BACKGROUND

Nutrients are blowing in the wind!

Did you know that massive amounts of tiny mineral particles originating from continental deserts are transported by the wind over distances of thousands of km to be deposited on soil, plants and into the ocean [1]? It happens that such desert dust typically carries nutrients (nitrogen - N, phosphorous - P; iron - Fe, zinc - Zn) that are essential for plants and marine phytoplankton to perform photosynthesis and convert atmospheric CO2 into oxygen. This is especially important in the “ocean deserts” where nutrients are too deep in the water column of the ocean, where light levels are often too low for allowing the algae to perform photosynthesis [2]

 NASA models and supercomputing have created a colorful new view of aerosol movement. Source: https://svs.gsfc.nasa.gov/10977

NASA models and supercomputing have created a colorful new view of aerosol movement. Source: https://svs.gsfc.nasa.gov/10977

The dust ballasting effect

Dust deposition in the ocean is also thought to promote the sequestration of carbon from the productive photic layer down to the deep sea by acting as a mineral ballast of sinking particles and organic matter aggregates, thereby accelerating the so-called "biological carbon pump" [3]. In fact, it has been recently demonstrated that more abundant and faster-sinking aggregates are generated when formed from a natural plankton community that has been exposed to atmospheric dust deposition, compared to less abundant and slower-sinking aggregates that are formed without dust [4].

The Fe Hypothesis

By fertilising the ocean (nutrient source) and by accelerating the biological carbon pump (ballasting organic matter), dust deposition is likely to change the Earth’s climate and atmospheric CO2. This so-called “Fe hypothesis” was first raised by John H. Martin in 1990, arguing that Fe input by desert dust may have played a major role in stimulating phytoplankton production in the past, leading to a significant drawdown of atmospheric CO2 during glacial times [5]. In addition, because phytoplankton are primary producers, changes in the patterns of their productivity are also likely to influence remaining oceanic food chain, thereby modifying the marine ecosystems as well as fishing stocks. 

Sahara desert: THE WORLD’S LARGEST SOURCE OF ATMOSPHERIC DUST

Of all the continental deserts, the Sahara is the world’s largest source of atmospheric soil dust. Around 200 million tons of Saharan-dust blown into and over the Atlantic every year are thought to supply nutrients for marine phytoplankton in the tropical North Atlantic, where N-fixation by marine phytoplankton is co-limited by Fe and P [6]. In spite of the tropical North Atlantic acting as a natural sink of dust transported all the way from Africa, little is known yet about its effects on phytoplankton communities across this region.

 Huge plumes of storm-blown Saharan dust as revealed from satellite images. Source: https://www.voanews.com/a/study-saharan-desert-dust-feeds-amazon-rainforest/2676025.html

Huge plumes of storm-blown Saharan dust as revealed from satellite images. Source: https://www.voanews.com/a/study-saharan-desert-dust-feeds-amazon-rainforest/2676025.html

WHY USING COCCOLITHOPHORES AS PROXIES FOR DUST-FERTILIZATION?

Coccolithophores (Haptophyta) are amongst the most important groups of phytoplankton in the nutrient-depleted (also called oligotrophic) regions of the global ocean, and hence they are natural inhabitants of the tropical North Atlantic [7]. They differ from other groups in that they cover their cells with tiny calcite plates (the coccoliths). This  provides them the ability of directly interacting and influencing the carbon cycle not only through photosynthesis (CO2 sink), but also via calcification (CO2 source) and carbon-burial in deep-sea sediments (acting as a mineral ballast for the export of organic matter, in a similar way to that of dust particles) [8]. Being both photosynthetic and calcifying, coccolithophores can be studied directly from the photic layer of the ocean where  phytoplankton typically thrives, and from sediment trap devices collecting particles (hence, also coccoliths) over longer periods of time. By also including more opportunistic taxa that quickly respond to short-term changes linked to nutrient input, coccolithophores provide interesting perspectives as indicators of ocean fertilization by dust, and its contribution to the organic and inorganic carbon pumps of te ocean.

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References

[1] Scheuvens and Kandler, 2014. On Composition, Morphology, and Size Distribution of Airborne Mineral Dust. In Mineral Dust: A Key Player in the Earth System. P. Knippertz and J.-B.W. Stuut (eds.), Springer Science C Business Media Dordrecht 2014, 15-49. [2] Jickells et al., 2005. Global iron connections between desert dust, ocean biogeochemistry and climate. Science (308), 67–71. [3] Pabortsava et al., 2017. Carbon sequestration in the deep Atlantic enhanced by Saharan dust, Nat. Geosci., 10, 189–194. [4] van der Jagt et al., 2018. The ballasting effect of Saharan dust deposition on aggregate dynamics and carbon export: Aggregation, settling, and scavenging potential of marine snow. Limnol. Oceanogr. 00, 1–9. [5] Martin, 1990. Glacial Interglacial CO2 change: The Iron Hypothesis. Pale oceanography 5(1), 1-13. [6] Goudie and Middleton, 2001. Saharan dust storms: nature and consequences. Earth. Sci. Rev. 56, 179–204. [7] Winter, et al., 1994. Biogeography of living coccolithophores in ocean waters. In: Coccolithophores, edited by: Winter, A. and Siesser, W., Cambridge Univ. Press, 161–177. [8] Rost and Riebesell, 2004. Coccolithophores and the biological pump: responses to environmental changes. In: Coccolithophores from molecular processes to global impact. H. Thierstein; J. Young (Eds.) Berlin, Springer, 99-125.