Millions of years ago, the wasteland that we know today as the Sahara desert was an orchard. A region of wide grasslands and dense forests, with permanent lakes. A distinctive feature of the water masses that receive water from areas as productive as forests is that they accumulate a large amount of organic matter and mineral salts. These nutrients are the perfect breeding ground for the development of a wide variety of algae, among which one group stands out: diatoms.
What are diatoms?
Diatoms are a large, highly diversified group of unicellular algae. Unlike plants, which have a cellulose cell wall, each diatom is encapsulated in an exoskeleton, called a frustula , made up of two structures called thecae , made of silica minerals, which fit one into the other like a box and its lid . Theca are often elaborately ornamented with various pore patterns that are specific to each species, and are very useful for identification.
With the exception of the Dead Sea, no body of fresh or salt water is known without diatoms. Of course, they are photosynthetic organisms that retain carbon and release oxygen into the atmosphere. However, their greatest ecological virtue happens when they die. Porous ornamentations are optimal structures to adsorb substances dissolved in water. In this way, the frustules retain an enormous quantity of nutrients even after the diatom is dead: carbonates, calcium, phosphorus, the very silica of which it is composed, and even the organic matter that was part of the living diatom.
If the remains reach the bottom, they can end up buried along with the rest of the sediment and fossilize, forming a type of rock that we call diatomite , from which the well-known “diatomaceous earth” is obtained.
However, in the Sahara another phenomenon happened. When the water evaporated and the lakes dried up, the diatoms’ exoskeletons, dry and retaining all those nutrients, turned to dust and became part of the desert sand. Of all the lakes, the one known as the Bodélé depression in Chad stands out, currently considered the largest source of dust on the planet.
The journey to the new world
The sand of the Sahara is one of the finest known. It is easy for the wind to carry up to 60 million tons of dust a year. Being so light, it remains suspended in the atmosphere, and can travel long distances with the wind, hundreds and even thousands of kilometers, crossing seas and oceans and falling on other continents.
That is what has recently happened in Spain. We call it haze , and it causes episodes of orange haze that can be deposited covering everything. If the situation arises, it can rain mud.
However, the most common course taken by the suspended dust is not towards Europe, but towards South America. On his trip, he passes through the Canary Islands, where the calima phenomenon is well known. And all that dust ends up landing in the Amazon River basin.
The needs of the Amazon rainforest
The contribution of African dust is essential for the maintenance of the forest, and to prevent the depletion of its most precious nutrient, phosphorus.
It is one of the most limiting nutrients in the Amazon basin. The biological part of the phosphorus cycle has a very defined direction, and is given by the drag of the water. Plants and animals can exchange phosphorus relatively easily —through food and feces—but water always ends up carrying it out to sea. There it is assimilated by the algae and enters a new trophic cycle from which it can hardly get out, beyond the small amounts provided by the guano of seabirds. Millions of years are necessary so that, after the algae die, settle and fossilize, they re-emerge as rocks that the water can erode.
However, the Amazon has an invaluable source that provides huge amounts of phosphorus each year. They are those diatom algae that died in the lakes of the Sahara and, turned into dust, have flown thousands of kilometers with the wind until they precipitate with the rains. It is this haze that feeds the Amazon rainforest with phosphorus. It is estimated that the amount of Saharan dust received by the Amazon basin is around 30 tons per hectare per year. And with it, up to 23 grams of phosphorus are deposited per hectare and year to fertilize the basin. The amount of phosphorus that the basin loses through the river is very similar.
the circle closes
Of course, everything that the Amazon rainforest produces, phosphorus included, ends up in the waters of the Amazon, the longest river in the Americas. Its mouth of slow and calm waters is the ideal place for the proliferation of algae, such as diatoms . Algae that retain much of the nutrients dissolved in the water.
Perhaps in millions of years, the world will change so much that the orchard we know today as the Amazon rainforest ends up turned into a wasteland, and its dust, rich in diatom remains, feeds another jungle somewhere else on the planet , providing it with the same phosphorus that once the Sahara granted her with its haze.
Battarbee, RW et al. 2002. Diatoms. In JP Smol et al. (Eds.), Tracking Environmental Change Using Lake Sediments (Vol. 3, pp. 155-202). Kluwer Academic Publishers. DOI: 10.1007/0-306-47668-1_8Fontes, JC et al. 1985. Freshwater to marine-like environments from Holocene lakes in northern Sahara. Nature, 317(6038), 608-610. DOI: 10.1038/317608a0Washington, R. et al. 2006. Links between topography, wind, deflation, lakes and dust: The case of the Bodélé Depression, Chad. Geophysical Research Letters, 33(9). DOI: 10.1029/2006GL025827Yu, H. et al. 2015. The fertilizing role of African dust in the Amazon rainforest: A first multiyear assessment based on data from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations. Geophysical Research Letters, 42(6), 1984-1991. DOI: 10.1002/2015GL063040