The great mechanics of the Earth's climate is an extremely complex system, based on a subtle balance of different micro-processes. Among these, carbon exchanges at the ocean surface play a predominant role. Researchers at the EPFL and the Institut Méditerranéen d'Océanologie (MIO) in France recently discovered a new component: a source of biological phosphorus that potentially acts as a major source of food for phytoplankton, the populations of microalgae that evolve on the surface of the water and perform functions essential to life on Earth. This source, the exact extent of which has never before been studied, could have major implications for the climate and greatly refine the effectiveness of climate forecasting models. This discovery was recently published in the journal npj Climate and Atmospheric Science.
To grow, phytoplankton need a variety of chemical elements, mainly iron, nitrogen and phosphorus. This cocktail enables these microscopic plants, which live in large sheets in the surface layers of lakes, seas and oceans, to thrive and carry out their favourite activity: photosynthesis. In doing so, they play a crucial role in regulating the climate, capturing large quantities of carbon dioxide (CO2) from the atmosphere, storing it in the water in the form of biomass and releasing the oxygen that is so precious to living beings. Phytoplankton also plays a third essential role: at the base of the food chain, it enables many marine species to survive.
Phosphorus inputs and availability therefore condition the development of phytoplankton, photosynthesis rates and the overall amount of CO2 absorbed. This is why it is important to identify all the sources of fertilisation in marine ecosystems in order to gain a better understanding of climate mechanisms and how human activities can affect them.
Complete the table
"It had already been established that a significant amount of inorganic phosphorus is delivered to remote marine ecosystems by airborne mineral dust in the form of minerals or phosphate ions, but this picture was in fact not complete," explains Kalliopi Violaki, lead author of the study and a researcher at the Laboratory of Atmospheric Processes and their Impacts (LAPI), which is part of the EPFL's Faculty of the Natural, Architectural and Built Environment (ENAC).
During a year-long sampling campaign in the eastern Mediterranean, at the Cretan site of Finokalia, this scientist discovered the existence of a major source of phosphorus contained in airborne biological particles, such as viruses, bacteria, moulds, plant fibres and pollen grains. While the exact quantities are not yet fully established, researchers already know that they are at least comparable to those of inorganic phosphorus carried by dust aerosols. What's more, this biological phosphorus is largely in the form of phospholipids, a key component of cell membranes.
"The fact that terrestrial ecosystems can fertilise marine ecosystems by means of bioaerosols, i.e. atmospheric microparticles of biological origin, is a completely new concept that changes our understanding of the active mechanisms controlling the carbon cycle, and therefore the climate", points out Athanasios Nenes, director of the LAPI and co-author of the study.
A major parameter
Currently absent from models, this flow of organic phosphorus could prove to be a major new parameter, providing a much more detailed understanding of how marine ecosystems respond to climate change. Global warming is already increasing the density of ocean layers. The consequences are a slowing down of the mixing between these strata - which differ in terms of density, temperature, oxygen levels and salinity - and a disruption of the oceans' capacity to absorb CO2. This increase in stratification also prevents the distribution of nutrients between the seabed and the surface, potentially leading to a dangerous change in the natural environment and food sources for a large number of marine organisms. Taking account of this new source of phosphorus could therefore change our idea of how the seas and oceans will react to climate change.
This LAPI research demonstrates the crucial role of atmospheric particles in the environment, and that changes in their flow can be just as many grains of sand in the overall climate mechanism. This is why the study will continue to determine the exact influence of this new source.
References
"Bioaerosols and dust are the dominant sources of organic P in atmospheric particles", published on 3 December in Nature Climate and Atmospheric Science. Authors: Kalliopi Violaki, Athanasios Nenes, Maria Tsagkaraki, Marco Paglione, Stéphanie Jacquet, Richard Sempéré, Christos Panagiotopoulos.