We Blogged It!
Lat/Long: 04.54N 051.22.53W
Air Temp:28.6C 83.4F
Surface: 29.6C 85.28F
Salinity: 21.75 psu
Today we reached what is probably the farthest point south from the US we will travel (see lat /long above). We are about 20 miles off the coast of French Guiana. Just a couple of days ago we were sampling in depths of 3000m; today we’re sampling in 50m waters! And the color of the sea has changed again under the influence of the fresh water in the plume- its back to a brownish-green color. The water samples we took this afternoon were teaming with microbial life-especially diatoms.
Dr. Doug Capone, professor of Environmental Biology at USC explains that the Amazon plume waters bring excesses of key nutrients necessary for marine phytoplankton growth to the ocean. These nutrients and their penetration out to sea will determine what types of phytoplankton we’ll find in the plume. Combined nitrogen (as nitrate, one type of nutrient) from river waters is used by coastal phytoplankton and is removed early on in plume evolution.Ammonium (another form of combined nitrogen) is important to phytoplankton in the open sea. It’s produced by other microbes which recycle it from organic matter and is generally found in higher concentrations in the river than in the open ocean. Another nutrient from the river, phosphorus (as phosphate) can be critical in constraining the productivity of a system. It’s taken up quickly and recycled (like ammonium). There are very low concentrations of phosphorus in the upper water column. In fact, the N. Atlantic is severely depleted in phosphorus.
The Capone Group is determining the availability of these compounds in the plume and the ocean below it and assessing how phytoplankton are using nitrogen and phosphorus. Doug’s group includes Research Lab Specialists Troy Gunderson and Matt Taihla, and Grad student Laila Barada. Using enriched isotopic tracers of the compounds, they can “trace” its entry into organic matter as its taken up from the surrounding environment.
Doug adds “When combined nitrogen concentrations become vanishingly low, nitrogen fixing organisms (termed diazotrophs, organisms which can use the massive pool of dissolved nitrogen gas in the water, rather than depending on combined nitrogen such as ammonium or nitrate) gain a competitive advantage and proliferate. We are also working in collaboration with the Montoya group to determine the importance of nitrogen fixing organisms in the nitrogen cycle of the plume. We are examining the contribution of specific subgroups such as the non-heterocystous cyanobacterium, Trichodesmium, the diatom-diazotrophic associations (DDAs) and smaller single celled nitrogen fixers. We are also examining other phosphorus compounds (other than phosphate) for their possible importance in supporting phytoplankton (and diazotroph) growth.”
The Montoya Group is led by Dr. Joseph Montoya (Georgia Tech) and includes
Dr. Jason Landrum and Grad students Rachel Horak, Julie Grosse, and Sarah Weber.
Working closely with the Capone group, they are interested in the movement of nitrogen and carbon through ecosystems, specifically nitrogen fixation and controls within the Amazon plume.
They use a mass spectrometer to measure the presence of the rarer 15N and 13C stable isotopes relative to the much more common 14N and 12C isotopes. Remember when you learned that the molecular weight of carbon was 12 grams per mol? Well, it turns out that 12C has a very rare cousin that behaves pretty much the same but weighs just a bit more. When organisms take up carbon and nitrogen, they tend to favor the lighter version because it is easier to handle. Measuring the ratio of 13C:12C or 15N:14N in phytoplankton then tells you about how actively they are fixing carbon (photosynthesis) or nitrogen.
A mass spectrometer is also a powerful tool for measuring rates of activity directly using the heavier isotopes as tracers (like a dye). They add 15N and 13C to each sample ( like a dye) and incubate for a period of time to see what the organisms do with that dye. Are they fixing the nitrogen or carbon and incorporating that dye into their cells, or not?
Rachel and Julia are also measuring nutrients for the team. In the natural environment you can also observe what the organisms are doing by looking at the nutrient chemistry (the concentration of nitrates, phosphates, silicates and nitrites) which give the basic parameters for the influence of the plume and offshore waters.
Question of the Day
- Do the bacteria in the water make us sick?
Only a few of them. Bacteria are in every habitat on Earth, growing in soil, hot springs, radioactive waste, water, as well as in organic matter and the live bodies of plants and animals. Bacteria recycle nutrients, with many steps in nutrient cycles depending on these organisms, such as nitrogen fixation.