The HOG Sampler

One of the major outcomes of this expedition has been the success of the HOG sampler. The large volume sampler for Hydrothermal Organic Geochemistry was designed and built by Chief Scientist Susan Lang and her lab at the University of South Carolina. It was the main workhorse of this expedition, collecting large volumes of hydrothermal fluids (we use the word “fluid” to describe the water, minerals, and gases coming out of hydrothermal vents) from Lost City chimneys that we will use to address our primary research questions. Susan and her team have been designing, building, and testing the HOG sampler for the past two years, and the Lost City 2018 expedition was the maiden voyage of the fully functional sampler. The fact that the HOG performed so well on its first big test is a testament to the Lang lab’s preparation and attention to detail, particularly by Bryan Benitez-Nelson who was meticulous in everything from choosing fittings, calculating how many bottles would fit in a given space, and designing bench tests. The HOG’s sturdy build is thanks to Allen Frye in the UofSC machine shop, who translated pen and paper sketches into elegant solutions.

The HOG collects fluids through a long, titanium nozzle that is inserted into the source of the fluid to be sampled (e.g. a chimney). Collecting water into a container is one of those simple actions that is trivial when conducted by a human on land but an incredibly difficult and complex physics problem when conducted by a robot on the seafloor. In addition to the usual challenges of working in the deep sea (pressure at depth, saltwater corrosion, the need for all fittings and materials to be water-tight), collecting water from a hot spring on the seafloor has the inherent complication that the water you want to sample is already mixing with the surrounding seawater as soon as you are able to see it.

For this reason, the precise positioning of the HOG’s intake nozzle was critical to the quality of the sample and usefulness to our research. If the nozzle is one centimeter too far in any direction, we could be sampling boring, normal seawater and not hydrothermal fluids venting from the subsurface. The ROV Jason pilots were superb at positioning the HOG’s nozzle in the exact spot we requested at each chimney. With some samples, we were able to get real-time feedback on the quality of the sample from an in-line temperature probe connected to the HOG’s nozzle. A scientist sitting in front of the temperature display would call out temperatures while the Jason pilot would gently nudge the nozzle one millimeter here or there until we found the highest temperature (indicating mostly hydrothermal fluid and not seawater).

Once the hydrothermal fluid was drawn into the nozzle, it would be suctioned into one of several collection bags installed in the main body of the HOG. The collection bags are water-tight, mostly gas-tight and hold up to 2 liters of water. We also had some success collecting fluids into huge bags with a maximum capacity of 11 liters. On the ship, each HOG water sample was split into more than a thousand subsamples that will be subjected to a comprehensive array of analyses including measurements of pH, alkalinity, dissolved ions, metals, nutrients, organic compounds, hydrogen and methane gas, helium, radium, strontium, and microbial cell and viral abundances. HOG samples will also be used for microbial cultivation experiments and environmental DNA sequencing studies. One of the key analyses for this expedition will be the measurement of carbon-13 and carbon-14 isotope ratios in specific organic compounds dissolved in the hydrothermal fluids, which should give us a good idea of which organic compounds are derived from the Earth’s mantle (and possibly as old as Earth itself), which organic compounds are mixed in from seawater, and which of these organic compounds are eaten by specific microbes. The variety and depth of these analyses is enabled by the large volumes of water that the HOG sampler collected.

In addition to collecting samples, we used some of the HOG bags as incubation experiments. We added an isotopically-tagged molecule (e.g. methane gas) into the bag prior to the dive so that when fluids were drawn into the bag, the resident microbes were exposed to the tagged molecule. Our laboratory analyses of these bags will test whether the microbes consumed the tagged molecule and released it as a byproduct, and if so, investigate which species of microbes were involved. By integrating these experimental results with the comprehensive chemical analyses to be performed on the pristine samples, we hope to trace the flow of carbon from the basement rocks underneath Lost City to the metabolism of specific members of the microbial community.

During our six days at Lost City, the HOG collected 101 samples of hydrothermal fluid and seawater, comprising a total of 272 liters. These samples were split into >1000 subsamples to be analyzed in our respective laboratories. We also collected many other kinds of samples by other means. See the previous blog post for a short list. We will be busy.

In this video, the HOG intake nozzle is carefully positioned in the precise location where the hottest, highest pH hydrothermal fluids are exiting the chimney before mixing too much with seawater.