With global warming, large forest fires, like the ones that devastated Australia in 2020 are expected to occur with increasing frequency. However, these fires can have an unexpected impact. They can grow algae thousands of miles away.
Green spots on the ocean, spots which put end to end have an area equivalent to that of the Sahara (more than 9 million km2), this is what appeared on satellite images between Australia and the southern coasts. Americans, at the beginning of last year. These green traces corresponded in fact to an astonishing proliferation of phytoplankton, microalgae. For the first time, a study published in the journal Nature was able to establish that this appearance of algae was directly linked to the Australian forest fires that occurred several thousand kilometers away.
These microalgae were able to proliferate exceptionally, feeding on the ashes of the fire. Clearly, the plumes of smoke, rich in iron particles produced by the combustion of trees, fertilized the ocean and by falling, in an area usually poor in iron, allowed this exceptional development of microalgae.
The story does not end there. The other observation is that these microalgae, by developing thanks to photosynthesis, have absorbed part of the carbon released by these fires, explains Morgane Perron, co-author of the study. 700 million tonnes of additional CO2 were released into the atmosphere, recalls this researcher from the Institute for Marine and Antarctic Studies (IMAS), University of Tasmania in Australia. These algae absorbed most of it before disappearing after four months, when the ashes of the smoke stopped feeding them.
However, this phenomenon does not occur systematically. We already knew that the ocean was a carbon sink, it absorbs a third of the CO2 due to human activities, but there were exceptional circumstances for these algae to appear: wind, temperatures, season. The other side of the coin is that these plumes of smoke also carried heavy metals, lead, zinc, mercury, which also fell back into the water.