In drylands around the world, human activity has harmed soil quality, in part by damaging biological soil crusts (biocrusts) composed of photosynthetic organisms like cyanobacteria, algae, mosses, and lichens that grow as biofilms. Biocrusts make up a “living skin” that covers around 12% of Earth’s surface and provide essential ecosystem services to drylands. Trampling by livestock and off-road traffic damages established biocrusts, which grow only after rare rain events that moisten the soil. The loss of biocrusts can result in degraded ecosystems.
Scientists are working to restore these damaged biological communities by planting biocrust-forming organisms in disturbed areas. The challenge lies in biocrusts’ slow growth; natural recovery can take years to decades depending on the environment and frequency of disturbance. However, recent research published in Applied and Environmental Microbiology suggests that enlisting the help of beneficial bacteria may give biocrusts their second wind.
The research, led by Ferran Garcia-Pichel, a professor at Arizona State University, aims to restore damaged biocrusts in the southwestern United States by growing cyanobacteria in nurseries and lab cultures. Garcia-Pichel’s team then plants their cultivated biomass in disturbed ecosystems.
To produce microbial material suitable for biocrust restoration (inocula), biologists create environments conducive to faster cyanobacteria growth. Shading, increased frequency of watering, and soil stabilizers at on-site nurseries can create Goldilocks conditions. However, it can still take up to 3 months to obtain sufficient biomass for inocula. Now Garcia-Pichel’s research suggests that growing heterotrophic bacteria with cyanobacteria can further improve growth in biocrust nurseries.
Soil contains many different heterotrophic bacteria (those that consume organic matter), but the bacteria that Garcia-Pichel’s team use are special. They physically attach to the cyanobacteria, establishing a mutualistic relationship. The heterotrophic bacteria provide the cyanobacteria with essential nutrients, while the cyanobacteria feed the heterotrophic bacteria with carbon produced during photosynthesis.
Low Investment, High Returns
Can restoration biologists use such bacterial partnerships to aid biocrust restoration? Garcia-Pichel’s team tested this idea by adding cyanobacteria to desert soils in the lab and in outdoor nurseries. Then they compared the amount and speed of cyanobacteria growth with and without the heterotrophic bacteria.
The heterotrophic bacteria from dryland biocrusts improved the speed and amount of cyanobacteria growth under laboratory and nursery conditions, acting as a probiotic of sorts. In some soils, adding the heterotrophic bacteria alone was sufficient to increase biocrust growth.
The research findings suggest that adding these probiotic bacteria alone may help remnant cyanobacteria in disturbed biocrusts recover in some cases. This could benefit biocrust restoration, because the heterotrophic bacteria require much less time and effort to grow than cyanobacteria. According to Garcia-Pichel, “[the time investment is] nothing—it’s overnight cultures.” And, while not always effective in restoring biocrusts alone, adding the probiotic bacteria with cyanobacteria inocula could give the cyanobacteria an easier time in more degraded areas.
“For sure, I think this is beneficial,” said Anita Antoninka, a biocrust ecologist at Northern Arizona University who was not involved in the research. “[The study shows that] you can increase the growth of [individual] strains more quickly by adding heterotrophic bacteria.”
So far, biocrust probiotics have been used only under nursery conditions, and further assessment of their effectiveness on restoration in nature is an important next step. According to Antoninka, whole biocrust inocula obtained from more pristine sites could already have sufficient heterotrophic bacteria present. So the effectiveness of probiotics in biocrust restoration is likely dependent on the inoculum and the extent of degradation.
—Derek Smith (@djsmitty156), Science Writer