By Veronica Lance, LDEO
If you have been following the science of the Southern Ocean GasEx project, you know that our major goal is to measure and better understand what controls the rates of exhange of carbon dioxide (and other gases) between the atmosphere and the surface ocean. Where do phytoplankton fit into this picture?
Phytoplankon, the microscopic single-celled “plants” of the sea function similarly to terrestrial plants – that is they take up inorganic carbon, then, using the energy of sunlight and an important enzyme (“RuBisCO”) form chemical bonds among carbon atoms leading to the production of simple organic carbon molecules such as carbohydrates. This “fixed” carbon plant material forms the base of the food chain – small metazoans and zooplankton feed on phytoplankton, which in turn feed fishes and marine mammals. In the Southern Ocean, this “food chain” has been found to be quite short and direct as compared with many other ocean environments…. As few as 3 links to the “top”: Diatoms … krill … whales!
Our understanding is, that over geological time scales, the ocean has been acting like a big sponge soaking up some of the high concentrations of atmospheric carbon dioxide during warm periods and perhaps ventilating carbon dioxide out of the oceans back into the atmosphere during cold, glacial periods. In modern times (the “Anthropocene Age”) the ocean has been soaking up some proportion of the huge amounts of carbon released into the atmosphere by the burning of fossil fuels and by other respiration processes accelerated by industrialized methods (for example, large scale agricultural practices). In an imaginary bathtub-in-a-closed-room model ocean, the gases in the atmosphere (room air) will equilibrate with those in the water (bathtub) – that is, the water will absorb the gases until it can absorb no more and the conditions will be steady or stable. If we make now make our bathtub very, very deep and add some phytoplankton, the conditions are no longer stable. The phytoplankton will use up some of that dissolved inorganic carbon. Eventually some of that carbon material (now in organic forms such as dead phytoplankton cells or fecal pellets from animals farther up the food chain or bacterial clusters of decaying matter) will sink very deep and become isolated from the surface waters where it was formed. As the organic carbon disappears from the surface ocean, more carbon dioxide from the atmosphere has a chance to be soaked into the surface ocean. The fate of the sunken carbon is that it could be respired again on a relatively short time scale (geologically speaking, years to hundreds of years) and be circulated back into the surface ocean (for example by upwelling) OR it could be buried into the sediments for relatively long time scales (thousands to hundreds of thousands of years). This process has been given the descriptive phrase “biological pump” (see picture below). ￼
In 1952, Steeman-Nielsen described the method he developed for estimating organic carbon productivity in the sea using radioactive carbon (14C) as a tracer. In light of the many high-tech methods being used on this cruise, this one feels old-fashioned now – but the beauty and satisfaction of this method is that it pretty much always works! There are some finer details about what this method actually does or does not measure which I am not getting into here, but in a general sense, we can get a good estimate of the rate of net carbon uptake by phytoplankton throughout the depths of the ocean where sunlight penetrates (the “euphotic zone”) on a daily basis in a given water mass (“primary productivity”). These rates will go into the bigger carbon flux equations that the SO GasEx project is all about.
I’m sure my colleagues think I am nuts, but I enjoy doing these incubations. While we are at sea, I see almost every sunrise and many sunsets in all kinds of weather. Some of the scientists aboard collect water samples and bring them back to the lab for analysis. My 14C samples get analyzed “live” and so it is satisfying to be able to walk off the ship at the end of the cruise already knowing something about my observations and being able to start thinking about how they fit into the bigger pictures of the SO GASEX project and the regulation of primary productivity in the Southern Ocean (see Fig. 2 for example of data).
For the SO GasEx project, I do several different kinds of measurements, but I am doing the primary productivity work as a collaboration between my Lamont group (with Bob Vallaincourt and John Marra) and Pete Strutton (aka “bottle cop”) of Oregon State University. Basically, I collect water from the CTD rosette from several representative depths in the ocean into clear, polycarbonate, well-washed bottles. Next, I inoculate the water sample with a small amount of the radiotracer 14C and put the “spiked” jars into the on-deck incubator. The incubator (see pic below) is simply a plexiglass box which contains several tubes which are shaded to imitate light levels at the respective depths of the ocean from where we collected the samples. Seawater is pumped through the box to keep the samples at ambient temperatures. The spiked sample jars go into their respective light tube and there they will stay from dawn to dusk or from dawn to the following dawn (depending on the precise observation desired – I am doing both types on SO GasEx). The community of organisms trapped in the bottle are both “fixing” and respiring carbon – and some of the tracer 14C along with it. At the end of the incubation, the jars are gathered from the incubator and the seawater samples are filtered. The filters collect all the organisms in the jar – some of which have now incorporated some of the radiotracer. The filter goes through a few more procedures and then is placed into a liquid scintillation counter which is a way of determining the disintegration rates and ultimately the amount of the 14C tracer in the sample organisms. With a few more calculations, we end up knowing something about the rate of carbon uptake in the water column. A plot of an early station in SO GasEx is shown below. At the same time, Pete sets up similar dawn-to-dawn incubations using the stable isotope 15N as a tracer to determine the rate of nitrate fixation (perhaps the subject of another blog someday). The ratio of carbon uptake to nitrogen uptake gives us some clues as to how fast the fixed carbon might be sinking into that very deep bathtub ocean. Every morning at 0430, Pete knocks on the door of my rad van where I have been prepping my samples. Together in the pre-dawn greyness, we go out to the incubator to harvest our previous days’ samples and put out the next set (see pics below).
￼p.s. All this talk about radioactivity might provoke some health and safety concerns. We are concerned, but more for scientific reasons than health or human safety. This is why: First, the amount of radioactivity we work with is very small with respect to human health concerns. Second, humans are not plants, so any 14C tracer to which we might be accidentally exposed is not taken up into our bodies. Third, it is easily washed off of surfaces with a bit of soap and water – which is later properly disposed. Our biggest concern is not to leave any traces of 14C on surfaces of the ship. Why worry about the ship? Because other scientists who use the Ronald H. Brown measure ambient concentrations of carbon isotopes including 14C. Natural 14C concentrations are very low so that even miniscule contaminations, hardly noticed by our scintillation counting methods, could skew the measurements of the natural concentrations. So, out of respect for future science, we go through several steps to insure no radioactive carbon escapes out of our control. Our work is done inside a special container van (“rad van”) placed outside on the deck of the ship (see pic below). We wear special lab coats and booties which remain inside the rad van – it’s kind of like going through an air-lock. The samples which go out to the incubators have a short trip of only a few meters from the van to the incubator and are well-sealed.
The “biological pump” cartoon drawn by Zackary Johnson.
Light tubes in incubator with flowing seawater.
Harvesting an incubation just before sunset.
Pete and I setting out our daily 24-h 14C and 15N sets at dawn.
Rad van being set into place on the deck of the NOAA ship Ronald H. Brown.
At work inside rad van. The red lights are to keep the phytoplankton “asleep” before they go into the incubator for the day.
Thanks to Bruce Hargreaves, Bob Vallaincourt and Paul Schmeider for photos.