The Iron Eaters of Loihi Seamount

From June 25th to July 7th, Brian Glazer, an oceanographer at the University of Hawaii Manoa (UH) led an expedition to Loihi, whose base remains largely unexplored. He and his team from UH, as well as colleagues from the University of Minnesota, IFREMER Centre de Brest, and Woods Hole Oceanographic (WHOI), surveyed the seamount’s deeper reaches using Woods Hole Oceanographic’s Sentry autonomous underwater vehicle (AUV), and collected water samples to better understand the processes impacting delivery and dispersion of hydrothermal fluids from Loihi to the Pacific Ocean. Originally, the team was scheduled to use Woods Hole’s innovative Nereus hybrid remotely operated vehicle (HROV), but tragically the vehicle was lost earlier this year.

Using Sentry’s high-resolution camera and sonar systems, as well as chemical sensors, the team was able to gain a better understanding of processes underway at two known sites where bacteria are feeding on or oxidizing iron. They also gathered clues as to how widespread such bacterial communities might be in unexplored areas. The team also used the vehicle to gather new data from the seamount’s relatively more explored summit to better understand connections between it and activity toward the base.

Watch the video produced with footage from this cruise:

Discovery

Glazer and his colleagues first discovered the iron-oxidizing bacterial formations at the base of Loihi by accident. They were looking for some bare rocks to compare against samples they had been collecting at the summit. But then, in between a few basalt boulders they found huge microbial mats—given away by telltale signs of orange layers, or flocs, between volcanic rock formations. The color comes because the bacteria are essentially just rusting—or oxidizing—iron from hydrothermal fluids and seafloor rocks.

Further investigation would reveal that in places there were ponds of microbial material a meter or more deep, and that a hardened, blackish layer of oxidized iron and manganese covered the mats. These features were much larger than anything seen at Loihi’s summit or other hydrothermal vent fields. They named the site FeMO Deep (Fe= the abbreviation for iron, M=Microbial, and O=Observatory), and would return to it three times on subsequent expeditions. The team never had the opportunity to fully explore the surrounding areas, and numerous questions remain about what they have already seen.

What’s Going On?

Mats of iron-oxidizing bacteria are not uncommon in hydrothermal vent fields with warm or hot water flows, but there the mats are not especially extensive. What Glazer and his colleagues found appeared to be something new. There were no vents around and only a slight 0.2°C temperature change. That indicates that hydrothermal fluid—seawater that cycles down into the seafloor, gets warmed by Earth’s internal heat, then reemerges—is flowing out of the seafloor in the vicinity. But, given the temperature, that flow has to be exceedingly diffuse. So it’s not at all clear yet why or where the mats form.

Glazer would eventually come across an old cruise report from the 1990s written by a Japanese and American team describing similar iron-rich deposits at Loihi’s base, but with hydrothermal waters that were about 6°C higher than background seawater. There have also been more recent reports of potentially related mats at places like the Gakkel Ridge in the Arctic, and near the Galapagos, but there is only limited information available on these sites.

Using Sentry, the team was aiming to map about 40 square kilometers around the seamount’s base, or roughly ten times more than had ever been explored. Temperature, oxygen, and pH sensors, along with photos, were used to identify telltale signs of hydrothermal water flows, that might reveal new iron oxidation or related sites. The researchers hope to return to the base in coming years with a remotely operated vehicle (ROV), to study promising sites, particularly to confirm whether there might be additional iron mat concentrations.

The team also configured Sentry during certain dives to filter water samples and analyze for more specific details about how iron and other chemical components cycle around the seamount. Related work at the summit, including water sample collections using Falkor’s CTD rosette allowed comparisons between the chemistry of the two areas, and provided more information about iron cycling around the seamount, and how much iron the hydrothermal activity there is contributing to the surrounding ocean.

Why It’s Important

On short timescales, iron-oxidizing microbial activity like what is occurring at Loihi could make a previously unappreciated contribution to the ocean’s iron and carbon cycling—a critical driver in overall ocean balance—particularly if exploration proves the activity is more widespread. There are also implications for longer timescales as well.

Reddish geological features found on land and in the seafloor (umbers) contain high concentrations of iron and manganese, just like the formations at Loihi. These umbers also have structures similar to the crusts at Loihi, with alternating layers of oxidized iron and manganese. Loihi may just give scientists a glimpse of how umbers formed. If so, the work could help answer lingering questions on whether umbers formed biologically, or if they were simply the result of geology and chemistry.

Beyond better understanding of Earth’s geological record, the work aboard Falkor, combined with follow-up research on future expeditions, could have implications for the search for life elsewhere. If, for example, researchers can identify a definitive chemical signature for geological features formed by microbes like those around Loihi, it could ultimately allow scientists to decipher whether similar geological features on places like Mars were biologically produced—a potentially simpler task than finding living cells with limited exploration.

by Mark Schrope