Magazine article Oceanus

Revealing the Ocean's Invisible Abundance: Scientists Develop New Instruments to Study Microbes at the Center of the Ocean Food Web

Magazine article Oceanus

Revealing the Ocean's Invisible Abundance: Scientists Develop New Instruments to Study Microbes at the Center of the Ocean Food Web

Article excerpt

Microbes. They are invisible to the naked eye, but they play a critical role in keeping our planet habitable. They are everywhere, in abundant numbers, but are still difficult to find. They come in a multitude of varieties, but too often are difficult to distinguish from one another.

Wherever there is water (fresh or salt), there are usually microbes--microscopic, single-celled organisms. In the ocean, they form an unseen cornucopia at the center of a food web that ultimately nourishes larger organisms, fish, and people.

Their fundamental role in the ocean's food supply makes them critical targets for study, and scientists would like to know much more about them. They would like to identify them and count them. They would like to learn more about how marine microorganisms (part of what we call plankton) eat, grow, reproduce, and interact with other organisms. They would like to determine how changes in the ocean might affect the microbial communities' vitality and viability.

Finding minuscule life forms in a seemingly infinite ocean isn't trivial. But in recent years, oceanographers have been developing new techniques and instruments to identify and count marine microorganisms. Year by year, we are learning more about them and discovering that they are even more numerous, varied, and important than we thought.

A diverse microbial community

Some marine microbes are bacteria, or prokaryotes--simple cells with no specialized organelles, which are among the smallest of living things. Others are eukaryotes--larger and more complex cells with a nucleus, mitochondria, and other organelles.

Eukaryotic microbes, also called protists, include both producers, such as algae, and consumers, such as protozoa. They thrive in a variety of habitats--living suspended in the water, in bottom sediments, or on other objects. They form communities, or assemblages, of different species that photosynthesize, consume each other, and are, in turn, consumed by other things in the ocean's food web.

In the last few years, we have considerably advanced our knowledge of the structure and function of these assemblages--particularly planktonic assemblages that we sample by collecting the water they inhabit. We now know that these plankton assemblages are diverse, composed of species with widely different sizes, growth rates, and nutrition. Not surprisingly, we know more about the larger protists (greater than 100 microns) than the smaller ones (under 20 microns). Larger protists are easily visible using light or electron microscopes. They have features that remain intact throughout procedures to sample, preserve, and examine them, which can break or distort cells. These features are often lacking in the smaller organisms; and if they are present, they are harder to see and characterize.

Identifying protists has always involved some type of microscopic analysis, with someone looking at the shapes, or morphology, of the cells. But now we also use molecular methods--techniques that give scientists the ability to detect and identify the presence of even small protists based upon their DNA in water samples. Scientists have begun to describe the genetic composition of communities of species that live and interact in the same water. Our next objective is to overcome several technical challenges so that we can routinely monitor changes in protist populations over time.

Sampling the invisible

So far, all of our detection and identification techniques, both morphologic and molecular, have relied on collecting samples from remote sites and analyzing them in laboratories. But these techniques don't give us all the information we need.

Collecting samples from ships means physically taking separate water samples, at separate times, in separate places. Samples taken this way are, quite literally, just single samples--of one location at one time. They don't provide a continuous picture of protists in a given area of the ocean. …

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