Plans to curb climate change by using plankton to draw carbon dioxide into the world's oceans have been boosted.
A spectacular natural algal bloom in the Southern Ocean helped to "lock" carbon away into deep sea sediments, according to a study in Nature journal.
But the amount of carbon stored was not nearly as high as some artificial "geo-engineering" schemes had predicted.
Plans to "seed" plankton blooms by adding iron to oceans are strongly opposed by many green groups.
The international research team behind the Crozex study say their findings have "significant implications" for plans to mitigate climate change.
They come as scientists resume a controversial ocean fertilisation experiment in the Scotia Sea, east of Argentina.
The Lohafex study had been suspended by the German government after environmental groups protested that it violates the terms of the UN Convention on Biological Diversity.
They fear that adding iron to oceans may damage ecosystems.
Using algae as a "biological carbon pump" has been touted as one of the more promising "geo-engineering" schemes for mitigating global warming.
Plankton act as a natural sponge for carbon dioxide - drawing the greenhouse gas down out of the atmosphere and into the sea.
When plankton die, they sink to the bottom of the ocean, locking away some of the carbon they have absorbed.
Experiments suggest that "seeding" oceans with iron can stimulate the growth of plankton - particularly waters which are rich in nutrients.