The number of plankton in the seas is almost beyond comprehension. A single teaspoonful of ocean water holds several million of these microscopic drifters, and in recent years, scientists have discovered plankton are involved with everything from the health of the water to global warming.
Now, a team of researchers, led by marine microbial ecologist Mary Ann Moran at UGA, has discovered a bacterial “switch gene” in two groups of plankton. This gene helps determine whether certain marine bacterioplankton convert a sulfur compound to one that rises into the atmosphere and affects the Earth’s temperature or remains climatically inactive in the seas.
“This new gene offers a powerful tool to study the question of how bacterioplankton are involved with ocean-atmosphere sulfur exchange,” said Moran.
The study was published in the journal Science. Other authors of the paper include: William B. Whitman, of the UGA department of microbiology; Erinn Howard, James Henriksen, Alison Buchan and Chris Reich, present or former doctoral students at UGA; current UGA postdoctoral associate Helmut Bürgmann and former postdocs Kimberly Mace and Jose González; former UGA undergraduate Rory Welsh; UGA lab coordinator Wenying Ye; Samantha Joye, biogeochemist in the department of marine sciences at UGA; and Ronald Kiene of the department of marine sciences at the University of South Alabama.
Much of the sulfur in the atmosphere comes from the surface of oceans and a compound called dimethlysulfide or DMS. Marine bacterioplankton control how much sulfur rises into the atmosphere by converting a compound called DMSP either to DMS or to sulfur compounds that are not climatically active.
Moran and her team discovered, in two groups of bacterioplankton, the gene that controls whether or not these sea drifters create DMS that rises into the air or another compound that doesn’t. The implications are considerable, since atmospheric sulfur is involved with everything from acid rain to global warming.
“This breakthrough in the microbial physiology of DMSP metabolism opens the door to finally understanding the biology and ecology of this globally important process,” said Whitman. “These are necessary steps in learning how our planet works. They have very practical implications as well and could help ameliorate the climate change caused by emissions of greenhouse gases.”
Moran agreed. “We knew bacteria played a key role in the cycling of marine sulfur, but the discovery of this gene allows us to pinpoint how and when bacteria control this process,” she said. This flow provides a key feedback loop in theories of global climate regulation for which biotic processes are central elements.
The researchers discovered that the bacterioplankton in the Roseobacter and SAR11 groups are the primary plankton involved with directing DMSP away from DMS and thus making sulfur unavailable to atmospheric processes. (Roseobacter largely grow in coastal waters, while SAR11 are more prevalent in open seas.)
Plankton can be divided into broad functional groups. Phytoplankton live near the water’s surface and flourish through photosynthesis. Zooplankton are small protozoans that feed on other plankton, and bacterioplankton include bacteria and Archaea (a relatively newly discovered category of sea life) that remineralize organic material down the water column.
While researchers had known for some time that phytoplankton can degrade DMSP to DMS, efforts to predict global patterns of ocean-atmosphere DMS flux based solely on the abundance of phytoplankton had been unsuccessful. Moran and her team thus became interested in the other plankton that were involved.
“We had an advantage, because we are able to grow Roseobacter in the lab,” said Moran. “But much less was known about how SAR11 ‘make a living.'”
Dramatic advances in understanding how these bacterioplankton work have occurred in the past few years with the availability of new genomic data. The scientists searched genome fragments of bacterioplankton that grow in the Sargasso Sea, looking for specific gene sequences that would show how these sea creatures use sulfur compounds. (The Sargasso Sea is an elongated region in the middle of the North Atlantic surrounded by ocean currents.)
“This project has brilliantly come full circle,” said Matthew Kane, program director with the National Science Foundation, which supported the research. “Isolation and discovery of a novel, keystone bacterium from the ocean, and sequencing of its genome enabled the team to work out the genes involved in the DMSP cycle. They now have been able to show the distribution and abundance of one of these genes back in environmental populations, thus revealing the previously hidden role that marine microbes play in the global sulfur cycle.”
The discovery of the bacterial gene switch in these two groups of bacterioplankton will open new areas of research, since DMSP synthesis may account for almost all of the marine-created atmospheric sulfur.