New Study Shows Group Of Marine Bacteria Dominate Offshore Waters Of Southeast, May Play Key Role In Ecosystem Processes

A new study by marine scientists at the University of Georgia is uncovering intriguing and unexpected clues about marine bacteria. Most interesting may be the dominance of bacteria from the so-called "marine alpha group" in the near-shore waters and estuaries of the Georgia coast. As much as 30 percent of the bacteria in the area belongs to a single group named marine alpha bacteria, but the reasons for it - and its significance - are still unclear.

"Right now, an important goal in marine microbiology is understanding the connection between the structure of bacterial communities and their ecological function," said Dr. Mary Ann Moran. "We are studying a group of bacteria that are closely related but which may be very diverse functionally."

The research was published late last year in the journal Applied and Environmental Microbiology, and it has been funded by grants from the Georgia College Sea Grant Program and the National Science Foundation. Moran has collaborated with postdoctoral associate Dr. JosŽ Gonz‡lez on the research.

Scientists agree that coastal bacteria play a substantial role in biogeochemical processes and that these bacteria could one day be important in business and industry as well as in maintaining the health of near-shore ecosystems. A number of problems, however, have kept the composition of bacterial communities largely unknown. Researchers have been reluctant to dive into the vast array of marine bacteria because they are devilishly hard to culture in laboratories. And even the ones that have been grown often are unrelated to the bacteria that are ecologically important in marine ecosystems, making scientific connections and inferences difficult.

That has all changed in the past few years with the advent of new techniques to identify bacteria at the level of their basic building blocks. Using specific gene sequences as targets for identification, scientists can recognize and identify bacteria without the need to culture them first.

"The target for our work is called the 16S ribosomal RNA gene," said Moran, "and it's a good gene to focus on because all living things need ribosomes to make proteins."

It's all about what happens inside the cell. Ribosomes are small cellular components where protein synthesis takes place, and they are composed of specialized ribosomal RNA (which is abbreviated rRNA) molecules. The genes coding for rRNA are an essential component of the genetic material of all prokaryotes (cellular organisms such as bacteria that lack a limiting membrane), but they vary enough to give each species a unique name tag.

Moran and Gonz‡lez found preliminary evidence that a cluster of marine bacteria may be particularly important in coastal seawater of the southeastern U. S. and then used the key rRna gene to quantify just how abundant they may be. In work that is drawing considerable interest from marine scientists worldwide, they designed a "probe," a molecule that is labeled in some way - usually with radioactivity or fluorescence - to seek out the 16S rRNA gene of marine alpha bacteria in seawater. The probe looks for similar sequences of base pairs (the chemical building blocks of the double-stranded DNA) and marks them by hybridizing to them.

Using this process, the marine scientists were able to discover, to their surprise, that up to 30 percent of the bacterial genes present at a number of locations off coastal Georgia over a three-year period were from members of the marine alpha group of bacteria. The presence of the marine alpha bacteria dropped off in the fresh-water areas of estuaries, such as where the Altamaha River empties, indicating they are exclusively marine and specialized for the salty water found along the Georgia coast.

Moran really needed more proof that the probe was correctly targeting marine alpha bacteria, however. So she and Gonz‡lez used a technique called polymerase chair reaction (PCR) to greatly amplify the genetic sequences of the 16S rRNA genes for study, while at the same time successfully culturing the bacteria in low-nutrient seawater agar medium.

"In addition to providing further evidence for the abundance of marine alpha bacteria in coastal seawater of the southeastern United States, successful culturing of bacteria from this group furnishes organisms for studies of the physiology and ecology of this important cluster," said Moran.

Now that it's clear that marine alpha bacteria are abundant in near-shore marine systems, the question remains: What role do they play? As it turns out, these bacteria could have an important future place in industry, since at least one described for the first time and named by Gonz‡les and Moran (Sagittula stellata) can break down lignin, the polymer that acts as a natural binder and support for cellulose fibers of woody plants. The potential use of such bacteria in the pulp and paper industry is obvious. And lignin and related compounds also have many natural sources in coastal waters.

Also, there is evidence that some of the bacteria may be involved in sulfur cycling - an important natural process that is also used in business and industry. Much more remains to be known before these marine bacteria can be used by humankind, however. Indeed, scientists are still trying to completely understand the role of these bacteria in the seas.

These are only two aspects of the expanding research, and Moran has just received a three-year, $273,000 grant from the National Science Foundation to discover more.