Biological Soil Crusts

Definition: Surface-bound assemblages of microorganisms consolidate soils into mm to cm-thick crusts that occur on arid lands wherever the lack of water restricts the settlement and development of plant cover. To know more about Biological Soil Crusts, please explore the further sub-sections.

Introduction :

Biological soil crusts (BSCs) are also known as cryptogamic or cryptobiotic microbial communities. They are complex microbial communities dominated by cyanobacteria as primary producers that build crust on the top layer of arid lands soils.

Facts numbers:

  • 35% of the total Earth’s continental surface is covered by arid lands, BSC usually cover these areas
  • 30 to 350 kg C ha-1 is the annual range of carbon input in BSC
  • 1 to 100 kg N ha-1 is the annual range of nitrogen fixation in BSC
  • 4th is the position of Microcoleus vaginatus in the World ranking of the most abundant cyanobacteria
  • 54 x 1012 g of Carbon is the biomass of microbial primary producers in BSC

In spite of their geographic extent and ecological importance, many aspects of the biology of BSCs remain unknown; this is why we are studying them!

BSC formation, story of a very slow process

1) Crusting is initiated by growth of filamentous cyanobacteria (e.g. Microcoleus sp.) during episodic events of available moisture.
2) As they grow, these cyanobacteria produce a high amount of slime (extra-polymeric substances) that traps mineral particles.
3) This process result in the formation of a pioneer light-crust
4) Once the crust is stabilized other microbes colonize the crust. For instance other cyanobacteria (e.g. Nostoc sp.) forms black colonies on the top of the crust
5) Later on, other organisms, such as lichens, eukaryotic microalgae, and mosses, may be integrated as dwellers of the crust.

Greening deserts: the incredible ability of Microcoleus sp.

When BSC get wet they rapidly turn green.
This is due to Microcoleus sp. that moves to the surface of the crust once exposed to water. These bacteria are doing oxygenic photosynthesis and carry green-blue photosynthetic pigments; this is why the surface of the soil turns green when they move there.
The answer of Microcoleus sp to wetting event corresponds to a quick metabolic shift that allow the turn ON of metabolism pathways.
Once the crust get dry, Microcoleus sp go back inside the crust, a few millimeters below the surface. Microcoleus sp filament can survive for long periods in this hostile environment ( dry, with low light and low nutrients input) through days, months or years…until the next wetting event.
The behavior of Microcoleus sp. and more generally of the whole crust microbial communities through these wetting-drying cycles is studied in collaboration with research groups from LBL and JGI.

Objective :

We are interested in the description of how the community switch ON when the crust get wet and then switch OFF and get prepared to long periods of dessication when the crust dry.


To describe this process we are using a combination of cultivation, direct DNA/RNA sequencing, metabolites identification, in silico modelling and imagery based techniques.
If you want to know more about this project, please contact us!
You can also visit the webpage of our collaborators in Laurence Berkeley laboratory.

References :

1) F Garcia-Pichel, J Belnap, S Neuer, F. Schanz – Algological Studies , vol. 109, 2003 Estimates of global cyanobacterial biomass and its distribution
2) F Garcia-Pichel, O Pringault – Nature, 2001 Microbiology: Cyanobacteria track water in desert soils

Destruction and recovery

Human disturbances in Biological Soil Crust (BSC) often create severe environmental problems. The dust storms or “Haboobs” that have been increasingly striking Phoenix area in the recent years are a good local example. Biological Soil Crusts are responsible for maintaining soil cohesion and stability in arid areas. They also improve the soil fertility and play an important role in the germination, growth and survival of native species of plants.
Our main research objective is to facilitate the recovery of degraded arid and semi-arid lands through the restoration of the biological soil crusts. This project is developing laboratory methodologies and establishing a nursery for testing the inoculation techniques and biocrust formation. The inoculation techniques will also be tested and monitored in the field, operating in a degraded biocrust from the Department of Defense Training Areas. Special attention will be paid on whether biocrust restoration may facilitate the function of native plant vs. colonization of exotic plants.


Garcia-Pichel, F., Lopez-Cores, A., and Nubel, U. (2001) Phylogenetic and morphological diversity of cyanobacteria in soil desert crusts from the Colorado Plateau. Applied and Environmental Microbiology 67: 1902-1920.

Belnap, J., and Lange, O.L. (2001) Biological soil crusts: Structure, function, and management. Spirnger-Verlag. Berlin, 479 p.

Zaady, E., Gutterman, Y., and Boeken, B. (1997) The germination of mucilaginous seed of Plantago coronopus, Reboudia pinnata, and Carrichtera annua on cyanobacterial soil crust from the Nagev Desert. Plant Soil 190: 247-252

Garcia-Pichel, F., and Belnap, J. (1996) Microenvironments and microscale productivity of cyanobacterial desert crust. Journal of Phycology 32: 774-782.

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