The Arctic winter is cold and dark everywhere. But under the sea ice, strange things happen. At the bottom of the food chain, phytoplankton, which fix inorganic carbon into usable sugars, don’t grow—there’s no sunlight for photosynthesis. Yet in these inhospitable conditions, some marine bacteria have found a surprising way to eke out an existence.
Heterotrophic bacteria, which normally eat the carbon they need to survive, have a problem if they are living underneath polar ice: how to survive the long winters, where food, in the form of photosynthetically derived organic carbon, is severely limited? While carbon fixation in the dark has been studied in the oxygen-scarce twilight zones of the deep ocean, it hasn’t been considered important in shallower, well-oxygenated waters. Laura Alonso-Sáez, currently at the Centro Oceanográfico de Xixón in Spain, and colleagues have discovered that Arctic heterotrophic bacteria are doing a surprising amount of carbon fixing in the dark (ISME J, 4:1581-90, 2010).
To find out just how important these dark C-fixation processes were, Alonso-Sáez took Arctic seawater that had been depleted of organic carbon and measured both the uptake of inorganic, radiolabeled bicarbonate and bacterial growth. “We suspected that many [species of bacteria] would have to have different types of metabolism” in order to survive, she says. Three months before these experiments, a colleague had collected buckets of icy seawater from the Amundsen Gulf in northwestern Canada, filtered it, and stored it in the dark on board the research ship CCGS Amundsen. When Alonso-Sáez arrived aboard the ship, she re-filtered the aged seawater to remove any bacteria that had grown during storage, inoculated it with freshly collected Arctic bacteria and radiolabeled bicarbonate, and incubated the mixture at 2°C in the dark for 25 days, until the bacteria reached a stationary phase of growth.
The Arctic bacteria incorporated the bicarbonate in the water, but when Alonso-Sáez got back to her lab at Uppsala University in Sweden, she and coauthor Pierre Galand analyzed the genomes of the bacteria they’d collected to see which types of species had grown, and found that heterotrophs, and not autotrophs, were doing most of the inorganic carbon fixing, and thus growing, in the dark. “It was quite shocking for me,” Alonso-Sáez says, to realize that so many heterotrophic bacteria “were not only able, but were incorporating inorganic carbon so actively.” Melissa Garren at the Scripps Institution of Oceanography agrees: “The magnitude of the process they documented was surprisingly large.”
Alonso-Sáez doesn’t think that this happens all the time, but is probably most important under conditions of starvation. Still, it could be a widespread phenomenon in the world’s oceans. Ferdinando Boero at the Università del Salento in Italy notes that in the dark of winter, life either temporarily stops—goes into diapause—or carries on with organisms that can survive under these conditions, until things get better. “The more we study the system in the ‘bad season,’” he says, “the more we will find surprises.”
Most of Alonso-Saez’s research was done aboard the CCGS Amundsen. She is no stranger to shipboard life, having been on polar research cruises in the spring and summer. Three months of Arctic winter was “an interesting experience,” she says. “You try to sample the water, and it’s freezing!” But it was a “great opportunity” and the only way to find out what is going on under the ice, Alonso-Sáez adds. Garren says that setting up all the experiments was a “strong undertaking” from a logistical standpoint, “which [Alonso-Sáez and her team] did well and did carefully.”
The study raises many other questions, such as: How general is dark carbon fixation by heterotrophs? What happens over longer time periods, or wider areas? And how does heterotrophic carbon fixation fit into the ocean’s carbon cycle more generally? Back in Spain, Alonso-Sáez is currently working at a more molecular level, looking at the diversity of bacterial genes involved in the degradation of marine organic matter in the Bay of Biscay, and how that process changes over the year. She hasn’t been put off by her experience in polar waters, though, and plans to return to the Arctic or Antarctic in a year or two to perform more icy experiments.
A Hidden Jewel refers to an article, published in a specialist journal, which has been evaluated in Faculty of 1000, a post-publication peer review service of the Science Navigation Group. Read the evaluation of Alonso-Sáez’s article.