A plenitude of ocean life: a new census of the sea is revealing that microbial cells thrive in undreamed-of numbers. They form an essential part of the food web

Natural History, May, 2003 by Edward F. DeLong

The Polar Duke, our ice-worthy Norwegian vessel, was immobilized--beset, to use the correct nautical term--by enormous sheets of sea ice. It was early August 1995, late winter in Antarctica, and the two-meter skin of frozen seawater that enveloped us was a seasonal expression of the Southern Ocean. Our destination was Palmer Station, a research station run by the National Science Foundation and situated on Anvers Island, off the Antarctic Peninsula. Evidenfiy, though, our group of American scientists and support staff had set out just a little too soon. It took ten days for a change in wind and the breakup of the ice pack to free the ship, but by then we were low on fuel and forced to return to Chile to be resupplied. When we finally made it to Palmer Station, we were a month behind schedule. Only two months were left of our field season, and that was spent largely on cross-country skis, hauling sleds laden with carboys full of seawater.

So went the first visit of my research group to Antarctica. Our aim was to search out and quantify the range and biomass of a peculiar group of microorganisms known as archaea. The wisdom of the day was that the critters should not be present at all in the cold, oxygen-rich waters of the Southern Ocean. But a sample of Antarctic seawater collected in early 1990 at Palmer Station, carried to California, and given to us for analysis suggested otherwise. We hoped to show that archaea were major players even below the pack ice.

Archaea (originally dubbed archaebacteria) were not even recognized as a separate branch of life until the 1970s, when the microbiologist Carl R. Woese and his colleagues at the University of Illinois at Urbana-Champaign made a thorough analysis of their ribosomal RNA. This kind of RNA, which plays a role in protein synthesis, occurs in the small structures called ribosomes that exist in every known kind of cell. Because of its ubiquity, ribosomal KNA can serve as a kind of universal bar code for all organisms, placing them in proper historical relation to one another on a single evolutionary tree. Woese concluded that Archaea is one of three major evolutionary branches of life, as deeply rooted as Bacteria and Eukarya. (Eukarya, whose cells contain a nucleus and other structures, encompass plants, animals, fungi, and protists--protozoa, algae, and lower fungi.)

Apart from their evolutionary heritage, archaea appeared to have one thing in common: they thrived in extreme environments. At the time of our expedition, we knew some lived in saline lakes five times saltier than the ocean; some lived in anaerobic (oxygen-free) habitats, where even trace amounts of oxygen would prove lethal; and some lived in hot geothermal environments that would cook most organisms to a crisp. Among them was Pyrolobus fumarii, which could grow in anaerobic deep-sea hydrothermal vents at temperatures as high as 235 degrees Fahrenheit.

Our surveys of the frigid, aerobic Antarctic waters turned up archaea in great and unexpected numbers. Indeed, we have learned that cold-adapted cousins of heat-loving archaea appear to be flourishing in marine waters both shallow and deep and at all latitudes--polar, temperate, and tropical. They turn up in the guts of abyssal sea cucumbers and in sediments at the bottom of the sea. Quantitative surveys now show that archaea comprise between 20 and 30 percent of all the microbial cells in the ocean.

The discovery and enhanced understanding of so many new microbial groups stems not only from the quest to look in new places. Modern-day microbe hunters also have new, high-tech tools for identifying and counting microbial life. In the past the method of choice had simply been to culture a sample of, say, seawater and then see what grew. Although that approach is still being perfected, many cells stubbornly refuse to grow under laboratory conditions. The new techniques, some based on the tricks of molecular biology, enable biologists to find out what is in the samples by direct observation.

Microbial life is proving to be far more diverse than cultured samples could suggest. A lot of the newly recognized life in the oceans is so small that its size is reflected in its name: picoplankton. The plankton comprises the floating "wanderers" of the sea, single-celled and multicelled plants and animals (including many immature larval forms) that move primarily by drifting with the currents [see illustration at left]. Anything smaller than 0.05 millimeter but larger than 2.0 microns, capable of passing through fine-mesh nets, is considered nanoplankton (the prefixes "nano-" and "pico-" do not literally correspond to such measurement units as the nanometer or the picometer; they arise instead from naming traditions in marine biology). The picoplankton comprises the smallest cells, ranging between 0.2 and 2.0 microns across (between 1/500th and 1/50th the diameter of a human hair).

Until the 1970s, picoplankton was thought to be an insignificant element of the marine microbial food web; its biomass seemed much too low to play a primary role. But estimates of the numbers of microscopic planktonic organisms climbed dramatically in the late 1970s, when the so-called epifluorescence microscope was developed. This instrument, coupled with the use of fluorescent dyes that Cause individual microbial cells to glow under ultraviolet light, enables the cells to be easily seen and counted, Technically, the process is an easy one. You simply add the dye, which binds to DNA in a sample of seawater, wait five minutes, collect the seagoing microorganisms on a filter, and observe them under the microscope. It is now known that the density of microorganisms ranges from tens of thousands per milliliter in the deep ocean to millions per milliliter in the energy-rich waters near the surface.