CORVALLIS, Ore. – Researchers at Oregon State University have discovered what could be a new, limiting nutrient in the world's oceans.
In a publication today in the journal Nature, they report that chemically "reduced" sulfur is a nutrient requirement for SAR11, the smallest free-living cell known and probably the most abundant organism in the seas.
This may be another important step forward in understanding all the factors related to phytoplankton production – what has been called the "Holy Grail" of marine ecology, since phytoplankton are the base of the marine food chain.
If reduced sulfur is sometimes in short enough supply, it could limit growth of SAR11 and any other organism with the same unusual requirement, the scientists said. These results raise the possibility that sulfur may turn out to be as important to some organisms as nitrogen, phosphorus, and iron are already known to be for most marine organisms.
The findings may have implications for everything from understanding ocean ecology to bacterial genetics and global climate function.
SAR 11 was first discovered by OSU researchers in 1990. There is great interest in understanding how this obscure bacteria works, because it dominates microbial life in the oceans and plays a major role in the cycling of carbon on Earth. Although these bacteria may have been thriving for a billion years or more, they have the smallest genetic structure of any independent cell.
That small genetic structure, in fact, may be why SAR 11 has to “borrow” its reduced sulfur as a waste product from other nearby microorganisms.
“This appears to be part of the genomic streamlining that has made SAR 11 such an evolutionary success,” said Steve Giovannoni, a professor of microbiology at OSU. “It’s a very simple, lean machine, and by using sulfur produced by other sources it doesn’t have to expend the energy to reduce this nutrient itself. It may have traded independent function for simplicity and energy efficiency.”
Sulfur in various sulphate chemical combinations is abundant in the oceans. Virtually all other marine life forms, the researchers said, have the genetic and biological capability of “reducing” it to the chemical form they need as a nutrient. SAR 11 can’t do that. Unless something else produces the sulfur in the form it needs, it dies.
“SAR 11 has a very small genome, and some genes that we routinely find in almost every other life form simply aren’t there,” said James Tripp, a research associate at OSU and author of this study. “It had been thought that this gene which reduces sulfur was pretty much universal, but when we looked for it in SAR 11, we couldn’t find it.”
There are no other aerobic organisms known that have this genomic structure, the scientists said.
“This is just really, really unusual,” Giovannoni said. “It also raises the question of what other bacteria and phytoplankton have unsuspected nutrient requirements that we know nothing about.”
The findings are of more than academic interest, researchers say. Even though the basic mechanisms of phytoplankton production in the ocean are known, it’s not really clear what all the factors are that control the process. But that process is essential to marine life, a breathable atmosphere and global climate.
Oxygen in the Earth's atmosphere is largely created and maintained by photosynthesis, in which plants convert sunlight into biological energy through a process that requires chlorophyll. In the oceans, SAR 11 is a partner in this process. It recycles organic carbon, and produces the nutrients needed for the algae that produce about half of the oxygen that enters Earth's atmosphere every day.
The function of SAR 11 may also affect climate in more specific ways. One of the major sources of sulfur used by phytoplankton is referred to as DMSP – it’s the compound that puts the “ocean smell” in salt air, and it’s important in climate models since it helps form clouds that ultimately cause rain. If SAR 11 were not using much of this sulfur compound, it conceivably could have a major effect on cloud formation and ultimately global climate.
“There’s a lot we still need to learn about the basic functions of marine ecology, because they can affect so many other things,” Giovannoni said. “We certainly did not expect sulfur to be so important in this situation. When we look more, there will probably be more surprises.”
This work was supported by grants from the Marine Microbiology Initiative of the Gordon and Betty Moore Foundation, and the National Science Foundation.