Editor's Note: Journalist and crew member Kathryn Eident and scientist Jeremy Jacquot are traveling on board the RV Atlantis on a monthlong voyage to sample and study nitrogen fixation in the eastern tropical Pacific Ocean, among other research projects. This is the fourth blog post detailing this ongoing voyage of discovery for ScientificAmerican.com.
RV ATLANTIS MAIN DECK—Of the metals readily available on Earth, iron is one of the most abundant. We come in contact with iron in some form every day—whether it's in the air we breathe, or the metals we use in our tools, in our buildings and our vehicles. It's even in our bodies.
The world's oceans get much of their iron supply from deserts. Winds carry dust from the wilds of Africa and the dunes of the Middle East and seed vast bodies of water with this essential nutrient for photosynthesis. But, as scientists are learning, the winds can only carry dust so far, and portions of the southeastern Pacific are a virtual dead zone of iron.
This iron deficiency has forced scientists at sea on the RV Atlantis to ask the question: does the lack of iron affect nitrogen fixation? What makes this question doubly hard to answer is the fact that the waters they are sampling in, the Eastern Tropical South Pacific, are also nitrogen deficient, which may mean nitrogen fixers aren't here at all.
"We are trying to understand which nutrients control nitrogen fixation in this area," said Sophie Bonnet, a marine biogeochemist studying the impact of various nutrients on nitrogen fixers. "I have the feeling they [nitrogen fixers] could fix at higher rates if some nutrients were here."
Iron is crucial for the ocean's nitrogen fixers, those filamentous and uni-cellular organisms that convert nitrogen gas into more usable forms such as nitrate or ammonium. As they make the conversion, these organisms can use anywhere from 10 to 100 times more iron than a creature that simply needs iron for the photosynthesis process, said Bonnet, who hails from the Institute of Research and Development in Marseilles, France.
Together with Julien Dekaezemacker, a PhD student at the University of Marseilles, Bonnet has set up an elaborate system of water filtration and incubation, spiking dozens water samples (adding measured amounts of varying nutrients) to stimulate both photosynthesis and nitrogen fixation, and later measure the outcomes.
At each station, Bonnet and Dekaezemacker set up pumps that bring water onto the ship from 30 feet below the ship's keel. They work together with Dr. James Moffett (USC), using his specially-designed trace metal-free conductivity, temperature and depth (CTD) rosette, an instrument that can gather water from varying depths in the water column.
After holding aside one set of bottles as a control, they then add varying amounts of iron, phosphorus and even glucose to see what combination of nutrients organisms may respond the most and let them incubate for 24 hours.
"We fertilize them [some bottles] with iron to try and simulate the concentration of dust," Bonnet said. "And in some of them we add nutrients to try to understand...when it's not iron limited anymore, what is the next limiting factor? Is it more nitrogen? Is it more phosphorus?"
Then they add tracers (in this case, varying forms of nitrogen) and incubate the samples for another 24 hours. Finally, they filter the water onto small, glass-fiber filters that will record what changeshave taken place in that time span. In all, they use 60 bottles at each station, which adds up to hundreds of samples, Bonnet said.
They have to be extremely careful with their experiments, too. Since iron is such an abundant substance—especially on a metal ship—Bonnet and Dekaezemacker have to ensure that no part of their experiment, from gathering water, to bottling it, spiking it, filtering it and storing it, becomes contaminated with traces of iron.
That means they can't use anything that may have come into contact with iron, like metal instruments, and they can't even expose their samples to the air. They work in what they jokingly call " the bubble": a plastic enclosure with highly filtered air, and they use only specially-designed plastic tools and bottles. Before entering the space, they take off their shoes, and they don lab coats and gloves to ensure they haven't tracked iron traces in with them too, Bonnet said.
This work, and their collaboration with many of the scientists on this trip, will eventually allow them to measure a variety of parameters and collaborate with many of the scientists on this voyage. They'll measure how much of the nutrients are consumed in each bottle, calculate nitrogen fixation rates, assess whether there is an increase in biomass, and identify what types of organisms are responding to the experiments.
"Who liked iron the most?" Bonnet said. "It's helpful to know who was happy about that."
Traditionally, scientists interested in nitrogen fixation have focused on one of the most famous nitrogen fixers—an organism on which Chief Scientist Doug Capone is an expert—trichodesmium. This multi-cellular organism, shaped like filament or a piece of human hair, floats on the surface in visible brown patches. But these organisms need particular conditions such as a lot of iron, high water temperatures and low levels of oxygen to fix nitrogen, Bonnet said.
"It's true that when they are here, they fix a lot of nitrogen," she said. "But they are very complicated."
Bonnet thinks there are other organisms that also use nitrogen to flourish. For instance, much smaller uni-cellular organisms like pico and nano-plankton and some bacteria may fix nitrogen deeper within the water column, which is why they are sampling with the CTD cast, she said.
"My feeling is that here in these waters we'll encounter more of these little guys than tricho," she said. "We now realize that these small nitrogen fixers account for a large part of the nitrogen fixation in the global ocean."
For all the work of carefully gathering, spiking and measuring their samples, Bonnet and Dekaezemacker won't even know if their experiments are working until they process the data back home in the lab.
"I don't know if there are any nitrogen fixers here, though we have indirect evidence of their presence. If there are, they're smaller fixers," she said. "But still, that can be very interesting."
But if the 28 scientists on board the RV Atlantis do find nitrogen fixation, they will have contributed some of the first ground-breaking data to be compiled about the process in this region.
"That's why we're measuring," she said. "Just to see, just to see."