Scientists on Sunday said that methane which leaked from the 2010 oil-rig blowout in the Gulf of Mexico persisted in the sea for months beyond a presumed cleanup of the gas by marine microbes.

As much as half a million tonnes of natural gas, 80 percent of it methane, leaked into the deep sea as a result of the blowout on April 20, 2010, on BP's Deepwater Horizon rig.

The leak triggered a surprising "bloom" of marine bacteria that feasted on the gassy hydrocarbon plume.

The bugs performed a valuable environmental service, helping to prevent gas from lingering in the sea -- where it would contribute to ocean acidification -- or from escaping to the air, where it would add to the greenhouse-gas problem.

The bloom was so dramatic that, by the end of August, tests suggested all the gas had been mopped out by these natural little helpers.

But in a study published in the journal Nature Geoscience on Sunday, US marine scientists said the bloom abruptly declined at the end of June, even as methane concentrations remained about 5,000 times above background levels.

The bugs did indeed remove a significant amount of the gas, but their population crashed while the leak was still in progress, it said.

Engineers eventually capped the blowout on July 15, after 83 days. In addition to the gas, around four million barrels of oil escaped into the Gulf of Mexico.

Data from research expeditions that ran from May to December 2010 suggest that the residual plumes dispersed, according to the study.

Above-normal methane concentrations from the well, carrying a telltale carbon isotope signature, were found over a large area north and northeast of the wellhead, and this persisted until the end of the year at least, the study said.

The investigation, headed by Samantha Joye at the University of Georgia, did not estimate how much gas was not gobbled up by the microbes.

In addition, it was not designed to assess any environmental damage.

Why the microbial bloom crashed is unclear, but the fact that it happened underscores the many uncertainties in the complex marine environment when a gas leak occurs, it said.

Potential factors in these blooms include the availability of other nutrients for the bacteria, currents, other microscopic marine life and chemicals used to disperse oil slicks.