Leg 3: March 8, 2012
Day 8: Analyzing samples
Today we’re transiting south to the Guaymas Basin, so most of our day was spent in the lab processing and organizing samples before the next Doc Ricketts dive. There are many different ways that we look at, test, analyze, and compare what we find here. Our samples include imagery, water, sediment, organic matter, and animals. With these samples we test and evaluate various parameters like biological community structure, chemical composition, grain size, oxygen level, pH, the amount of organic matter present, and biomass (the amount of living matter) on the seafloor.
One of the ways we compare deep-sea biological communities is to video record long “transects” of the seafloor using high-definition cameras. When we get back to MBARI, these recordings will be reviewed and analyzed in great detail, identifying and counting each and every organism. In addition, we document the behaviors and interactions we observe, the geological features of the habitat that animals live in, and the physical properties of the overlying water. It is always quite exciting to find a deep-sea fish or invertebrate that has never been seen before. Just as we found yesterday, many larger animals live under the sediment and are not necessarily visible on video.
In addition, the sediment itself holds an astounding number of small living organisms. We sample the mud to look for these animals. Our sediment cores hold about as much mud as you would find in a soda can. You’d never know just by looking at that small amount of mud, but there can be hundreds of animals inside. Most of these small organisms can only be seen under our microscopes, and include many different types of crustaceans, worms, clams, snails, and other rarer things. Imagine how many of these small organisms exist given that two-thirds of the earth is covered in deep-sea mud!
Some of our sediment cores are used to look at even less conspicuous constituents—chemical content and mud grain size, among others. On this day’s dive, as on previous days, we collected about 30 cores. Once back on the ship they are taken from the ROV and stored in a walk-in cooler kept at a chilly five degrees Celsius (41 degrees Fahrenheit). In the R/V Western Flyer wet lab, we use open-ended syringes to take centimeter-deep (about 1/2 inch) sub-samples from the mud they contain, like miniature cores within the core. Some sub-samples are saved for later analysis; some are analyzed immediately, as we do for adenosine triphospate (ATP), a rapidly degrading molecule common to living cells, thus a measure of living biomass. ATP is chemically extracted from a small amount of mud and mixed with a luminescent compound obtained from fireflies. A sensitive meter measures the light given off when ATP reacts with firefly extract, the emitted light being proportional to the ATP present.
A few cores are kept cold and enclosed in a nitrogen-flushed glove box which helps maintain the mud’s in-situ oxygen characteristics, which would be altered if exposed to air. Inside, slight finger-turns of tiny gears lower electrodes on a micro-manipulator fractions of an inch through the overlying water and into the mud. Temperature, electrical resistance, and oxygen concentration are recorded as the probes are simultaneously stepped down through the mud’s upper two inches. The resulting data profile the oxygen available to the sediment community biota and movement of O2from the water above. The mud’s porosity, the O2 concentration in the water at the sediment interface, overlying water flow, and other factors control the sediment depth where the community uses—respires—all of the O2. This balance point is usually only a few millimeters below where water and mud interface. These data provide valuable context and fascinating comparison for the animals we sift from the sediments of the warm, oxygen-rich Salsipuedes Basin and the cold, oxygen-poor Guaymas Basin
—Linda Kuhnz and Chris Lovera