Bloodworms
© Christian Lott / MPI Bremen / HYDRABloodworms are just one type of water-dwelling animal that produces laughing gas.

They may be no match for methane-burping cows, but bloodworms are doing their best to make a name for themselves with climate scientists. New research shows that their guts leak "laughing gas" - a powerful greenhouse gas - albeit in amounts too small to significantly affect the climate.

Previously, no water-dwelling animal was known to produce the gas, more properly known as nitrous oxide (N20).

Some land invertebrates such as earthworms are known to produce nitrous oxide, so to see if water invertebrates are also a source, Peter Stief and his colleagues at Aarhus University in Denmark surveyed seven aquatic sites including freshwater creeks, lakes and the seashore.

They collected a wide range of worms, larvae and bugs, placed them in closed vials, and analysed what came off. They found not only that N20 is produced, but that the amount increased with time.

The researchers suspected that microbial "bioreactors" in the animals' guts might produce the gas. To test their hypothesis, they took a closer look at the bloodworm (Chironomus plumosus), a ruby-red midge larva that is a favourite food of trout.

Mud munchers

Bloodworm larvae live in U-shaped burrows in sediment at the bottom of lakes and rivers. They wriggle within these tubes in order to draw in oxygen- and nutrient-rich water through one opening and pump it out through the other. For food, they munch on organic material from the sediment.

Along with their food, the larvae also ingest bacteria that live in the mud. These bacteria are not digested, however, and can survive in the larvae's guts by metabolising nitrates, producing N20 as a by-product.

The team dissected bloodworms, placed the guts in vials, and then looked for changes in gas concentrations.

They found that in the absence of oxygen, bloodworm gut produces the same amount of laughing gas as the whole larva, demonstrating that processes within the gut are solely responsible for the leak. Adding nitrate to the vials boosted the amount of N20 that leaked out of the guts; adding oxygen decreased it.

By tracking the movement of nitrous oxide in the dissected guts, the researchers showed that the gas leaks through the gut wall, then through the skin and out into the environment.

No laughing matter

Steif's experiments suggest that the presence of bloodworm larvae and other animals contribute less than 1% of the N20 in the atmosphere. The bulk of it is emitted by fossil-fuel burning.

Maija Repo of the University of Kuopio in Finland says the finding adds to our understanding of how aquatic systems respond to certain chemical changes in the environment, such as nitrate pollution.

Sadly, Steif was unable to determine if the laughing gas made the larvae happier than they otherwise would have been. "I tried to see if they smiled when they were all crowded into sealed test-tubes," he jokes.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0808228106)