An interesting paper come out recently by a group at the University of Gothenburg in Sweden. Working at Svalbard, a common Arctic study site for European researchers, they measured the flux of organic compounds called halocarbons (or organohalides) from young sea ice and frost flowers. Halocarbons are a class of chemicals defined by a halogen atom or atoms (fluorine, chlorine, bromine, or iodine) bound to a chain of carbon and hydrogen atoms. The simplest halocarbons are the methylhalides, which contain only a single carbon.
Halogens are important reactive chemicals in the atmosphere, playing a role in ozone depletion (at ground level, which is good, and in the stratosphere, which is bad) and cloud condensation. Halocarbons, especially the lower molecular weights ones like the methylhalides, are a source of halogens to the atmosphere. When bound up in an organic molecule the halogen atoms are less reactive, and less prone to reacting with large, heavy molecules than when free. This enables them to leave their site of origin (sea ice, seawater, leaf litter, a tree…). Once in the atmosphere the light, unreactive halocarbons can be split apart via sunlight driven reactions, freeing the halogen as a reactive ion.
The concentration of common halocarbons in the atmosphere. Note the strong seasonal cycle for methylchloride, an indication that it is derived from photosynthetic organisms. Image from http://www.esrl.noaa.gov/gmd/hats.
Lots of people are interested in what produces the halocarbons that end up in the atmosphere. There are many sources, including anthropogenic ones, but a major source is marine phytoplankton and algae. Like terrestrial plants, these organisms produce oxygen during photosynthesis. Oxygen, and the energy required to produce it, is dangerous stuff to have in a living cell. Like gasoline in a car it can do a lot of damage if the stored energy gets released in the wrong way. The damage oxygen does to cells is called oxidative stress, and cells stockpile antioxidants as a defense against it. There are also certain enzymes that have antioxidant functions, including a class of enzymes called haloperoxidases that act on hydrogen peroxide (a highly oxidizing byproduct of photosynthesis). As the name implies, halogen atoms play a role in the reaction catalyzed by haloperoxidases. The product of the reaction is a halogen that is short an electron, making it particularly reactive. It can react further with another molecule of hydrogen peroxide regenerating the original halogen ion. Alternatively it can receive a carbon atom from another enzyme, making it a methylhalide.
The general reaction of haloperoxidase and hydrogen peroxide (Butler and Walker, 1993).
What does all the have to do with frost flowers and young sea ice? Phytoplankton in the water column are preferentially entrained in young sea ice as the ice forms. There, close to the light, they continue photosynthesis. Halocarbons produced by their haloperoxidases are released into the brine network in sea ice rather than seawater. As continued ice growth pushes this brine to the surface (some of which ends up in frost flowers) the halocarbons may end up in the atmosphere much quicker than those produced by phytoplankton in the water column. The data seems to support this, halocarbon concentrations in young sea ice peak near the surface several days after ice formation (perhaps when the entrained phytoplankton become adapted to their new home and resume photosynthesis). After several days however, halocarbon concentations at the ice surface are greatly diminished, possibly due to the loss of connectivity in the brine veins as the ice temperature decreases.
Bromoform, the most abundant halocarbon measured in the study, peaks close to the ice surface at around day 12 (Granfors et al., 2013).
There is also a possible bacteria source of halocarbons in young sea ice and frost flowers, tying this work to our recent publication in EMIR (Bowman and Deming, 2013). There we reported the presence of an unusual community of bacteria that we hypothesize are symbionts of phytoplankton trapped in the ice. Many of these bacteria belong to taxonomic groups known to produce halocarbons using haloperoxidases similar to those contained in phytoplankton. They do this to alleviate the oxidative stress of their phytoplantkon partners, which enables the latter to undergo increased levels of photosynthesis (which ultimately benefits the bacteria).
We have not observed many phytoplankton within frost flowers or at the ice surface, conditions there are probably too hostile for them, but there is an abundance of hydrogen peroxide and other oxidizing compounds produced by sunlight at the ice surface. Perhaps the bacteria in frost flowers, for the short time they are able to survive, are engaged in their normal task of eliminating these harmful compounds and producing halocarbons. Unfortunately we have not yet undertaken measurements of halocarbons and other biogenic compounds in parallel with analyses of the frost flower microbial community. Until we do this we can only speculate!
