In stable neighborhoods, there’s often very little turnover of residents. But a new study surprisingly suggests that around hydrothermal vents, considered to be some of the most stable environments on Earth, microbial communities in some parts of a hydrothermal system can undergo dramatic demographic changes over time.
Environmental conditions on the deep ocean floor rarely change: No sunlight reaches the sediment there, and water temperatures typically hover near freezing. In such an environment, hydrothermal vents — which spew immense amounts of warm, often nutrient-rich water — offer an oasis for microbes and the creatures that consume them. Even within these systems, however, conditions, such as the mineralogy of the hydrothermal chimneys and the chemical composition of vent fluids as those oases age, can change. These changes, in turn, can cause dramatic shifts in microbial communities, microbiologist John A. Baross and his colleagues report online January 11 in the Proceedings of the National Academy of Sciences.
Baross, of the University of Washington in Seattle, and the other researchers studied microbial diversity in rock samples collected by the deep-sea rover Alvin from the Lost City hydrothermal field, a vent system that has been active in the North Atlantic for at least 30,000 years. Active vents there discharge hot, highly alkaline fluids that are rich in methane and hydrogen. Biofilms cover many of the vents’ carbonate chimneys, and each gram of that highly porous rock typically contains between 1 million and 1 billion microorganisms, says Baross.
Although the team’s analyses suggest that there are more than 800 genetically different types of microbe living today around the vents, those analyses also show that more than 80 percent of the individual microbes found there belong to a group called Methanosarcinales, which includes some organisms that make methane and others that consume it. “It’s a tough world to live in,” Baross says of the conditions there. “It’s not surprising to see just one dominant group,” he adds.
The team compared microbes living on recently formed rocks to those living on much older deposits. Presumably, the researchers say, populations living on young rocks are similar to those that lived on the older deposits when they too were young. In one sample deposited just 34 years ago, more than 90 percent of the organisms present were Methanosarcinales, the researchers found. Two other geologically fresh carbonate rock samples, deposited 43 and 128 years ago, show similar levels of Methanosarcinales dominance.
But a different type of methane-consuming microbe is predominant in a sample deposited about 1,250 years ago, a microenvironment where today the rock isn’t bathed with hot alkaline water and where conditions now more closely resemble those of the surrounding seafloor. That microorganism is present in the fresh samples of carbonate, too, but only in small numbers, the researchers note. So the team suggests that the differences between the microbial communities around old vents and new ones didn’t necessarily result from the evolution of new species.
Instead, Baross and his colleagues speculate, the shift stems from changing conditions: When the environment changed, species that once were barely making a living were able to reproduce and thrive, while those that had been dominant began to languish. “It’s like the [microbial] community is preprogrammed to survive,” says Baross.
The team’s findings show that the microbial communities have changed with time but don’t indicate precisely when or specifically why that shift occurred, says Jack A. Gilbert, a molecular ecologist at the Plymouth Marine Laboratory in England. It’s possible, he notes, that some changes in microbial diversity at the vents could stem from season-to-season changes in the amount of organic material falling from surface waters. “The ecosystem is so horrendously dynamic, it’s hard to tell just from these few data points,” he notes.
Even so, he adds, the team’s results offer some support for the “everything is everywhere” notion of microbial ecology. According to that idea, many types of microbes are present in a wide variety of environments, but sometimes only in small numbers. Rare varieties can rise to prominence, however, given the right shift in environmental conditions.
