Hydrothermal chimneys can be found wherever you have a mid-ocean ridge spreading center. Like geysers on the sea floor, chimneys are formed when hot, mineral-laden sea water emerges from beneath the crust and deposits those minerals in a (sometimes towering) column rich in metals and sulfur. (The one pictured at the right is 9 meters tall.) Microbiologists have studied these chimneys for a while now, examining which bacteria and archaea predominate and what they might be doing while the gushers flow, but a group at the University of Southern California and the University of Minnesota, Twin Cities have taken a look at the communities that exist on chimneys that have long since plugged up and stopped - dormant chimneys, referred to as "inactive sulfides". They found some telling differences between the communities on active chimneys and the ones on chimneys that no longer flow - differences that show inactive chimneys are actually active biogeochemical engines that continue to cycle sulfur, nitrogen, and iron on the ocean floor.
The changeover to becoming a non-productive vent is a significant one, says Katrina Edwards, the senior author on the study. When a chimney stops spouting sea water, "the chemistry [of the chimney] goes from being dominated by fluid emissions - venting of hydrothermal fluids that are not in equilibrium with bottom seawater - to a mineral-chemistry based system" says Edwards. So, active chimneys are dominated by reactions between materials in venting sea water and the surrounding sea water and dormant chimneys are dominated by reactions between sea water and the minerals in what is left of the chimney.
This makes for a big difference in habitats, differences that are reflected in the 16S rRNA sequences of the organisms they find in these two types of chimney. One glaring difference is the ε-proteobacteria and Aquificae: the predominant groups on active chimneys, they are rare on inactive chimneys, where they are probably replaced by α-, β-, δ-, and γ proteobacteria and Bacteroidetes, which seem to thrive there and probably participate in redox transformations of sulfur, nitrogen and iron. Other observations also indicate the communities are undergoing ecological succession, in which old species and processes are slowly replaced by others.
And how long have these chimneys been dormant while the accompanying bacterial communities are busy chugging away within? Co-author Jason Sylvan says they don't know the age of their particular sample sites, but that others working at the 9°N East Pacific Rise site have used lead isotope ratios to date the dormant features in the area from several decades to 20,000 years old.
"Given that the structures persist for thousands of years," says Sylvan, "all the while supporting active microbial communities, it is certainly possible that these communities have an impact on ocean chemistry."
Edwards hopes to do continue her work with the chimneys. "The deposits that are created through hydrothermal venting are poorly understood over long time frames. We know they stick around for multiple decades, but what influence they have on biological and chemical oceanographic processes is uncertain."
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