Researchers Discover Unusual Symbiosis In Marine Microbes

April Flowers for – Your Universe Online

A study published in this week’s issue of Science shows how tiny, single-celled algae and a highly specialized bacteria have formed a partnership that helps keep the ocean fertilized.  This symbiotic pair takes nitrogen from the atmosphere and “fixes” it into a form that other organisms can use.

The results of the study emerged from the investigation of a mysterious nitrogen-fixing microbe that has a very small genome.  Jonathan Zehr, a marine scientist at UC Santa Cruz (UCSC), first detected the marine microbe in 1998, but it now appears to be the most widespread nitrogen-fixing organism in the oceans.

The project was a collaboration between scientists from UCSC´s Institute of Marine Sciences, the University of Hawaii, the Max Planck Institute for Marine Microbiology, the Station Biologique de Roscoff, the National Science Foundation and the Pierre and Marie Curie University.

Even though the microbe belongs to a group of photosynthetic bacteria known as cyanobacteria, it lacks the genes that are needed to actually carry out photosynthesis. This is where the relationship with the algae comes in.

“The cyanobacterium is a nitrogen-fixer, so it provides nitrogen to the host cell [the algae], and the host cell provides needed carbon to the cyanobacterium, which is lacking the machinery to get its own,” explained Anne Thompson, a researcher at UCSC.

The symbiosis between these two microorganisms has been undiscovered until this time, according to Matt Kane, program director in the National Science Foundation’s (NSF) Division of Environmental Biology.

“Genomic analysis indicates that the partnership between these organisms in some ways models the one that led to the evolution of plant organelles,” says Kane.

From an evolutionary perspective, the team says that this is an interesting symbiosis, “because it can be seen as analogous to an early stage in the endosymbiosis that led to chloroplasts in plants.”

Endosymbiosis is a process whereby chloroplasts, which carry out photosynthesis in all plants, evolved from symbiotic cyanobacteria that were eventually incorporated into host cells.

Previously, Zehr and his team studied the cyanobacteria in samples that were processed at sea and brought back to the lab. Although the team was able to sequence the microbe’s complete genome and discover that it was missing the genes for several key metabolic pathways, they were only able to see the symbiotic relationship when they sorted freshly collected seawater samples onboard a research vessel.

“Our collaborators at the University of Hawaii, Dave Karl and Ken Doggett, put a cell sorter into a portable laboratory — a lab in a box — so now we can take the machine to sea and sort cells that minutes before were in their natural environment,” says Thompson. “That’s how we found the association.”

The researchers admit that it is difficult, at this point, to estimate the contribution of this symbiosis to the global carbon cycle.  There are other algae in more abundance which may be more important in terms of the ocean’s carbon cycle.  But, according to Zehr, the cyanobacteria partners likely make this a significant contribution to global nitrogen fixation in the oceans.

“Planktonic symbioses are very difficult to study,” says Foster. “The associations are often fragile.  Here we used multiple tools to identify one of the first examples of this kind of partnership in plankton.”

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