Newly Discovered Nitrogen-Fixing Cyanobacterium May Alter Understanding of Ocean Carbon and Nitrogen Cycles

13 November 2008

An unusual microorganism discovered in the open ocean may force scientists to rethink their understanding of how carbon and nitrogen cycle through ocean ecosystems. A paper describing the new findings appears in the 14 November issue of the journal Science.

A research team led by Jonathan Zehr, a marine scientist at the University of California, Santa Cruz, characterized the new microbe by analyzing its genetic material, even though researchers have not been able to grow it in the laboratory. Zehr said the newly described organism seems to be an atypical member of the cyanobacteria, a group of photosynthetic bacteria formerly known as blue-green algae.

Unlike all other known free-living cyanobacteria, this one lacks some of the genes needed to carry out photosynthesis. The microbe can, however, fix nitrogen from the atmosphere into a form useable by other organisms—i.e., it can provide a sort of fertilizer.

For it to have such an unusual metabolism is very exciting. We’re trying to understand how something like this can live and grow with so many missing parts.

—Jonathan Zehr

Earlier research by Zehr’s group had revealed surprisingly large numbers of novel nitrogen-fixing cyanobacteria, including the one that is the focus of this study, in the open ocean.

Although 80% of Earth’s atmosphere is nitrogen, most organisms cannot use it unless it is “fixed” to other elements to make molecules like ammonia and nitrate. Because nitrogen is essential for all forms of life, nitrogen fixation is a major factor controlling overall biological productivity in the oceans.

The new microbe is one of the most abundant nitrogen fixers in many parts of the ocean, Zehr said. New DNA sequencing technology provided by 454 Life Sciences enabled rapid sequencing of the organism’s genome.

The results showed that it is missing the entire set of genes needed for photosystem II—large complex of multiple proteins and chlorophyll molecules—and carbon fixation, essential parts of the molecular machinery that carries out photosynthesis in plants and cyanobacteria. The genes for photosystem I appeared in the sequencing data, as did genes for both photosystems from the small numbers of contaminating cyanobacteria in the sample.

That has multiple implications. It must have a ‘lifestyle’ that’s very different from other cyanobacteria. Ecologically, it’s important to understand its role in the ecosystem and how it affects the balance of carbon and nitrogen in the ocean.

—Jonathan Zehr

During photosynthesis, photosystem II generates oxygen by splitting water molecules. Because oxygen inhibits nitrogen fixation, most nitrogen-fixing cyanobacteria only fix nitrogen at night, or do it in specialized cells. The lack of photosystem II enables the new microbe to fix nitrogen during the day, Zehr said.

But without photosynthesis, it can’t take carbon dioxide from the atmosphere and convert it into sugars, and so it’s not clear how the new microbe feeds itself. Either it has some way of feeding on organic matter in its environment, or it lives in close association with other organisms that provide it with food, Zehr said. “It would make a perfect symbiont because it could feed nitrogen to its host and live on the carbon provided by the host,” he said.

Zehr said he plans to continue research on the new microbe and fill some gaps in the present knowledge. Efforts are currently underway to map the microbe’s presence in the oceans and determine its global abundance. Zehr is also interested in how its metabolism differs from other known cyanobacteria. If it can be cultured, there may be ways to exploit this organism’s unusual metabolism in biotechnology applications, he said.

Zehr’s coauthors on the paper include graduate student Shelley Bench, researcher Brandon Carter, and postdoctoral scholars Ian Hewson, Tuo Shi, and James Tripp of UCSC, as well as Faheem Niazi and Jason Affourtit of 454 Life Sciences.

The work was supported by the National Science Foundation and the Gordon and Betty Moore Foundation; sequencing was provided by 454 Life Sciences.

Resources

• Jonathan P. Zehr, Shellie R. Bench, Brandon J. Carter, Ian Hewson, Faheem Niazi, Tuo Shi, H. James Tripp, Jason P. Affourtit (2008) Globally Distributed Uncultivated Oceanic N₂-Fixing Cyanobacteria Lack Oxygenic Photosystem II. Science 14 November 2008: Vol. 322. no. 5904, pp. 1110 - 1112 doi: 10.1126/science.1165340