a very long time.