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Volume 5, Number 1
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Mutual Arrangements
Small-scale Symbiosis of the Seas

By Erica Goldman

Bleached coral reef - photo copyright Wolcott Henry 2005
© Wolcott Henry 2005

Microbes enter into collaborative concords that
make them far more potent than they might be alone.

Bacteria live ecologically complicated lives. They inhabit some of the most extreme environments on earth and can break down some of the most noxious compounds known. They compete with each other for limited resources (see Of Microbes and Messes). But they also enter into collaborative concords, intricate symbioses with other organisms that make them far more potent than they might be alone.

Like members of the visible world, microbes can enter into mutually beneficial treaties with other organisms. Picture the clownfish that darts unscathed around a sea anemone's poisonous tentacles or the Egyptian plover that rides astride a crocodile, freeing it from parasites while reaping a ready source of food. Now picture these relationships unfolding on a scale many orders of magnitude smaller than the eye can see.

When the infamous marine dinoflagellate Pfiesteria piscicida roared into the headlines in 1997, with reports that it released a fish-killing toxin in the Chesapeake Bay, microbiologist Robert Belas went to work to identify the different bacteria associated with Pfiesteria. His hypothesis: microbes that clustered around this dinoflagellate might play a role in its suspected "toxicity." Understanding the nature of these novel symbioses would be the first step toward validating this idea.

As part of a team of scientists from the University of Maryland Biotechnology Institute's Center of Marine Biotechnology in Baltimore, Belas readily found that the notorious dinoflagellate consorts with an impressive group of followers, over 30 different species associated by some sort of symbiotic link. In a story of discovery that would unfold over the next decade, Belas embarked on a journey that would lead him from Pfiesteria to oceanic sulfur metabolism to coral bleaching, ultimately steering him towards possible links between bacteria and rising global temperatures.

Belas soon found that sulfur was the great common denominator between the associated bacteria and Pfiesteria. He discovered that a group of bacteria called Roseobacter dominated his cultures and that this group uses sulfur (by oxidation) to make its nutritional way in the world.

Bob Belas at  his lab in Baltimore - photo by Erica Goldman

Following a long trail from Pfiesteria to the open ocean, UMBI microbiologist Robert Belas (above) has tracked down the ways in which bacteria may use antibiotics to protect a range of hosts, including corals. Corals that lose their symbiotic algae (zooxanthallae) look ghostly white or "bleached" (shown top of the page). Photo by Erica Goldman

Wondering why sulfur seemed to link these bacteria to Pfiesteria, Belas and his graduate students found themselves drawn deeper into the basic question of what made this association tick. For the time being, the path led away from the immediate question of Pfiesteria's toxicity and into the mysterious world of oceanic sulfur metabolism.

Todd Miller, Belas's graduate student, began to focus on an organic sulfur compound known as DMSP (dimethylsulfoniopropionate), one of the most abundant sources of sulfur in the marine environment. Since many unicellar dinoflagellates and algae make DMSP, Miller realized that the next logical step would be to determine if Pfiesteria also makes the sulfur compound.

When Miller found that Pfiesteria does in fact make DMSP, it seemed likely that some of these sulfur-oxidizing bacteria might be drawn to Pfiesteria because of the sulfur compound. Miller homed in on one of these sulfur-oxidizing microbes, classified it, and named it. He called the bug Silicibacter sp. TM1040 (for Todd Miller).

The connection between TM1040 and Pfiesteria turns out to be a tight one. TM1040 has sensory mechanisms to detect DMSP in the surrounding water. It can swim toward the source (Pfiesteria), and firmly attach itself to the dinoflagellate. This close physical association, explains Belas, suggests an evolutionary adaptation on the part of the bacteria to get close to their source of nutrition.

TM1040 also seems to have evolved a trick to keep competitors away from it, says Belas. With the help of an undergraduate student supported by Maryland Sea Grant, Belas's group found that TM1040 produces an anti-bacterial compound (an antibiotic) that inhibits pathogens such as Mycobacterium marinum, Vibrio anguillarum, and Vibrio cholerae, which can affect fish and algae.

Does antibiotic protection hold the key to the close association between Pfiesteria and TM1040? Maybe, says Belas. The bacteria (TM1040) seem to benefit Pfiesteria with their antibiotic activity by keeping pathogens away and in turn receive open access to a source of carbon and sulfur. But this work is still preliminary, he cautions. The research team has not yet shown that the antibiotic is even produced when the bacteria is in contact with Pfiesteria. The group is working to develop an assay to measure gene expression in order to test this concept, he says.

Commercially, TM1040's antibiotic could be developed for aquaculture purposes, to add to the water as a probiotic to protect fish from diseases such as Mycobacterium, says Belas. But antibiotic production by TM1040 and related bacteria may have even broader implications, he says, for the health of the seas.

Coral Connection

The antibiotic-producing prowess of TM1040 could have effects that reach far beyond local waters. Antibiotic production by bacteria like TM1040 may protect corals from bleaching and counteract some of the effects of rising global temperatures, says Belas.

Bacteria similar to the TM1040 live in the mucus that surrounds corals. And the antibiotic produced by TM1040, Belas found, can kill two of the pathogens connected with coral bleaching (Vibrio shiloi and Vibrio coralliilyticus).

Again sulfur seems to play a key role in the story. Corals harbor algal cells inside their body (called zooxanthallae) that in turn depend on the coral for survival — a symbiosis of clear co-dependence. Like Pfiesteria, these algal cells also produce the sulfur compound DMSP. If the relationship between these TM1040-like bacteria and corals proves similar to the one between TM1040 and Pfiesteria, sulfur would be the carrot that draws the bacteria close.

But a link is still missing. Belas suspects that TM1040-like bacteria may keep the coral healthy by producing antibiotic compounds that protect them from other pathogens. He does not yet know whether TM1040 itself lives in the mucus surrounding corals, only that similar species do.

If bacteria do provide antibiotic protection to keep corals healthy, can they still thrive if the world's water temperatures rise?

All microbes have an optimum temperature range at which they function best. TM1040 is no exception. It stops growing at a temperature of 32°C and stops producing antibiotic at 29°C, says Belas. Coincidentally, most coral bleaching occurs when waters rise to 30°C or above. Preliminary evidence also suggests that bleached corals do not have TM1040-like organisms as part of their normal mucus flora and will often have pathogenic Vibrio in their place.

Following this train of logic, rising water temperatures may thwart the ability of the bacteria to make antibiotic, which makes corals vulnerable to bleaching. "I have no data to support this yet," cautions Belas. "It is entirely coincident."

But Belas is not alone in his thinking. Additional support for the idea that antibiotics may help prevent coral bleaching comes from Kim Ritchie, head of the Coral Microbiology Program at Mote Marine Laboratory in Sarasota, Florida. She's found that when water temperatures are cool, corals have a tremendous diversity of bacteria in their associated mucus, including some that have the ability to produce antibiotics. Bleached corals in warmer waters lose this bacterial diversity and tend to harbor an overabundance of pathogenic Vibrio in their place.

"It's a good hypothesis," says Ritchie of Belas's suggestion, "but it will be difficult to prove." Just because bacteria produce antibiotics in the lab, does not mean that they produce them in nature, she explains. "There is so much going on in communities of organisms associated with the coral's symbiotic algae (zooxanthallae)," Ritchie says.

The emerging picture involving microbes, coral mucus, and temperature change is complex, agrees Garriet Smith, a marine microbiologist at the University of South Carolina. "Temperature changing over time affects everything, the zooxanthallae, the biota, interactions with bigger organisms," says Smith. "Any change that weakens one component and strengthens another is potentially important."

What started as a quest to figure out the source of Pfiesteria's suspected toxicity has become part of a much bigger story that links the smallest of organisms (bacteria) to the largest of Earth's problems (rising global temperatures). Today, the Pfiesteria conundrum remains an open question. But Belas's work promises to recalibrate our perception about the scales over which different organisms connect to each other. With each new discovery, we come closer to understanding the complex role of tiny microbes in a global context, closer maybe, to exploiting their talents to preserve the delicate balance of the global ecosystem.

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