Cool Hydrothermal Vent Fluids Fuel Sediment Microbes

Lukewarm fluids from cool hydrothermal vent systems supply nitrate and oxygen to microbial communities in overlying sediments.

High-temperature hydrothermal vent systems, like black smokers, have generated of lot of interest from scientists in recent years, but cool hydrothermal systems (CHS) are a common feature of the ocean floor that have received far less attention. These cooler vents emit clear, lukewarm fluids, making them much harder to spot than black smokers.  However, scientists estimate that they spew out enough water with dissolved salts into the oceans to rival the input of these salts from rivers and streams. 

The arm of Alvin, an HOV (human operated vehicle) drives a tube into the sediment over the Dorado Outcrop to collect samples by push coring. Credit: 2013 Woods Hole Oceanographic Institution. Photo courtesy of Geoff Wheat, NSF OCE, DSV Alvin, 2013 ©Woods Hole Oceanographic Institution

To understand how fluids from cool hydrothermal vents might be affecting their local microbial communities, Deep Life Community members Laura Zinke (University of California Davis, USA), Brandi Kiel Reese (Texas A&M University, USA), Beth Orcutt (Bigelow Laboratory for Ocean Sciences, USA), and Jan Amend (University of Southern California, USA) analyzed bacteria and archaea from sediments from the Dorado Outcrop, a seamount off the west coast of Costa Rica that is perfused with these fluids. In a new paper in Frontiers in Microbiology [1], the researchers report that oxygen and nitrate dissolved in the vent fluids appear to fuel a distinct microbial community that differs from those in nearby sediments and seawater. These microbes may be involved with nitrogen cycling, and their activities have the potential to affect carbon storage in seafloor sediments.

“Cool hydrothermal systems are fundamentally different from the hot hydrothermal systems that we normally think of, and that is evident in their microbial communities,” said Zinke. “These systems are probably prevalent in the seafloor and if they’re all like the Dorado Outcrop then there could be more nitrogen cycling than we’ve realized.” 

Orcutt, along with co-author C. Geoffrey Wheat (University of Alaska, USA) and colleagues, confirmed the first cool hydrothermal vent system at the Dorado Outcrop. During a cruise funded by the National Science Foundation (NSF) in 2013, they used a remotely operated vehicle (ROV) to explore the seamount. At first they didn’t see any fluids, just a lot of octopus, but eventually spotted what resembled a “soaker hose” of diffuse fluid spilling out from the crust. They realized that instead of a giant vent, the fluids emerged in faintly shimmering streams from the outcrop. According to Orcutt, the octopuses, which gather around the vents to incubate their eggs are probably the easiest way to spot a CHS.

Octopuses are an excellent indicator that cool hydrothermal fluids are venting nearby. They incubate their eggs near the slightly warmer streams of water. Photo courtesy of Geoff Wheat, NSF OCE, DSV Alvin, 2013 ©Woods Hole Oceanographic Institution

The CHS fluids at the Dorado Outcrop are similar to the surrounding seawater, but slightly warmer, measuring about 10 to 15 degrees Celsius instead of 2 degrees Celsius. “The fluids are moving through the ocean crust so fast that they’re mining a lot of heat, but chemically, the fluids are not changing a whole lot. It still has oxygen and nitrate in it, and those are important substrates for microbes to use,” said Orcutt. 

After the discovery of the Dorado Outcrop, Zinke was lucky to get a spot on the second NSF-funded research cruise to the location in 2014, as a graduate student in Amend’s lab. She was interested in the microbiology of the sediments and collected core samples from the sediments over the CHS and at a nearby “background” location off of the outcrop. Zinke spent long hours processing the cores in the ship’s cold room. Back on land, she sequenced and analyzed the microbial communities with funding from the DCO through the Census of Deep Life and the Center for Dark Energy Biosphere Investigations (C-DEBI).

Zinke’s analysis of the microbial communities showed that there was significant overlap between the CHS sediments and the background sediments. The CHS communities, however, had elevated numbers of microbes believed to be involved in nitrogen cycling, likely stimulated by the nitrate and oxygen carried by the vent fluids. Considering that there may be thousands or millions of similar CHS systems in seamounts worldwide, Zinke suspects that there may be “cryptic nitrogen cycling” going on at numerous locations along the seafloor.

Now, co-authors Wheat and James McManus (Bigelow Laboratory for Ocean Sciences, USA), are looking into how the extra oxygen and nitrate impacts carbon cycling in the CHS. Microbes can “breathe” oxygen and nitrate, and these compounds give them more energy than other compounds that microbes commonly breathe in sediments, like iron and sulfate. The researchers want to know if the extra energy will enable the microbes to take in more carbon to make biomass, or if it will allow them to eat organic carbon compounds more rapidly, which would impact long-term carbon storage in the sediments.  

Orcutt cautions that the Dorado Outcrop is the first CHS to be studied in detail, and that it may not be representative of cool vents worldwide. “We need to go find other sites with cool hydrothermal vent systems and figure out if there are similar patterns,” said Orcutt. “That’s really the next challenge.”

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