Taking the Ocean's Pulse
3 Apr 2007
(Source: Astrobiology Magazine)
By Henry Bortman
Near the town of Moss Landing, California, midway between Santa Cruz to the north and Monterey to the south, the Salinas River empties lazily out into the Pacific Ocean. What makes this otherwise unremarkable juncture unique is that it lies at the mouth of Monterey Canyon, one of the longest and deepest underwater canyons in continental North America. Virtually unknown, and largely unexplored, the sheer size of Monterey Canyon puts it squarely in league with its celebrated upcountry counterpart, the Grand Canyon. Which explains why not one but two major marine research centers - Moss Landing Marine Laboratories and Monterey Bay Aquarium Research Institute (MBARI) are headquartered in Moss Landing.
There are other extensive canyon systems dotted about the floor of the world's oceans. But it is rare for scientists to be able to walk across the street, hop on a boat, and within three-and-a half hours, be poised to drop a submersible vehicle overboard for a journey more than 4,000 meters (2.5 miles) down into the deep.
So when MBARI's Chris Scholin invited me out for a day on the water, to watch as he performed the a deep-water test of his Environmental Sample Processor (ESP), I eagerly said yes. Then he told me the boat would be departing at 5:30 am.
The ESP is designed to slurp up a liter or two of ocean water, filter it to capture any microorganisms that may be present (a fair amount of gunk comes along for the ride), break open the microbial cells, tag the DNA of pre-selected organisms with a luminescent chemical, and capture a digital image of the resulting glow. It does all this within the confines of the core unit, a metal cylinder that looks like a downsized oil drum - roughly 16 inches in diameter and 3 feet tall. Its innards are a labyrinth of plastic tubing, valves, a couple of syringes to push the water around from place to place, IV bags filled with various types of aqueous chemistry, an Intel ARM processor, 32 megabytes of RAM, a handful of controller boards and a digital imaging device raided from a camera designed for use by amateur astronomy buffs. The ESP's casing, which is machined from a solid block of Nibral, a corrosion-resistant bronze alloy, has 1.5-inch-thick walls and weighs about 700 pounds.
Inside the ESP, all this processing takes place within small pucks, squat metal cylinders the width of a quarter and half an inch tall. Several different types of pucks are used. The first one contains a 22-micron filter - 22 microns is about one-tenth as wide as a tiny grain of sand - that extracts all of the particulate matter, including microbes, from the ocean water. A total of one liter of water is pushed through the filter by a syringe, 25 milliliters at a time, a process that takes more than an hour. What follows is an intricate sequence, during which various fluids are squirted out and sucked up by a pair of syringes, through the various pucks. The final puck houses a DNA probe array containing four dozen biochemically treated dots, each capable of snagging a strand of ribosomal RNA from a specific microorganism. Once all the sucking and squirting is done, the dots on the array glow with varying intensity. If a dot glows brightly, it means the water sample contained a high concentration of the particular genetic sequence the dot was primed to detect.
The top of the final puck has an opening through which the ESP's camera captures its image of the probe array. While the ESP is working, up onboard the Point Lobos, the research team sits around waiting for the device to send an image. The picture isn't all that impressive: it's a grayscale picture of dots. What's impressive is that the ESP performs this entire sequence, from ingesting the initial water sample to sending home a polka-dot postcard, without human intervention. With that postcard in hand, the researchers know what organisms are living in the water. (Actually, during these test runs, they already know what organisms are living in the water. That's how they know what DNA sequences to probe for.)
Last year Scholin's team tested the device in the surface waters of Monterey Bay. That was the first step. The second step was going deep. As we headed out into Monterey Bay, we were embarking on day two of a three-day series of deep-water tests.
To perform in deep water, the ESP needs a front end, a separate piece of equipment that can collect high-pressure water and depressurize it before sending it to the core. All of the fluids in the core unit are kept at close to one atmosphere, slightly above 15 pounds per square inch. The pressure at 1,000 meters is 15 hundred pounds per square inch. If such highly pressurized water found its way into the ESP, it would break its low-pressure seals, flood the instrument and destroy it. The sampler consists of two metal cylinders, each with an internal piston. As the pistons move up and down, they alter the pressure in the cylinders. The sampler first collects high-pressure water in the larger of the two cylinders; then the pistons move, siphoning off some of the water into the smaller cylinder, which reduces the water pressure to a usable level. The smaller cylinder then feeds 25 milliliters of water at a time to the core. On the first day of testing, the ESP successfully acquired and processed a sample at 500 meters (1,640 feet). On day two, the goal is 1,000 meters, and things aren't going so well.
For deployment in the ocean, the entire ESP - both the core and the sampler - are mounted on a "sled," a four-foot-tall open aluminum frame, which is attached beneath the Ventana, one of MBARI's underwater ROVs (remote operated vehicles). The Ventana is connected via a cable to the ship's power and electronics systems, and in turn the ESP is connected to the Ventana. The entire assemblage, which weighs about 3,200 kilograms (about 7,000 pounds) is lifted over the side of the boat by a massive crane and then released into the water. As the ROV descends, its yellow-clad umbilical cord unwinds slowly from a massive spool.
Belowdecks is a sophisticated control room - it looks like a compact version of JPL's Mission Control - full of floor-to-ceiling banks of computer and video screens that display various status indicators and underwater images. This is where the team waits for the ESP to send them its picture. But on this test run, before the dots arrive, they are watching a different image. It comes from a camera, mounted on the sled, trained on metal actuators - they look like a short stack of small, shiny knobs - that track the motion of the sampler's pistons. While the sampler is depressurizing its water, everyone sits around the control room watching the actuators move up and down. By noting how fast the actuators move, and in what pattern, the ESP team can tell whether or not the depressurization process is proceeding as it should. And today, unfortunately, it is not.
Scholin is unfazed. He explains that this is precisely what field tests are for: to shake out bugs. After several hours of experimentation, the problem is solved - or at least identified. Two things appear to have gone wrong. The first is that one of the bags of fluid used by the ESP during its multi-step procedure was never filled. The good news is that it will be an easy problem to fix, and won't require disassembling the unit. The second problem is more subtle: the pressure sensors in the sampler aren't staying in synch with the motion of the pistons; the piston in the smaller cylinder keeps overshooting its target, oscillating back and forth between too much and too little pressure. There is already a plan to fix the problem: swapping out some of the unit's narrow tubing with wider conduits, which will allow the water to move between the cylinders, and the pressure to change, more quickly. For now, though, there is nothing to be done but to haul the Ventana back up on deck and head for shore.
Later in the week I emailed Scholin to see how the third day had gone. He was thrilled. "We were able to pull a full volume sample at 1,000 meters and the probe array looked great ," he wrote back. It was, he said, a "great illustration of doing something for the first time, in the field, not having the benefit of being able to test the integrated system in the lab."
Scholin has been working on various incarnations of the ESP for nearly 14 years. Back in 2004 he applied to NASA's ASTEP (Astrobiology Science and Technology for Exploring Planets) program for a three-year grant to develop the ESP-II, the generation he's now testing. Next year, a unit will be sent into the deepest region of the Monterey Canyon, between 3,000 and 4,000 meters, using an ROV for deployment. Following that, it will be connected to MARS (Monterey Accelerated Research System - no relation to the planet), a network of deep-water monitoring equipment currently being installed at a depth of 860 meters in the Monterey Canyon. It will remain there for a short period of time, sporadically collecting and processing water samples, in response to chemical and temperature changes in the deep-sea environment. This deployment will enable MBARI researchers to track how microbial populations respond to environmental changes, such as temperature and ocean chemistry.
A similar installation will then take place in the Pacific Northwest, near Axial seamount, an active underwater volcano on the Juan de Fuca Ridge, where the ESP will eventually remain submerged for a full year. Marine researchers in Washington State and Canada are constructing an underwater observation network, NEPTUNE, on the Juan de Fuca tectonic plate, with one of its nodes at Axial. Deploying an ESP unit on the NEPTUNE network will allow scientists to study how fluid flow from the seafloor affects the abundance of different organisms in the deep-sea microbial ecosystem. It may also teach them about organisms that live in the ocean crust, organisms that are usually inaccessible, but that can be ejected into ocean water during an eruption.
The ESP as it exists today is unlikely ever to be used on another planet; none in our solar system is known to have large bodies of liquid water. But Jupiter's moon Europa is believed to harbor a vast ocean beneath its icy crusts, and it is possible that some day a mission to Europa could melt a hole in the moon's surface ice and send a miniaturized derivative of the ESP down to explore the ocean depths for signs of life. The ESP's search strategy would have to be modified a bit. It wouldn't make sense to look for specific terrestrial organisms on Europa; rather, a more generic set of chemical tests would have to be employed. With modification, the ESP could also be used in surface operation, to analyze powdered rock, soil or ice.
But imagine the excitement if an ESP-like instrument onboard a Europa deep-sea explorer one day sends back a picture of an array of bright dots. It might not be much to look at, but it could carry a profound message: that we are not alone in the universe, that there is life on another world.