The Role of Microbes in the Tropical Rainforest

Have you ever wondered how a Tropical Rainforest can sustain itself, without fertilizers and maintenance from Humans?

Well the organic layer, in which the trees are rooting, is in most cases just 30 to 50 cm thick on average and because the sub-soil frequently consists of Rock, Clay Mineral (Kaoline) or a layer not suitable for rooting, the roots, except for crevices,  are mainly growing horizontally. The trees are depending on its others support. Still the full-grown rainforest does not need the help from Humans. Humans are doing harm to the rainforest, most of the time due to poor knowledge of the processes, which are taking place above and more so below the soil. The Amazon forest is according to some scientist 110 million years old, while others estimate it to exist 450 million years. The 110 million would coincide with the time the American continents drifted to the west, until it bumped into the Pacific Plate forming the Andes Mountains/Rocky Mountains. See: 180 million years ago the supercontinent Pangea began to break up into two continents, Laurasia and Gondwanaland, at Nature.org. At the same time, the Atlantic Ocean was formed. I believe the 110 -150 million is about right, although some flora and fauna compare with that of Africa and Europe for that matter.

Cross section of a nodule. See the membrane between the vascular system of the tree and the nodule

The rainforest depends on processes in symbiosis with microbes, like Mycorrhizae (root fungi) and Rhizobacteria (root bacteria), which are predominantly endophytic (living in the host). Among the heterogeneous tree population are species belonging to the Leguminosae, a major group of plant families. These plants are of the utmost importance, due to the major need for Nitrogen, which they acquire out of the air in gaseous form (N₂). The absorption of Carbon Dioxide (CO₂) from the air also needs the help of the microbes and Nitrogen to convert it to nutrients for the forest, while releasing Oxygen in the air. The air consists of 78% Nitrogen (N₂) and 21% Oxygen (O₂). The remainder are various gases of which Carbon dioxide (CO₂) forms the majority.

Root fungus in symbiosis with the plant

Here comes the symbiosis with fungi and bacteria. The trees, forming nodules in their rooting systems, secrete flavonoids to attract bacteria. Most of the bacteria live inside those nodules and convert the Nitrogen gas into compounds, which are readily available for uptake by the roots, such as Nitrates (NO₃), Nitrites (NO₂) and Ammonia (NH₃). The trees are pumping these converted compounds in the soil for the benefit of all plant life, including the fungi. The processes in the forest also depend on some macro organisms; to decompose large trunks and other dropped phytomass. To name a few, beetles, caterpillars larvae, birds etc.

rhizobacteria

Nodules in the root system of the tree

Rhizobacteria in the rootzone

The fungi are transporting nutrients through the tree and between the trees so even those, not exposed to the sun, can benefit from the nutrients obtained through Photosynthesis by others. They are also forming shields around the hair roots, to keep them moist and aid in the absorption of nutrients. They further improve the soils keeping the aggregates together and forming bridges. This functions as absorption complexes to regulate the Air and Water Household in the rooting zone and protect against erosion. It is amazing the cooperation between all the species.

Fungi and bacteria, producing more nutrients for the trees, decomposing the leaves and branches, which fall on the forest floor. At given times certain genera of fungi, belonging to the Basidiomycetes, produce recognizable fruiting bodies like Mushrooms, small and large, which are popping out of the ground or growing on the lower trunks. The start of new growth without sufficient microbes results in a very dense vegetation with a few species and poor growth for many years.

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