Southern Ocean Iron Experiment (SOFeX) Cruise
January 5 - February 26, 2002
February 22, 2002: Day 49
Position: 51 degrees, 30 minutes South, 171 degrees, 7 minutes West
R/V Melville Log Entry: We have been focused upon those processes that act to move carbon from the surface waters to the deep sea. This inevitably involves the autotrophic fixing of carbon (plant growth) followed by the sinking of particles or particle products from the surface waters downwards (recall the fecal pellet express?). We have seen that iron is key to the process of taking up carbon and transforming it from a dissolved or gaseous species into a particulate phase (a phytoplankton), this is primary production. But there are many fates to a particle in the ocean and sinking out is only one of them. Particles may aggregate into "marine snow" (side bar: Marine snow is a term used to express the amorphous aggregates of organic material, detritus, phytoplankton, fecal pellets and a bunch of other stuff that is poorly describedit was coined this name by submersible pilots who watch it streak by their portholes like snow falling. I think of marine snow particles as underwater dust-bunnies, and they have been shown to be rich in bacteria. Ok, back to the story.) and sink out, zooplankton may consume or filter out the particles and carry them around, migrate with them in their guts, or excrete them as a fecal pellet. Many different heterotrophic process act to transform the particulate carbon back into the dissolved phase and when this happens, nothing fluxes.
The top dogs of these heterotrophic remineralizers (as geochemists call them) are the bacteria that number over a billion per liter. Bacteria, as marine microbial ecologists say, run the joint. Yep, they are everywhere and they eat reduced carbon products such as phytoplankton and zooplankton detritus. They are also capable of eating dissolved organic material that phytoplankton and zooplankton produce. If the organic material comes from phytoplankton we call it exudation. If it originates from zooplankton we call it, for lack of a better term, excretia. It doesnt sound like a glamorous life living off of fecal pellets, exudates, and excretia, does it? But then again, think for a moment about the bacterial community that lives in your gut (E. coli), the ones that live in your yard and make those fallen leaves disappear over the winter, and the ones that reside at your local waste water management facility and do, wellyou know what they do there. Their oceanic cousins do much the same out here in the Southern Ocean.
So, just as the oceans act as gardens in the surface waters, they also act as compost piles and it is the balance between these processes (autotrophy vs heterotrophy) that will determine the ultimate fate of carbon in the sea.
Director of composting aboard Melville is Jack Oliver of the Virginia Institute of Marine Science. Jack is looking at bacterial production to estimate the rate at which they incorporate substrate and grow. He is particularly interested in the way in which bacteria respond to both the increased carbon (as a result of increased phytoplankton production) and potential increased available iron in the water column following iron addition. Some of the questions he is trying to answer are: Are bacteria also limited by iron availability? Does bacterial abundance increase following iron addition? Can the compost pile keep up with the garden? Jack intends to find out. He is using tritiated leucine and thymidine (a radioactive form of an amino acid and DNA precursor, respectively) to see how much of this the bacteria suck up into their cells.
Bacteria are not unlike any other living cell on the planet (including those inside of you): 1. they contain protein and DNA; 2. they must synthesize new protein and DNA in order to grow and divide. To synthesize protein and DNA requires building blocks like leucine and thymidine. He spikes his samples with leucine and thymidine, lets them incorporate it over a period of time, then extracts the protein and DNA (hopefully with some of the radioactive leucine and thymidine in it). The extracted protein and DNA samples are then counted on a liquid scintillation counter that measures the light emitted from the decay of his tracer. Sparks alive! Their activity will play a big role in the fate of carbon falling from the surface waters and the ultimate removal - and respiration - of carbon dioxide. Bacteria bat last.
- Kenneth Coale, Chief Scientist