Life and dinner among the microbes
Welcome to the PROTATAX23 expedition to Axial Seamount. On board the research vessel Thomas G. Thompson, a team lead by WHOI microbiologist Julie Huber is getting ready to use the remotely operated vehicle (ROV) Jason to study microbes that form the foundation of the food web in the deep ocean.
In a few tides, the Thompson will sail into the Pacific and head for a location about 300 miles west of the Oregon coast. Axial Seamount is the best-studied underwater volcano in the world, with regular mapping expeditions and sensors from the Regional Cabled Array, a part of the NSF-funded Ocean Observatories Initiative that sends real-time seafloor images and oceanographic data back to shore. It is also a site where scientists accurately predicted an eruption in 2015 and where they are eagerly expecting the next eruption very soon.
But this science team isn't here to study the volcano itself. They are here to study some of the smallest members of the volcanic deep-sea food chain: bacteria and archaea and the single-celled microbial eukaryotes-or protists-that eat them.
Axial is home to many active hydrothermal vent fields, deep-sea hot springs where superheated water from Earth's interior mixes with cold seawater to create oases of life. The bacteria and archaea that live at vent sites can convert the chemicals in hydrothermal fluid into organic carbon-a process known as chemosynthesis. While these microbes thrive in the venting fluids, creating a hotspot of activity in the deep ocean, they also become prey for the protists.
Exactly how many get eaten, and at what rate, is unknown, but it is important to get a more comprehensive understanding of how carbon and other nutrients move within and out of deep-sea hydrothermal vents. To do that, this team is going to need water samples. A lot of water samples
Over the course of the expedition, Jason will dive to vent sites around the seamount to collect samples and run experiments, all the while sending back video from the seafloor to the Jason Team and the science party in the vehicle's control van.
The afternoon before sailing, Jason is being fitted with a new sampler to enable the scientists' research. The SUPR Sampler-which stands for Suspended Underwater Particulate Rosette-was built by WHOI Engineer Kaitlyn Tradd. Based on another sampler developed at WHOI, this one is customized to gently draw in water at vent sites without destroying microbes before they reach the lab. It has custom filters, sample bottles, and a temperature probe.
In the ship's main lab, Sarah Hu, a co-principal investigator and professor at Texas A&M, is setting up a workspace where she and several graduate students will process samples acquired with SUPR. They will feed previously captured microbes that they have treated with fluorescent dye-visible under a microscope-as prey to the protists in the water samples and quantify the number of prey the protists consumed and how quickly they do that. They will also measure the microbial biomass and genomic content in each sample.
Across the room, WHOI scientist Maria Pachiadaki, also a co-principal investigator on the expedition, is setting up the miniSID-another sampler based on earlier models developed at WHOI-that will be deployed over the side of the ship and set up to work autonomously on the seafloor near Jason. The miniSID will run the same experiment that is happening on the ship, but it will do so while submerged at the vent site. Doing the experiment in situ and immediately after a sample of microbes will allow the team to understand the impact of the depressurization and manipulation on the samples they bring to the surface-and allow them to get the most accurate grazing rates possible.
While this team has some deep-sea mysteries to solve, there is a lot they already know, about deep-sea microbes in general, and the ones that live at the Axial vents in particular.
Axial is one of few places in the world where scientists like Huber have spent years studying the base of the food chain. That means that they have a wealth of historical data to work with and know exactly what sites to sample. Hu has studied surface water microbial food webs and wants to apply that knowledge in the deep sea. And Pachiadaki has studied the rates of microbe grazing-or feeding-in the water column of the deep sea.
"The whole ocean food web is driven by microbes," says Huber. "We understand that very well in the surface ocean-we can measure primary production through photosynthesis, grazing rates, and look at how carbon moves up through zooplankton and fish. Now we're trying to understand how it works in the dark at the bottom of the ocean."
- Hannah Piecuch
This NDSF blog series will follow the PROTATAX23 expedition to Axial Seamount, covering the science and scientists at sea, and the ROV Jason operations that make it possible. PROTATAX23 is funded by the National Science Foundation (NSF OCE Award #1947776).