Thick gelatinous jellyfish blooms may be unsightly and hazardous, especially for those partial to a dip in the sea. But new research published in today's Proceedings of the National Academy of Sciences shows that seas thick with jellyfish also squeeze fish, and shellfish, out of the marine food-chain. That matters because there has been an increase in these gelatinous blooms over recent decades, which means the humble jellyfish could be party to the reworking of entire marine ecosystems.

The research team, looking into effects of jellyfish swarms on coastal and estuary waters, was led by Rob Condon of the Virginia Institute of Marine Science (VIMS). They focused on the peculiar relationship between jellyfish and their decomposing bacteria - there are very few organisms, other than bacteria, willing to feast on their gelatinous bodies, and the large amounts of slime they produce.

The composition of that slime is key, to understanding why jellyfish can have such a big impact on the food-chain around them. The team have found that jelly-fish slime, that the creatures release continuously into the water, is extremely rich in carbon. In fact, it contains up to six times more carbon, relative to nitrogen, than other types of marine organic matter.

Experiments to test how this was digested by various bacteria were conducted in laboratories at VIMS - as well as France and Canada. They showed that this high-carbon material was decomposed very differently from other types of dissolved organic matter.

'The bacteria metabolized this carbon-rich material two to six times faster than they did with dissolved organic matter from water without jellyfish,' says Condon. 'This rapid metabolism shunted carbon toward respiration rather than production.'

Respiration by bacteria is similar to human respiration, where the carbon gets turned into CO2. As that that CO2 bubbles out to the atmosphere, it takes any potential energy, from the carbon, out of the food chain. Comparing jellyfish slime to the usual bacterial food - phytoplankton - is like comparing 'drinking Gatorade' to 'eating a hamburger' said co-author Deborah Steinberg, also from VIMS.

That change in food source had a direct bearing on the family of microbes found in the jellyfish slime-enriched waters. "Dissolved organic matter from jellyfish favored the rapid growth and dominance of specific bacterial groups that were otherwise rare," says Condon. And the changes are likely to work up the food-chain. With much carbon escaping out of the seas, the phytoplankton have less to feed on - and so too do their predators, fish and shellfish.

However the ultimate culprit in this story is not the jellyfish - but man. A range of factors, from overfishing and fertilizer runoff, to altered habitat and climate change, are thought to be driving the increase in blooms. That's something Condon has already seen locally. 'Indeed," he says, "we've seen this already in Chesapeake Bay. If these swarms continue to emerge, we could see a substantial biogeochemical impact on our ecosystems.'